Disseminated intravascular coagulation: new identity as endotheliopathy-associated vascular microthrombotic disease based on in vivo hemostasis and endothelial molecular pathogenesis

In 1924, Moschcowitz [9] first recognized a thrombotic blood disorder characterized by disseminated hyaline microthrombi in terminal arterioles and capillaries of many organs of a young woman who died of multiorgan failure. Later, Singer et al. [10] named this disorder as thrombotic thrombocytopenic purpura (TTP) and attributed it to generalized platelet thrombosis. Later, TTP was found to be caused by microthrombosis. TTP is characterized by thrombocytopenia, microangiopathic hemolytic anemia (MAHA) and often with dysfunction of the brain and kidneys. A quarter century later in 1950, McKay [1, 11] separately coined the term DIC for another blood disorder observed in a woman who died of disseminated microvascular thrombosis associated with hemorrhage progressing to multiorgan dysfunction syndrome (MODS). Because of thrombosis (coagulation) and hemorrhagic nature, the condition was termed “disseminated intravascular coagulation”. In retrospect, “DIC” ascribed by McKay and his late followers was very similar to TTP described by Moschcowitz in its pathologic feature of disseminated intravascular microthrombosis (DIT) and hematologic features of thrombocytopenia and MAHA as well as organ dysfunction syndrome.

“DIC” typically has occurred in association with critical illnesses such as sepsis and polytrauma, and often accompanied by hemorrhagic syndrome. The hemorrhagic syndrome has been inferred to be due to “consumption coagulopathy” resulting from disseminated fibrin clots formation and consumption of coagulation factors. Since acute promyelocytic leukemia (APL) is also characterized by consumption coagulopathy, this condition has been included in the column of “DIC” although hematologic features are very different from “DIC”. Several decades ago, clinical pathologists [12, 13] have found the pathology of “DIC” was consistent with “microthrombi” very similar to those of TTP, which are pathologically different from “fibrin clots” seen in DIC of APL. However, many coagulation specialists have used the terms “microthrombi” and “fibrin clots” interchangeably even though the true character of DIC of APL is made of fibrin clots and that of “DIC” observed in sepsis is made of microthrombi composed of platelet-ULVWF complexes, which character is the same to microthrombi occurring in TTP [14]. This interchangeable interpretation simply has reflected the ironclad concept that all “blood clots” (i.e., thrombi and fibrin clots) must be the same because the process of hemostasis and its end product of coagulation must be the same.

Until this date, in clinical settings, “DIC” and TTP are considered to be two distinctly different microthrombotic disorders, somehow each arising from utilization of yet undetermined but the same pathogenetic mechanism because “DIC” is clearly a hemostatic disorder causing hemorrhagic syndrome associated with conditions such as sepsis, and TTP is an enzymatic disorder due to ADAMTS13 deficiency. In addition, TTP-like syndrome has oftentimes been included in the column of TTP, and was called “thrombotic microangiopathy”, which term does not represent a disease [6]. Currently, the “thrombotic microangiopathy” has been designated when thrombocytopenia and MAHA are present without severe ADAMTS13 deficiency in an acquired medical condition. Nonetheless, pathologically all of them are characterized by microthrombosis in arterioles and capillaries. When sepsis presents with clinical features of thrombocytopenia, MAHA, MODS and abnormal coagulation profile, most of clinicians have called it “DIC”, but others classified it as TTP-like syndrome when coagulation profile was normal [6, 15]. Some theorists have insisted that “DIC” and “TTP-like syndrome only share the same pathophysiological mechanism because of different underlying diseases [16]. However, this author has affirmed “DIC” and TTP-like syndrome are the same disease after clinical, pathological, hematological and coagulation evaluation as well as atypical behavior of “DIC” to numerous anticoagulation therapies [5, 6, 17]. Previously, the conceptual difference between TTP and “DIC” was supported by the claim that TTP was due to excess of ULVWF as the result of protease ADAMTS13 deficiency, but “DIC” was due to a hemostatic disease associated with endothelial dysfunction as seen in sepsis. TTP is defined as a microthrombotic disease associated with ADAMTS13 deficiency resulting from either hereditary TTP (gene mutation-associated vascular microthrombotic disease [GA-VMTD]) or due to acquired autoimmune TTP (antibody-associated VMTD [AA-VMTD]). But conceptually “DIC” has been attributed to uncontrolled activation of disseminated intravascular hemostasis, leading to fibrin clot disease via activated TF path, which is only known thrombotic (coagulation) path in hemostasis at this time. Therefore, abnormal coagulation profile in “DIC” is thought to be the result of consumption and depletion of clotting factors during disseminated intravascular coagulation and secondary fibrinolysis [16, 18, 19].

