Incidence of thrombotic microangiopathies in Quebec: insight from a laboratory centralizing ADAMTS-13 testing

Thrombotic microangiopathies (TMA) are rare but life-threatening hematologic conditions mainly characterized by a severe thrombocytopenia, a microangiopathic hemolytic anemia (MAHA) and ischemic organ damage [1‐3]. The predominant primary forms of TMA include thrombotic thrombocytopenic purpura (TTP), typical hemolytic uremic syndrome (HUS) and complement-mediated TMA (CM-TMA) also known as atypical HUS [2]. Although clinical manifestations overlap between TTP and other TMAs, recent advances in the understanding of the pathophysiology led to a better discrimination of TMA forms, which is of importance to guide initial therapy [4]. TTP is associated with a severe deficiency of ADAMTS-13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) [5]. This deficiency leads to an accumulation of ultralarge von Willebrand factor (VWF) multimers and the unrestrained growth of VWF and platelet-rich thrombi occluding microvessels throughout the body [1]. TTP are mostly acquired forms due to the presence of ADAMTS-13 autoantibodies, whereas less than 5% of TTP are due to mutations in the ADAMTS13 gene (Upshaw-Schulman syndrome) [5, 6]. CM-TMA are generally associated with mutations affecting the function of proteins down-regulating the complement alternative pathway, although autoantibodies against complement-regulating factors such as anti-Factor H antibodies account for a significant proportion of CM-TMA. Typical HUS are most often triggered by Shiga Toxin-Producing Escherichia coli (STEC) [7]. In addition to these primary forms of TMA, secondary TMAs associated with different underlying causes of endothelial cell damages can occur. Aggressive treatment of malignancies and allogenic organ transplantation also increase the risk of secondary TMAs [8]. Because of overlapping clinical signs, the differential diagnosis of TTP and other TMAs remains challenging. Clinical point scores have been developed to help predict severe ADAMTS-13 deficiency in the absence of ADAMTS-13 activity testing. However, the low predictive value of these scores applied to unselected TMA reinforces the importance of early ADAMTS13 testing to discriminate TTP from other TMAs [9, 10].

Severe ADAMTS-13 deficiency, responsible for TTP, is defined by a plasma ADAMTS-13 activity ≤ 10% of the average activity found in the plasma of a healthy population [11, 12]. Therapeutic plasma exchange is rapidly initiated in the presence of TMA symptoms, most often before obtaining an ADAMTS-13 activity result. If TTP is confirmed by ADAMTS-13 activity ≤ 10%, presence of ADAMTS-13 autoantibodies and no family history suggestive of an Upshaw-Schulman syndrome, corticosteroid therapy with the addition of rituximab is recommended during both first and relapse episodes [13]. If ADAMTS-13 activity is > 10% and STEC-HUS excluded, the diagnosis is either CM-TMA or a secondary TMA. The latter may be drug-induced or triggered by cancer, malignant hypertension, transplantation or pregnancy. The therapeutic strategies in these cases aim to suppress the triggering element, treat the primary cause and provide supportive care. The diagnosis of CM-TMA is often difficult and time consuming as it involves functional, antigenic and genetic investigations to identify the factor causing the over-activation of the alternative complement pathway. Plasma exchange has shown no or little benefit to treat CM-TMA whereas therapeutic complement blockade at the level of C5 (e.g. eculizumab) has proven its therapeutic efficacy [14].

The information available on the incidence and prevalence of TMAs has so far been extrapolated from data obtained through national registries, reviews of the literature or multicenter retrospective analyses. These studies have reported an annual estimated incidence for TTP ranging from 1 to 6 per million population and an annual estimated prevalence of approximately 10–13 cases per million population [15‐24]. In the province of Quebec, the supply of medical biology tests is organized according to geographical criteria and expertise. As of 2013 the Quebec Ministry of Health has mandated that all requests for ADAMTS-13 activity and anti-ADAMTS-13 antibody testing be performed at hemostasis laboratory of CHU Sainte-Justine (CHUSJ). This structure gave us the opportunity to comprehensively study the incidence and prevalence of TTP and other suspected TMAs in the second largest province of Canada with approximately 8.5 million persons in 2019 (22.6% of the Canadian population) [25].

TMAs are associated with high morbidity. Even when properly managed, the mortality rate of TTP is up to 20%. Relapses and long-term effects of TMAs include depression, chronic kidney disease and cardiovascular diseases and require a continuing follow-up with long-term and expensive treatments [29].

We believe that this study constitutes an accurate description of TMA occurrence in Quebec. This was made possible by centralizing ADAMTS-13 activity testing.

The annual incidence of non-TTP TMA steadily increased between 2012 and 2019 with no difference between males and females. One must keep in mind that in the present study, the non-TTP TMA incidence is an approximation based on the postulate that ADAMTS-13 activity testing is a first line analysis prescribed for any patient with a suspected TMA, in order to differentiate TTP from any other non-TTP TMA. Several reasons may explain the increase of both ADAMTS-13 testing and non-TTP TMAs during the study period: (1) a growing awareness among physicians of the existence of TMAs and ADAMTS-13 testing [30]; (2) a better and unrestricted access to the test due to its inclusion in the province's official directory of medical tests [31]; (3) the requirement for the treating physician to exclude a diagnosis of TTP—i.e. to confirm a level of ADAMTS-13 activity level above 10%—in order to have access to the anti-C5 drug eculizumab for their patient with a suspected CM-TMA [32]; and (4) a true increase in non-TTP TMA. In recent years, new treatments for malignancies have become available, some of which are associated with an increased risk of TMA including vascular endothelial growth factor (VEGF) inhibitors and gemcitabine. In addition, the number of allogeneic stem-cell and solid organ transplantations have greatly increased [33] where TMA can be observed in graft-vs-host disease and with the use of calcineurin inhibitors.

