Association of platelet count with all-cause mortality and risk of cardiovascular and respiratory morbidity in stable COPD

Chronic obstructive pulmonary disease (COPD) is projected to become the third leading cause of death by 2030, representing 8.6% of global deaths [1]. Chronic systemic inflammation in COPD has been associated with poor clinical outcomes [2, 3]. Biomarkers have potential value as prognostic indicators in COPD, though their utility has been limited by disease heterogeneity and the influence of comorbidities [4]. Platelet count, a component of a routinely measured clinical assay, remains within a relatively narrow range among healthy individuals but can be significantly altered in the setting of both acute and chronic disease [5]. Notably, platelet count is correlated with levels of fibrinogen [6], an inflammatory biomarker identified as a means of selecting research participants with COPD at high risk for morbidity and mortality [7, 8]. A U-shaped association between platelet count and increased mortality has been recognized in the general population and the elderly [9‐14], though its role in COPD remains unclear. In addition to the well documented role of platelets in cardiovascular (CV) disease, platelets are now recognized as playing a role in many pathophysiologic processes such as inflammation, host defense, and tumor biology [15].

Overall, platelet count in COPD has been shown to be elevated compared with the general population [16, 17]. Previous studies have demonstrated an increased risk of in-hospital and 1-year mortality associated with thrombocytosis (defined as platelet count > 400 × 10⁹/L) measured at the time of acute exacerbation of COPD (AECOPD) [18]. In univariate analysis, thrombocytopenia measured on admission for AECOPD was also associated with in-hospital mortality, need for mechanical ventilation, intensive care unit admission, and length of hospitalization [19]. Our group has recently demonstrated that elevated platelet count (> 350 × 10⁹/L) measured in stable COPD is associated with higher likelihood of prior exacerbation and worse respiratory symptoms [20]. However, the relationship between platelet count measured in the stable phase of COPD and all-cause mortality has not been previously explored. Furthermore, comorbid COPD and CV disease has been associated with higher rates of morbidity than presence of either disease alone making this a particularly vulnerable population [21]. Using data from the Study to Understand Mortality and Morbidity in COPD (SUMMIT), a prospective randomized controlled trial enriched for presence of CV comorbidity and containing adjudicated events, we investigated post hoc the association of platelet count measured in the stable phase of COPD with all-cause mortality, CV and respiratory outcomes.

Among the subgroup of 1797 participants with platelet count measurement median platelet count was 219 × 10⁹/L (interquartile range [IQR] 181–260 × 10⁹/L). One participant with platelet count 1415 × 10⁹/L was excluded from analysis. Baseline characteristics are presented in Table 1 for the entire cohort and by platelet count quintile group (Q1: < 173 × 10⁹; Q2: ≥173 to < 205 × 10⁹; Q3: ≥205 to < 234 × 10⁹; Q4: ≥234 to < 272 × 10⁹; Q5: ≥272 × 10⁹). The higher platelet count groups included more females of younger average age with lower prevalence of obesity and cardiac comorbidities but higher prevalence of anemia and current smoking.

Patients were on study treatment for an average of 1.6 (± 1.1 SD) years for a total of 3848 person-years, during which time 86 (4.8%) experienced a CV event and 651 (36.2%) experienced an AECOPD, of which 159 (24.4%) subjects experienced a severe AECOPD. Patients were followed-up on and post treatment for an average of 2.3 ± 0.9 years for vital status during which time 105 (5.8%) participants died (Table 2). Four participants were lost to follow-up and did not have vital status recorded at the end of the study.

Participants in the highest quintile group of platelet count had the highest proportion of moderate/severe AECOPD (41%) and severe AECOPD (11%). CV composite events (6%) and deaths (8%) in the highest quintile were equivalent to the lowest quintile of platelet count but higher than the middle quintiles.

In this prospective study of patients with COPD and prior history or increased risk of CV disease platelet count measured in the stable state demonstrated a U-shaped association with increased risk of 3-year all-cause mortality, though a platelet count level above or below which risk of mortality was increased could not be definitively identified. Higher platelet count also showed trends towards increased risk for a composite outcome of CV morbidity and mortality though these results did not reach statistical significance. Platelet count did not appear to be associated with prospective risk for AECOPD. To our knowledge, this is the first study investigating the association between platelet count measured in the stable state of COPD and all-cause mortality. These findings expand upon prior studies reporting increased all-cause mortality among COPD patients with thrombocytosis and thrombocytopenia measured during an AECOPD [18, 19].