Understandably, contemporary dogmatic hemostatic theory based on TF path, which is only one pathway known in hemostasis in vivo, has been applied to the interpretation of “DIC”. It is defined to be coagulopathy caused by endotheliopathy in sepsis and other critical illnesses, triggering TF/FVIIa-initiated coagulation cascade that mediates systemic inflammation and coagulation/thrombosis, leading to blood clots containing fibrins. Because of coexisting inflammation and coagulopathy in endotheliopathy, the popular theory of “crosstalk” between inflammation and coagulation has been introduced and readily accepted. Also, the use of different descriptive terms of “thrombosis” on TTP/TTP-like syndrome and “coagulation” on “DIC” in the medical literature has kept the distance between two diseases even though their underlying pathology is the same “microthrombosis”. For TTP/TTP-like syndrome and “DIC”, more generalized character term “microvascular thrombosis” has been designated and accepted to be due to ADAMTS13 deficiency for the former (TTP) and endotheliopathy for the latter (“DIC”). But for “DIC”, the term “coagulation” is used and has implied that “DIC” is “fibrin clot disease” occurring as a result of uncontrolled hemostasis. These designations have strengthened the notion that TTP, TTP-like syndrome and “DIC” are three different diseases. TTP is an enzymatic disease, TTP-like syndrome is a condition associated with “thrombotic microangiopathy”, and “DIC” is a hemostatic disease. This misconception that fibrin clots are responsible for “DIC” has contributed to the delay for the medical community to recognize the genuine character of “DIC” and discovering the true mechanism of in vivo hemostasis.

Sometimes clinicians have been puzzled and fascinated by the “shared” pathogenetic mechanism not only among TTP, TTP-like syndrome and “DIC”, but also other hematologic disorders producing thrombocytopenia and MAHA, MODS such as poorly defined thrombotic microangiopathy, and hemolytic-uremic syndrome (HUS) [16, 18‐21]. However, these ambiguous concepts between “microvascular thrombosis” and “coagulation” have never been seriously questioned and explored, primarily due to unmistakably characteristic feature of hemorrhagic syndrome observed in acute “DIC”.

Not surprisingly, another conflicting concept about “DIC” has begun to encroach into the critical care medicine where the use of the term “DIC” in clinical diagnosis has been liberalized and expanded to include the cases of poorly defined “thrombotic microangiopathy” even with normal coagulation profile and absence of hemorrhagic syndrome [22, 23]. Many critical care clinicians have become comfortable in taking this proposition as chronic “DIC” whenever thrombocytopenia, MAHA and MODS in particular occur in critically ill patients even though no hemorrhagic syndrome is presented. Some coagulation specialists have been reluctant to use this identity as “chronic DIC”. In Japan, this additional term has been very enthusiastically accepted even with governmental funding and supports and is now designated as “chronic DIC”. European hemostasis/thrombosis community also has used these terms extensively, including “compensated” or “covert” “DIC” in contrast to “acute”, “uncompensated”, or “overt” “DIC” when significant hemorrhagic phenotype occurs.

Yet, some hematologists have stubbornly insisted the term TTP or TTP-like syndrome for coagulopathy-negative “DIC”. Furthermore, additional notable encounter was the perplexity of uncommon organ phenotypes in MODS such as acute renal failure, acute necrotizing pancreatitis, acute respiratory distress syndrome, fulminant hepatic failure, hepatic encephalopathy, cardio-pulmonary syndrome, hepato-renal syndrome, rhabdomyolysis, adrenal insufficiency, microvascular myocardial infarction and others [6, 24], which often have occurred in association with thrombocytopenia and MAHA without gene mutation of hereditary TTP or antibody production of acquired TTP. Some have called it TTP-like syndrome [6] and others used the term atypical acquired TTP, hemolytic-uremic syndrome, or thrombotic microangiopathy. Indeed, in clinical practice, EA-VMTD (i.e., TTP-like syndrome) is much more common disorder than GA-VMTD and AA-VMTD. Now, every TTP-like syndrome can be classified as EA-VMTD based on entirely different endothelial molecular pathogenesis from classical TTP that is caused by severe ADAMTS13 deficiency [6].

