Thrombocytosis in COVID-19 patients without myeloproliferative neoplasms is associated with better prognosis but higher rate of venous thromboembolism

Dear Editor, Platelet functions are well known to surpass thrombosis and hemostasis and platelets play an important role in inflammatory and profibrotic mechanisms. Thrombocytopoiesis is traditionally considered to occur in the bone marrow. However, thrombocytopoiesis was shown to occur in the lungs as well where circulating megakaryocytes release platelets and are responsible for up to 50% of total platelet production [1]. The specific role of platelets in the lungs is not fully elucidated. Platelets are the source of cytokines that were shown to support lung regeneration and embryonic development (SDF1, TGF-β1, IGF1), and fetal lung megakaryocytes in comparison to bone marrow megakaryocytes were shown to exhibit higher transcription of these factors [2]. Thrombocytopenia is recognized as a negative prognostic factor in COVID-19 patients [3]. Nevertheless, there are no data on the prognostic significance of elevated platelet count that can be encountered in COVID-19 patients, besides indirect conclusions based on reported outcomes in chronic myeloproliferative neoplasm (MPN) patients [4, 5]. We have retrospectively analyzed a cohort of 5876 consecutively hospitalized COVID-19 patients who were admitted to our tertiary-level institution (University Hospital Dubrava, Zagreb, Croatia) in the period from March 2020 to June 2021, had baseline complete blood count available, and did not have known prior or subsequent MPN. COVID-19 severity on admission was defined by the World-Health-Organization guidelines as mild, moderate, severe, and critical. Clinical and laboratory data used in the study are part of the hospital registry project and were obtained by analysis of written and electronic medical records. A total of 88.8% of patients received LMWH thromboprophylaxis in dose intensity per the assessment of treating physicians, 74.6% received steroids and 14.9% received remdesivir. Acetylsalicylic acid use corresponded to baseline indication for chronic use (18%). Platelet count and mean platelet volume (MPV) were obtained by Advia 2120i counter (Siemens-Medical-SolutionsDiagnostics-Pte-Ltd., Swords, Ireland). The normality of the distribution of numerical variables was tested using the Shapiro–Wilk test. Since the majority of numerical variables did not have normal distribution they were presented as the median and interquartile range (IQR) and were analyzed using the nonparametrical tests (the Mann–Whitney-U test and the Kruskal–Wallis-analysis-of-variance with the post-hoc test by Conover). Categorical variables were presented as frequencies and percentages and were analyzed using the Χ-test. Time to event analyses for survival were based on the Kaplan–Meier method. The log-rank test and the Cox regression were used for univariate and multivariate survival analyses. Follow-up was considered from the time of hospital admission up to 114 days, including the post-discharge period (to correspond to the followup time of the longest hospitalized patient). Clinical outcomes of interest (death from any cause, mechanical ventilation (MV), bacteriemia, venous and arterial thromboses proven by objective imaging and laboratory methods, major bleeding defined by the International-Society-on-Thrombosis-and-Hemostasis criteria) were considered during the hospitalization period and were evaluated as binary variables. The logistic regression was used for univariate and multivariate analyses for the prediction of nonsurvival outcomes. All analyses were done using the MedCalc statistical software, version 20.014 (MedCalc-Software-Ltd, Ostend, Belgium; 2021). Among 5876 patients, there were 3299 (56.1%) males. The median age was 72 years, IQR (62-81), and median Charlson comorbidity index (CCI) was 4 points, IQR (3–6). COVID-19 severity on admission was mild in 528 (9%), moderate in 281 (4.8%), severe in 4168 (70.9%) and critical in 899 (15.3%) patients. The median platelet count was 221 × 10/L, IQR (164–297), and median MPV was 9 fL, IQR (8.4–9.7). A total of 1120 (19.1%) patients presented with thrombocytopenia (platelets <150 × 10/L). Three-hundredand-six (5.2%) patients had platelets <100 × 10/L and 55 (0.9%) had platelets <50 × 10/L. A total of 270 (4.6%) patients had thrombocytosis (platelets ≥450 × 10/L). Sixty-three (1.1%) patients had platelets ≥600 × 10/L. Patients’ characteristics stratified by platelet count on admission are shown in Table 1. As shown, patients with thrombocytosis were significantly more likely to be younger, of female sex and to have lower comorbidity burden (CCI) than patients with thrombocytopenia (P < 0.05 for all comparisons) and of similar age, sex and comorbidity burden as patients with the normal number of platelets (differences not significant (n.s.)). Regarding specific comorbidities, arterial hypertension, diabetes mellitus, hyperlipoproteinemia, and