The U-shaped association between platelet count and mortality observed in this sample of stable COPD patients has been previously described in a general adult population [13, 14], postmenopausal women [9], and the elderly [10‐12]. Increased risk of all-cause mortality was consistently noted in these prior studies for platelet count > 300 × 10⁹/L but not thrombocytopenia (< 150 × 10⁹/L) [11‐13]. While higher platelet counts appeared to be associated with increased mortality in this study of COPD patients, platelet count ≥300 × 10⁹/L and thrombocytopenia trended toward increased risk of mortality, though did not reach statistical significance. This may be a consequence of a relatively small overall and group specific sample sizes compared with studies of non-COPD cohorts or risk for mortality present only at higher platelet counts, similar to those identified in prior studies of mortality following AECOPD (> 400 × 10⁹/L) and respiratory symptoms (≥350 × 10⁹/L, which we did not have adequate power to evaluate [18, 20].

Elevated platelet count has been associated with CV mortality in healthy middle-aged men but not the elderly [11, 12, 26], and with a composite CV outcome of myocardial infarction, cerebrovascular disease, or peripheral vascular disease in the general population [13]. Though we were not able to evaluate CV mortality independently due to low event rate, higher platelet count, evaluated as a continuous variable, appeared associated with a composite CV outcome in this sample of COPD patients with history or increased risk of CV disease, though higher platelet count categories did not reach statistical significance. Notably, elevated platelet count was not associated with AECOPD in this study which is in contrast to a prior study demonstrating association between platelet count ≥350 × 10⁹/L measured in the stable state and report of at least one AECOPD in the prior year [20]. Thus, it appears that platelet count may not inform risk of future AECOPD beyond prior exacerbation history. Additionally, in this higher risk sample of stable COPD patients with comorbid CV disease, platelet count appears to be a predictor of CV rather than respiratory morbidity.

Potential biological mechanisms for the association of elevated platelet count with all-cause mortality and CV morbidity in COPD include systemic inflammation, atherosclerotic plaque destabilization, and platelet activation. Previous studies investigating the prognostic value of biomarkers among patients with COPD inform the interpretation of the current findings. Fibrinogen, an acute phase protein elevated in systemic inflammation, has demonstrated an association with all-cause mortality in COPD [7]. A strong association between fibrinogen levels and platelet count has been reported [6]. Interleukin-6, a pro-inflammatory cytokine which has also been associated with increased mortality in COPD [27, 28], induces fibrinogen gene expression [29] and increases thrombopoetin production, which increases circulating platelets [30]. Several inflammatory mediators are known to activate platelets, which can then bind to vascular endothelial cells or adherent leukocytes, perpetuating or destabilizing a plaque [31]. The receptor for one inflammatory mediator, platelet activating factor, which perpetuates platelet activation and is associated with CV morbidity, is upregulated within airway epithelial cells of individuals with COPD suggesting a potential role in COPD with comorbid CV disease [32, 33]. Furthermore, in a cohort of individuals with COPD and stable coronary artery disease undergoing percutaneous coronary intervention, dual antiplatelet therapy with Ticagrelor improved markers of endothelial dysfunction and platelet activation suggesting that these mechanisms may be involved in the morbidity of coincident COPD and CV disease that was observed in this study [34].

A high prevalence of cancer related mortality was present in the lowest platelet count quintile, despite exclusion of patients with known malignancy from SUMMIT, which suggests presence of an underlying comorbidity with high cancer risk, subclinical malignancy or bone marrow suppression that may be driving increased mortality risk in this group. An association between thrombocytopenia and increased cancer mortality has been consistently reported in prior studies [9, 11‐13]. Thrombocytopenia is also associated with increased mortality risk when measured at the time of hospital admission for community acquired pneumonia, independent of infection severity [35]. In this population of COPD patients at high risk for respiratory infections, thrombocytopenia in the stable state likely places them at higher risk of thrombocytopenia and poor outcomes during future acute illness.

There are limitations to this study. Few patients were represented in the lowest and highest platelet count groups, which limited the power to make inferences about these groups. The infrequency of mortality events precluded statistical comparison of case-specific mortality between platelet count quintiles. The study sample included moderate COPD patients with CV disease history or risk factors, which may limit generalizability to all COPD patients. Finally, inflammatory biomarkers were not measured in this study, which precluded investigation of correlations between platelet count and inflammatory biomarkers and limited inferences regarding mechanism.

In conclusion, among patients with moderate COPD and CV disease history or risk factors, there is a U-shaped association between platelet count and all-cause mortality. Higher platelet count was also associated with trends towards increased risk of composite CV disease, although effect estimates for platelet count quintiles and categories were imprecise for all contrasts. Platelet count measured at baseline did not predict future COPD exacerbations.