Over the years, the enigmatic clinical features expressed by microthrombosis have aroused a concern in the mind of some clinicians, not only about the underlying disease, pathogenesis, diagnostic tests and treatment approach, but also about different clinical identity and phenotype amongst acute “DIC”/chronic “DIC”, TTP/TTP-like syndrome, HUS/acute renal failure, thrombotic microangiopathy/microvascular thrombosis, and MODS/biorgan syndrome [6, 16‐24]. Although TTP/TTP-like syndrome and “DIC”/DIT appear to be different in clinical phenotypes, the character of microthrombi is pathologically the same and microthrmbi are composed of platelet-ULVWF complexes [5, 14]. Thus, the term VMTD has been introduced to encompass the entire spectrum of “microvascular thrombosis” and “vascular microthrombosis” to include focal, multifocal, local, regional and disseminated VMTD [2‐6]. How ironical it is to note that the etiology, pathogenesis, diagnostic test and treatment are well established in TTP, which underlying pathology is VMTD, but we know nothing about the above clinical parameters in “DIC”, which underling pathology is also VMTD. The true mechanism of microthrombogenesis [2] causing “DIC” has eluded the scientists’ intellect in medical science and the clinician’s acumen of practice in medicine until this date. No doubt, it should be blamed to the fact that our understanding of in vivo hemostasis and thrombogenesis is not completely identified yet [3, 4].

When we look back the medical history since its initial description of “DIC” based on ill-founded concept, the medical literature is replete with published articles on the enigmatic nature of the pathophysiological mechanism of “DIC” from many bright minds of coagulation science and clinical medicine [25‐39]. Numerous theories had and has been proposed to explain the pathogenesis of “DIC” based on the mechanism of coagulation, which includes nitrogen bubble induction [26], cerebral ischemia [29], TF/FVIIa-initiated coagulation cascade [25, 30], cross-talk between inflammation and coagulation [30‐36], glycocalyx degradation [33], FXII-initiated consumption coagulopathy [34], neutrophil extracellular traps (NET)-induced immunothrombosis [35‐37], shock-induced endotheliopathy [38], brain regulation [39] and many others, but none of them has convincingly explained the unique clinical, laboratory and pathological features of “DIC”.

In retrospect, it is obvious that the incomprehensible nature of microthrombosis/fibrin clot disease has been due to yet incompletely identified in vivo hemostatic mechanism. Because of this limitation, clinicians and coagulation scientists have diverted their research efforts to look for the pathogenetic mechanism linking between “DIC” and microthrombosis, and have begun to apply new approaches. Science have moved away from the hemostatic model of the physiology of vascular system to molecular biological mechanisms of cellular/organ system dysfunction within immune system [35‐37], endocrine system [38], central nervous system [39], and even neuronal cells [29] to look for yet unknown complex molecular signal mechanisms. But, identifying the molecular pathophysiological mechanism of microthrombogenesis has been unsuccessful.

Recently, the research efforts have intensively focused on the role of neutrophil extracellular traps in hemostasis, identifying intravascular traps of circulating apoptotic blood cells and molecules within the vascular system, with enthusiasm and hope. The theory of immunothrombosis supported by NETosis and molecules from blood cell components has attracted many molecular scientists in search for the pathogenesis of thrombosis and microthrombosis. Hemostasis has been considered to be natural defensive mechanism against pathogen via its critical role utilizing cellular and molecular traps derived from immune cells as well as blood cells. However, the first essential principle that hemostasis and thrombosis must be activated only in vascular injury cannot be juxtaposed with immunothrombosis in both physiologic and pathologic hemostasis [3, 17]. The theory of NETosis is not consistent with this hemostatic fundamental because, without intravascular damage, tissue factor (TF) as well as ULVWF cannot be released to activate blood coagulation, and thrombosis could not be formed. No doubt, the key mechanism of in vivo hemostasis is determined by the response of vascular wall physiology following intravascular injury [3, 4, 17]. NETs cannot be the initiator of hemostasis, but happens to be the innocent bystander being trapped passively within the clots or perhaps secondary participant actively involved in trapping themselves within thrombogenetic scaffolds with a purpose [17]. NETosis is likely an auxiliary process only after initiation of hemostasis following intravascular injury. There are voices among molecular scientists cautioning the primary role of immunothrombosis in thrombogenesis [36, 37, 40, 41].

To date, the term “coagulation” is used to denote a “fibrin clots” forming process either in vitro and in vivo, which can be more precisely termed as “fibrinogenesis”. Fibrinogenesis can be defined as in vitro fibrin forming process via extrinsic cascade in the test tube and also a part of in vivo blood clotting process of hemostasis in vascular injury. Coagulation is conceptually interpreted as a process generating blood clots (i.e., primarily made of fibrin clots and blood cells) seen such conditions as deep vein thrombosis (DVT), pulmonary embolism and acute ischemic stroke. Coagulation community has been in general agreement with the conception that hemostasis is the same blood clotting process in both bleeding control of external bodily injury, leading to coagulation, and blood clot forming process (i.e., thrombogenesis) in intravascular injury, leading to thrombosis. However, a major dilemma is we cannot reconcile the different role of various coagulation participants in coagulation and thrombogenesis. This has been particularly true for ULVWF, which we know, is the main component interacting with the platelet and initiating in vivo coagulation via platelet plug formation in external bodily injury. The precise role of ULVWF, however, has not been identified in thrombogenesis in intravascular injury although it is suspected to play a significant role [42, 43]. To make the matters even more difficult, we have no clear understanding of the difference in the concept between coagulation and thrombogenesis in vivo for the role of interaction between ULVWF and fibrins.