The average incidence of TTP during the study period was 1.82 per million per year. This estimation is consistent with previous studies. In other Canadian provinces, the incidence rate was found to be 1 (Bristish Columbia) and 3.2 (Saskatchewan). Other countries reported annual incidences of 1.5 (France, Germany), 3.1 (Oklahoma, U.S.) and 6.0 (U.K.) [15, 17, 19, 20, 22]. This variability may result from demographic factors and definitions used to diagnose TTP [34]. In our study, the incidence of TTP evolved differently from that of non-TTP TMA. We observed a 2 to threefold increase in the incidence of TTP from 2013 to 2016 which plateaued from 2016 to 2018 then decreased in 2019. Noticeably, the temporal increase of TTP incidence is mainly seen in the female group. At this stage, it remains difficult to determine whether this variation is random or a significant epidemiological phenomenon. Potential interpretation bias should not be excluded: the initial increase and plateau could be due to the analysis by the laboratory of patients samples previously identified as TTP but for whom no ADAMTS-13 assay had been performed until 2016. The average annual prevalence of TTP (year 2012–2019) was 4.3 cases per million reflecting both acute and monitoring requests. This prevalence is lower than a previously reported estimation of 10–13 cases per million for TTP [17, 35], which could be due to differences in operational definitions between studies [36].

TMA can affect people of all ages and gender, but an average age at first presentation of 40 years and a female predominance (1 male for 2 to 3 female) has been previously reported for TTP in studies conducted in South Africa (15) and North American (Oklahoma) [19, 37]. In our study, the median age of non-TTP patients and TTP patients, who had no previous history of TMA was 56 years and 51 years respectively. We found a male–female ratio of 1:1.4 comparable to the ratio of 1:1.6 reported by a Canadian apheresis referral center (British Columbia) in patients with TTP [22]. In their study, Martin et al., found a median age at first presentation of 51 years, which corroborates our findings and suggest that differences in median age of patients with TTP between studies is due to demographic differences across countries. The predominance of females remained overall true when stratifying by age group, especially between the ages of 20 to 49 years. Indeed, the incidence was more than twofold higher in the age group of 30–39 years for females with non-TTP TMA and more than fourfold higher in the age group of 20–29 years for females with TTP compared to males. The clear trend for a higher incidence in females in the group of childbearing age was expected because pregnancy is a known risk factor of TMA [38]. However, non-TTP TMA incidence in females fades with increasing age whereas it increases in males beyond the age of 80 years. TMA affects a significant proportion of elderly patients with a peak incidence over 60 years of age in this study. Elderly patients present more comorbidities (mental alteration, stroke, renal dysfunction, cancer) than the rest of the population, which contributes to delay the diagnosis of TMA. They also seem to develop more TTP and secondary TMA associated with cancer or medication [39]. A better awareness of TMA-related complications could contribute to a faster diagnosis and a better management of these elderly patients whose greater frailty contributes to an increased mortality rate.

In contrast to the TTP TMA group for which severe deficiency of ADAMTS-13 activity is confirmatory, our estimates of the non-TTP TMA incidence may be overestimated. Indeed, non-TTP TMAs included true primary or secondary TMA and any other disease with TMA-like clinical presentation such as severe hypertension, severe sepsis with coagulopathy or mechanical causes [40]. Accordingly, TTP cases represented a mean of 20% per year of suspected TMA cases, proportion which is very close but lower to that obtained in the French, the Oklahoma and the Japanese registries who reported respectively a proportion of 24%, 23% and 21% for TTP among all TMA cases [17, 19, 41, 42].

The main limitations of our study are the lack of data regarding the medical reasons for TMA suspicion and the potential for misclassification of TMA. Patient registries allow access to reliable epidemiological data and offer the advantage of collecting standardized information as well as pooling data over time [43, 44]. However, the regulated structure and complex maintenance of registries can hinder the long term as well as for the recruitment and conservation of enrolled patients. Furthermore, an indirect selection bias can occur in the use of data from registries when access to the program is not mandatory. Missing data due to a low or inconsistent participation may also limit registry studies [45, 46], especially in rare diseases. The centralization of ADAMTS-13 activity testing in one reference center has the advantage of taking into account every case investigated in a given territory. Although the CHUSJ was designated as the only specialized center performing the activity assay, it is possible that some requests were sent to centers outside the province, although this would be rare for the following reasons: (1) the financial constraint imposed by the Ministry of Health in the event of non-compliance with this service corridor; (2) the optimization of turnaround time and testing cost made possible by the centralization of the analysis. For this study, we purposely stopped data entry in December 2019, so as not to include any patients infected by the SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) responsible for Coronavirus disease-2019 (COVID-19) as several reports described TMA-like presentations in infected patients [46]. The impact of the pandemic on the incidence of TMAs in Quebec is currently being investigated.

In conclusion, this study is useful both for the health system and for the management of patients affected by these rare and serious diseases. Further investigations on clinical presentation and underlying conditions associated with TTP and non-TTP TMA is needed to provide a complete clinical picture of TMA in the province of Quebec. The establishment of a regional registry may help to refine these results.