One question this author likes to ask is why McKay coined the term “DIC” instead of “disseminated intravascular thrombosis (DIT)”. In the lexicon of hemostasis, we call every intravascular blood clotting disorder as “thrombosis”, including DVT, TTP/TTP-like syndrome and heparin-induced thrombocytopenia with thrombosis, but the term “coagulation” is applied only in “DIC” for a disease designation. Traditionally the term “coagulation” has not been used in the describing a disease in each and every thrombotic disorder. From the start, this author believes the term “DIC” was incorrectly designated and has contributed false conceptualization of its pathogenesis.

Another concern is the character description of “blood clots” is imprecise although it carries the inclusive and perceptive meanings of “thrombus”, “microthrombi”, “fibrin clots”, and “hematoma” and supposedly represents more or less similar composites within their clots because, in current theory, all blood coagulation begins with activation of TF/FVIIa path and ends with “blood clots” composed of fibrin meshes, platelets, cellular traps and molecules, and other blood cells. From this proposition, the therapeutic approach has exploited the belief that the same hemostatic mechanism in the management of various thrombotic/coagulation disorders requires the same anticoagulation in every thrombosis and clot disease, and every clinical trial, but this therapeutic effort did not work well in “DIC” and hemorrhagic disease of APL as well as TTP-like syndrome with coagulopathy. Another approach was the counteracting therapy against expressed TF path biomarkers such as antithrombin, activated protein C, tissue factor pathway inhibitor and thrombomodulin. So far, every venture has shown no clear benefit in therapeutic trials.

When we look back these failures, it is no wonder why these approaches were not successful in sepsis-associated “DIC” and other microthrombotic disorders. It is because the characters of “microthrombi” composed of platelet-ULVWF complexes are very different from “macrothrombus” made of platelet and fibrin clots as well as blood cells and their components. On the one hand, microthrombi in sepsis are produced via activation of lone ULVWF path promoted by generalized exocytosis of ULVWF from damaged endothelial cells (ECs) in endotheliopathy (Figs. 1 and 2a) [5, 6]. On the other hand, macrothrombus in intravascular trauma (e.g., vascular procedures) is produced via combined activation of ULVWF path and TF path triggered by local release of ULVWF and TF from damaged ECs and subendothelial tissue (SET)/extravascular tissue (EVT) in intravascular injury [4, 17]. Here, vascular wall physiology plays a critical role in thrombogenesis in intravascular injury as well as hemostasis in external bodily injury.

The existence of these two different characters of “microthrombi” and “macrothrombus” had alerted this author that two different hemostatic paths must be present in vivo; one must be the result of activated ULVWF path and the other is that of activated TF path [3, 4]. Microthrombi must be promoted by microthrombogenesis, which is ULVWF-initiated thrombogenesis with recruitment of platelets. Additionally, fibrin clots are produced by fibrinogenesis, which represents TF/FVIIa- initiated coagulation cascade. Further, to make a complete form of macrothrombus, these two incomplete products, microthrombi made of platelet-ULVWF complexes and fibrin clots made of fibrin meshes, must be unified via macrothrombogenesis to produce macrothrombus [3, 4]. Based on this analysis and logic, this author has envisioned the framework of in vivo hemostasis and proposed that, in addition to currently known coagulation (TF) path, another path must exist, which can be called microthrombotic (ULVWF) path. These insights through the reinterpretation of false DIC have led for this author to identify novel “two-path unifying theory” of hemostasis and this proposal has given us a birth of complete picture of in vivo hemostasis in the context of vascular wall physiology, which can explicate every hemorrhagic disease and every thrombotic disorder.

Whenever abnormal coagulation profile is encountered in a critically ill patient, the differential diagnosis should include 1) acute “DIC”, 2) EA-VMTD with hepatic coagulopathy, and 3) primary fibrinolysis because they often present with thrombocytopenia, hyperfibrinogenemia or hypofibrinogenemia, prolonged prothrombin time and activated thromboplastin time, and elevated D-dimer. The critically ill patients with microthrombosis (i.e., EA-VMTD) are always characterized by markedly increased activity of FVIII and increased expression of VWF/ULVWF/VWF antigen following endothelial exocytosis of ULVWF/FVIII [44‐47]. Hyperfibrinogenemia in early phase of EA-VMTD-associated hepatic coagulopathy is due to release of fibrinogen from damaged hepatocytes. These findings rule out acute “DIC” as a correct diagnosis because it, according the concept of consumption coagulopathy, must be characterized by markedly decreased FVIII, decreased VWF and hypofibrinogenemia (Tables 1 and 2). Also, primary fibrinolysis can be ruled out due to the laboratory findings of hyperfibinogenemia and increased activity of FVIII, which should be decreased in fibrinolysis. The coagulation parameters of markedly increased activity of FVIII and increased expression of VWF/ULVWF/VWF antigen can be easily and perfectly reconciled with endotheliopathy-associated microthrombosis (i.e., EA-VMTD). Mild to moderately elevated D-dimer supports microthrombosis-induced liver cell damage (e.g., hepatic fibrinogenolysis) following activated ULVWF path rather than primary role of activated TF path. This differential diagnosis will be further discussed later in this article.

Additionally, unlike false DIC, “true DIC” (e.g., APL) shows the feature of consumption coagulopathy, which includes decreased fibrinogen level, and decreased FVIII and FV activity because fibrinogen is consumed, and FVIIIa and FVa become inactivated after modulation via activated TF-induced fibrinogenetic path. Strangely, in coagulation study of acute “DIC” in “real” patients, the activity of FVIII was markedly increased and VWF/ULVWF overexpressed [5, 44‐47]. These findings alerted this author to question the correctness of the concept of “DIC”. Certainly, the diagnosis of the coagulopathy based on changes in coagulation factors in acute “DIC” patients was more consistent with EA-VMTD with hepatic coagulopathy due to microthrombosis-induced liver necrosis. These laboratory findings summarized in Tables 1 and 2 are congruous with the concept that “DIC” and EA-VMTD are the same disease and support the underlying pathology is microthrombosis. This clarification on the role FVIII and ULVWF/VWF in microvascular thrombosis has corrected the major misinterpretation between the supposedly hemorrhagic disease of acute DIC and hepatic coagulopathy in EA-VMTD-induced liver necrosis [44‐50]. This error seems to be a trivial one, but this minor misrepresentation in medical community has hugely changed the history of medicine and contributed to the delay in identifying the mechanism of in vivo hemostasis.

The mechanism of alteration of coagulation factors between “true” DIC (i.e., APL) and false acute “DIC” are separately summarized with EA-VMTD-associated hepatic coagulopathy in Table 2 with explanation. Unlike EA-VMTD, consumption coagulopathy of APL with decreased activity of fibrinogen, FVIII and FV can be affirmed to be true DIC (fibrin clot disease) characterized by disseminated fibrin clots. The decreased coagulation factors in APL are the result of consumption of fibrinogen, and consumption and inactivation of FVIIIa and FVa. Increased D-dimer/FDP is the result of secondary fibrinolysis.

Markedly increased activity of FVIII and overexpressed VWF/ULVWF in patients presenting with EA-VMTD (e.g., TTP-like syndrome) have been mistakenly interpreted to be “hypercoagulable state” or “thrombophilic state” that causes thrombosis or coagulopathy. Instead, the correct interpretation is those changes are the result of endothelial exocytosis in EA-VMTD triggered by endotheliopathy. Better understanding for the role of ULVWF path and TF path in hemostasis in vivo should clarify many unaccountable “poorly defined coagulopathy”, “DIC-like syndrome” and “dysregulated coagulation system” in patients with thrombotic disorder and hemorrhagic disease. If a critically ill patient shows markedly increased FVIII activity and increased expression of VWF/ULVWF, please look for “masked” EA-VMTD and potential diagnosis of TTP-like syndrome.

Frequent coexistence of inflammation and coagulation disorder in sepsis and critical illnesses put forward the crosstalk theory to tie inflammation to microthrombosis [51‐53], but this interaction theory cannot explain the difference of molecular mechanism producing microthrombi and macrothrombi, and is unable to support the concept of in vivo hemostasis via cellular and vascular physiology occurring in intravascular injury. When sepsis progresses to endotheliopathy, both inflammatory pathway and microthrombotic pathway are activated simultaneously but independently as presented in “two-activation theory of the endothelium” (Fig. 1) [2]. Endothelial dysfunction promotes inflammation mediated by cytokines released from ECs and microthrombogenesis initiated by exocytosis of ULVWF from Weibel-Palade bodies located within ECs. However, no research has shown how cytokines alter hemostatic components ULVWF and TF that triggers to thrombogenesis. In sepsis and critical illnesses, clinical features of inflammation such as fever, malaise, myalgia and arthralgia, and those of coagulopathy such as microthrombosis and hemorrhagic syndrome can be better explained by each independent pathogenesis [2, 7]. Inflammation is not the cause of endotheliopathy, but is the result of endothelial dysfunction, which led to the release of various cytokines [17]. Even though inflammation and vascular microthrombosis often coexist together in endotheliopathy, inflammation has not shown to promote the activation of hemostasis components ULVWF or TF, nor acute and chronic inflammatory conditions are known to enhance hemostasis without endotheliopathy.

Additionally, it should be noted that inflammation does not occur in AA-VMTD (i.e., TTP) even though platelet-ULVWF complexes are the same microthrombi produced in EA-VMTD [17]. This observation also supports inflammation is primarily the result of endotheliopathy, and is not the essential component leading to coagulation or microthrombogenesis. Further, antiinflammatory agents targeting the inflammatory cascade have failed in clinical trials for sepsis-associated coagulopathy.

Until now, the pathogenesis “DIC” has been based on the ironclad concept of activated TF path forming “fibrin clots”. This notion has been further solidified in sepsis by altered expression of TF path markers as well as antithrombotic and prothrombotic endothelial markers. These include thrombin, antithrombin, thrombin-antithrombin complex, activated protein C (APC), tissue factor pathway inhibitor, thrombomodulin, and D-dimer [54, 55]. Even though altered TF path markers indirectly seem to support the role of activated TF path in “DIC”, these findings can be better explained by alternative mechanism of expression from the subendothelial tissue (SET) damage of vascular walls and extravascular tissue (EVT) damage associated with organ dysfunction. This tissue damage occurs due to microthrombi-induced hepatic necrosis in the liver and other organ damage, and also due to vascular injury resulting from surgery and vascular access procedures in the hospitalized patient. The SET/EVT damage would release TF and alter the expression of TF path related markers. Thus, altered TF path markers in “DIC” are suspected to be the result of secondary event that are unrelated to primary pathogenesis of activated ULVWF path. This complexity of various hemostatic path markers also has contributed to the misinterpretation of sepsis-associated coagulopathy.

The prevention of “DIC” in septic patients should be a key therapeutic target in reducing death from multisystem organ failure [56]. To date, no effective therapy for sepsis and septic shock has been found to reverse advanced MODS after many decades of labor intensive and highly expensive clinical trials based on contemporary theory of TF initiated hemostasis [57‐59]. Should we blame the failure of clinical trials to the intricacy of sepsis or lack of therapeutic advancement? This author can ascertain that our incomplete understanding of in vivo hemostasis was the main culprit of therapeutic failures [17]. To consider the above dilemma in perspective, in vivo hemostasis based on “two-path unifying theory” that is centered on vascular wall physiology will be briefly summarized as follows.

Nature has endowed human with only one normal hemostatic mechanism to be available for bleeding control in both external bodily injury and intravascular injury. Although hemostasis in external bodily injury protects human lives from life-threatening blood loss, it also creates thrombosis in intravascular injury harmful to human as shown in Fig. 2a and b [3].

The most important task of hemostasis is the nature’s law to sustain life through its activation in external bodily injury. However, an event of intravascular injury may damage to ECs which initiates exocytosis of ULVWF and recruit platelets, and activates ULVWF path of hemostasis. It may cause additional SET/EVT damage that releases TF into circulation and induces activation of FVII, and activates TF path of hemostasis. For example, sepsis typically causes limited but generalized damage to ECs via complement activation and pathogen-originated ligand adhesion to an endothelial cell receptor. It provokes disseminated endotheliopathy, which leads to lone activation of ULVWF and triggers microthrombogenesis without activation of TF path. However, severe local injury, vascular disease or surgery may cause not only ECs damage, but also SET/EVT damage that releases TF. It provokes activation of both ULVWF and TF paths, which together leads to macrothrombogenesis (Fig. 2b). Therefore, sepsis mediates disseminated microthrombosis (e.g., DIT; “DIC”), but extensive vascular wall injury produces local macrothrombosis (e.g., DVT, acute ischemic stroke) [60]. This simple physiologic concept of the vascular wall at different levels of damage succinctly defines the difference of the character between “DIC” and DVT. The only question to be identified is how microthrombosis and fibrin clots come together to form macrothrombus.

The theory on the mechanism of in vivo hemostasis has been proposed after philosophical, physiological and phenotypical contemplation about life in nature [4]. In the lexicon of hemostasis, at present the conceptual meanings of coagulation and thrombogenesis have been ambiguously defined. How can coagulation in external bodily injury be the same process to thrombogenesis in intravascular injury? The answer to this seminal question is very pertinent in the understanding of “how an underlying disease such as von Willebrand disease, hemophilia A, thrombocytopenia, thrombophilia and other diseases influences coagulation and thrombogenesis”. The fact is “coagulation” occurring in external bodily injury to stop the bleeding is one process. Still, it is the same process to “thrombogenesis” occurring in intravascular injury to produce to thrombosis. However, bleeding control is one event, but thrombogenesis contains three different sub-processes, which should be understood in the context of three different levels of vascular wall damage and activation of ULVWF and/or TF paths. Limited damage to ECs causes microthrombogenesis to generate microthrombi (e.g., “DIC”; transient ischemic attack [TIA]), limited activation of TF path causes fibrinogenesis to form fibrin clots (e.g., APL with consumption coagulopathy; tissue hematoma [e.g., subdural hematoma]), and combined damage to ECs and SET/EVT causes macrothrombogenesis to form macrothombus (e.g., DVT; acute ischemic stroke) [60].

The conception how could coagulation in bleeding of external bodily injury be one process of hemostasis in vivo, but thrombogenesis creates three different characters of blood clots in intravascular injury, is shown and summarized in Fig. 2b and Table 4. This very important question has been addressed by the physiological mechanism and phenotypical expression when the role of vascular wall damage was introduced in the framework of in vivo hemostasis. Please consider that contemporary in vitro hemostatic theory (i.e., extrinsic cascade) has been borne out from clotting studies in laboratory “test tubes”, in which the glass wall does not release ULVWF and TF, nor recruit the platelets from circulation.

Armed with this fundamental derived from vascular wall physiology, the concept of in vivo hemostasis is reconstructed to explicate how hemostasis is initiated and leads to blood coagulation to make bleeding stop in external bodily injury, and the mechanism of thrombogenesis is proposed to explain how three different paths of blood clot formation take place in intravascular injury through one hemostatic system.

Certainly, both coagulation and thrombogenesis are mediated by the same physiological mechanism of normal hemostasis. Thus, the term “coagulation” in vivo and in vitro, “thrombosis” (microthrombosis and macrothrombosis) in vivo, “fibrin clot” in vivo and in vitro, and “blood clots” in vivo and in vitro which have been utilized in the terminology of hemostasis and thrombosis have to be reconciled within three mechanisms of thrombogenesis in one hemostatic system. While trying to reconstruct and connect dots in normal physiological mechanism, the role of NETosis in hemostasis [68] came into question, and certainly it couldn’t be ignored. To integrate its partaking in vivo hemostasis, an updated version of “two-path unifying theory” is proposed as shown in Fig. 2b. In intravascular injury, activated ULVWF path leads to microthrombogenesis and activated TF path leads to coagulation – more discrete term for “coagulation” is “fibrinogenesis” – and ULVWF path and TF path must participate together in macrothrombogenesis. As shown in Fig. 2a, activated ULVWF path from damaged ECs produces “microthrombi” strings and activated TF path from damaged SET/EVT produces “fibrin meshes” as represented in the schematic vascular wall structure in Fig. 3. Hence, depending upon the level (depths) of vascular injury, different phenotypes of the thrombotic disorder must occur in intravascular injury (i.e., ECs; ECs/SET; ECs/SET/EVT) as explained in earlier subheading of hemostasis “vascular injury in hemostasis phenotypes” [60].

Philosophically and physiologically, it should be believed that nature must have endowed human with a simple, fast and efficient hemostatic mechanism to save lives in timely manner. Therefore, hemostasis must be a simple physiological mechanism evolved from biological adaptation and selection. In vascular wall injury, hemostasis should not be delayed by the process of complicated multiple bio-molecular steps following the injury. If there is any impediment, mediating through multiple activation of coagulation factors, and interactions of secondary coagulation components, cellular traps and molecular participation such as DNA and histones, this complexity of hemostasis is not compatible with human survival.

Instead, hemostasis should be an instantaneous and simultaneous process of activated ULVWF path and TF path starting at the gate of injured ECs and SET/EVT. We can observe these characteristics of blood clotting in external bodily injury site, bleeding time test in vivo and coagulation tests in vitro where no cellular traps and molecules are needed. Thus, this author would refute the concept of sequential hemostatic theory, theory of primary role of NETosis, and hypothesis of non-vascular organ system control as the main mechanism of hemostasis. Please note that the enthusiasm of NETosis has been cautioned and unconfirmed gaps remain in the interpretation among concerned molecular biologists [36, 37, 40, 41, 68].

Once the concept of ULVWF path is integrated into hemostasis to complement currently known one-sided TF path (activated TF-FVIIa cascade), normal in vivo hemostatic mechanism can be logically and systematically understood. According to “two-path unifying theory”, simultaneous activation of ULVWF path and TF path initiates hemostasis as shown in Fig. 2a. Two different clinical settings of vascular injury (i.e., external bodily injury and intravascular injury) require the participation of all five hemostatic components shown in Table 3-(2) to produce healthy hemostatic “blood clots” through the activation of cooperating ULVWF and TF paths.

In disseminated ECs damage from intravascular injury (e.g., sepsis-induced endotheliopathy leading to sepsis-associated coagulopathy), only ULVWF path is activated through platelet activation and endothelial exocytosis of ULVWF from Weibel-Palade bodies. The injury leads to excessive release of these prothrombotic multimers [43, 69‐71]. ULVWF become anchored to the membrane of damaged endothelial cells (ECs) as long elongated strings with support of collagen [72] and recruit activated platelets. Both components together form platelet-ULVWF complexes, which become “microthrombi” strings. This process forming microthrombi strings in intravascular space is called microthrombogenesis [2‐8]. The mechanism of ULVWF path in endotheliopathy mediating the expanded molecular response is presented separately in Fig. 1.

On the other hand, a local intravascular traumatic injury causing from ECs to SET/EVT damage (e.g., DVT), which results in additional local expression of TF, activates the TF/FVIIa complex-initiated fibrinogenetic path (i.e., TF path) as well as microthrombotic path (i.e., ULVWF path) at the local injury site. In contemporary coagulation theory, TF path, which fits properly with either the extrinsic coagulation cascade or cell-based coagulation [73], has been well formulated. To date, the fibrinogentic path in which TF-FVIIa-initiated coagulation cascade leads to the formation of fibrin clots [3] is considered to be the main body of hemostasis promoting every coagulation/thrombosis in vascular injury. Now, it becomes apparent that a newly incorporated ULVWF path shown in “two-path unifying theory” plays an equally or more important role in hemostasis in vivo [4, 17, 60, 67].

For example, “DIC” occurring in sepsis-initiated endotheliopathy is a partially activated hemostatic disease due to lone activation of ULVWF path, with serious pathophysiological ramification, resulting in DIT. Disseminated intravascular thrombosis is a form of EA-VMTD that may lead to hematologic phenotype TTP-like syndrome [17, 74].

Finally, how to differentiate 1) acute “DIC” (theoretical), 2) EA-VMTD-associated “hepatic coagulopathy” and 3) coagulopathy of APL? In hematologic evaluation of critically ill patients, the contemporary diagnosis of acute “DIC” has been established whenever an unexplained coagulopathy presents with laboratory features of thrombocytopenia, prolonged activated thromboplastin time and prothrombin time, positive D-dimers and decreasing fibrinogen level. However, this combination feature is not diagnostic of theoretical DIC.

To make accurate differential diagnosis of coagulopathy between acute DIC (theoretical) and EA-VMTD-associated hepatic coagulopathy, more specific laboratory tests are needed with proper interpretation based on pathogenetic mechanisms, which are listed in Tables 2 and 5 with following characteristic features.

In theoretical acute DIC, decreased FVIII and FV activity must be present due to inactivation of FVIIIa and FVa via activated protein C pathway and protein S pathway if TF-FVIIa coagulation cascade had been involved. However, the rest of coagulation factors FIIa, FVIIa, FIXa, and FXa should be normal because they are not inactivated [95, 96]. In hepatic coagulopathy, FII, FV, FVII, FIX, and FX may be mild to moderately diminished. In endotheliopathy, EA-VMTD is characterized by markedly increased activity of FVIII and overexpression of VWF/ULVWF due to their exocytosis from Weibel-Palade bodies.

An extremely interesting finding in acute hepatitis patients with severe hepatic coagulopathy, acute liver failure and fulminant hepatic failure has shown markedly increased FVIII activity and increased expression of VWF [46, 97‐104]. This unique finding of FVIII has not been explained to date. Some blamed it to “non-specific but acute phase reactant” in acute illnesses and insisted it is a contributing factor to hypercoagulable state. This author affirms both increased activity of FVIII and overexpressed VWF/ULVWF are the specific biomarkers for activated ULVWF path associated with endotheliopathy [105] occurring in EA-VMTD. Therefore, increased FVIII and VWF activity in critical illnesses such as sepsis (e.g., COVID-19) [75‐77] and trauma should alert the potential of underlying, but masked, EA-VMTD. This increased activity of FVIII is not the cause of hypercoagulable state, but is the result of on-going covert microthrombogenesis, leading to EA-VMTD, in critically ill patients. No doubt, sepsis-associated coagulopathy is not “DIC”, but is endotheliopathy-associated vascular microthrombotic disease.