active smoking were similarly distributed between subgroups of patients with different platelet count (n.s.) whereas patients with thrombocytosis were significantly less likely to be obese and more likely to have active malignancy (mostly gynecologic malignancy) than patients with normal platelet count, and less likely to have liver cirrhosis, chronic kidney disease or active anti-cancer therapy than patients with thrombocytopenia (P < 0.05 for all comparisons). COVID-19 severity on admission and presence of pneumonia did not significantly differ between subgroups of patients with different platelet count. However, the duration of COVID-19 symptoms significantly gradually increased in subgroups of patients presenting with increasing platelet count, with patients with


Dear Editor,
Platelet functions are well known to surpass thrombosis and hemostasis and platelets play an important role in inflammatory and profibrotic mechanisms. Thrombocytopoiesis is traditionally considered to occur in the bone marrow. However, thrombocytopoiesis was shown to occur in the lungs as well where circulating megakaryocytes release platelets and are responsible for up to 50% of total platelet production [1]. The specific role of platelets in the lungs is not fully elucidated. Platelets are the source of cytokines that were shown to support lung regeneration and embryonic development (SDF1, TGF-β1, IGF1), and fetal lung megakaryocytes in comparison to bone marrow megakaryocytes were shown to exhibit higher transcription of these factors [2]. Thrombocytopenia is recognized as a negative prognostic factor in COVID-19 patients [3]. Nevertheless, there are no data on the prognostic significance of elevated platelet count that can be encountered in COVID-19 patients, besides indirect conclusions based on reported outcomes in chronic myeloproliferative neoplasm (MPN) patients [4,5].
We have retrospectively analyzed a cohort of 5876 consecutively hospitalized COVID-19 patients who were admitted to our tertiary-level institution (University Hospital Dubrava, Zagreb, Croatia) in the period from March 2020 to June 2021, had baseline complete blood count available, and did not have known prior or subsequent MPN. COVID-19 severity on admission was defined by the World-Health-Organization guidelines as mild, moderate, severe, and critical. Clinical and laboratory data used in the study are part of the hospital registry project and were obtained by analysis of written and electronic medical records. A total of 88.8% of patients received LMWH thromboprophylaxis in dose intensity per the assessment of treating physicians, 74.6% received steroids and 14.9% received remdesivir. Acetylsalicylic acid use corresponded to baseline indication for chronic use (18%). Platelet count and mean platelet volume (MPV) were obtained by Advia 2120i counter (Siemens-Medical-Solutions-Diagnostics-Pte-Ltd., Swords, Ireland). The normality of the distribution of numerical variables was tested using the Shapiro-Wilk test. Since the majority of numerical variables did not have normal distribution they were presented as the median and interquartile range (IQR) and were analyzed using the nonparametrical tests (the Mann-Whitney-U test and the Kruskal-Wallis-analysis-of-variance with the post-hoc test by Conover). Categorical variables were presented as frequencies and percentages and were analyzed using the Χ 2 -test. Time to event analyses for survival were based on the Kaplan-Meier method. The log-rank test and the Cox regression were used for univariate and multivariate survival analyses. Follow-up was considered from the time of hospital admission up to 114 days, including the post-discharge period (to correspond to the followup time of the longest hospitalized patient). Clinical outcomes of interest (death from any cause, mechanical ventilation (MV), bacteriemia, venous and arterial thromboses proven by objective imaging and laboratory methods, major bleeding defined by the International-Society-on-Thrombosis-and-Hemostasis criteria) were considered during the hospitalization period and were evaluated as binary variables. The logistic regression was used for univariate and multivariate analyses for the prediction of nonsurvival outcomes. All analyses were done using the MedCalc statistical software, version 20.014 (MedCalc-Software-Ltd, Ostend, Belgium; 2021).
Among 5876 patients, there were 3299 (56.1%) males. The median age was 72 years, IQR (62-81), and median Charlson comorbidity index (CCI) was 4 points, IQR (3-6). COVID-19 severity on admission was mild in 528 (9%), moderate in 281 (4.8%), severe in 4168 (70.9%) and critical in 899 (15.3%) patients. The median platelet count was 221 × 10 9 /L, IQR (164-297), and median MPV was 9 fL, IQR (8.4-9.7). A total of 1120 (19.1%) patients presented with thrombocytopenia (platelets <150 × 10 9 /L). Three-hundredand-six (5.2%) patients had platelets <100 × 10 9 /L and 55 (0.9%) had platelets <50 × 10 9 /L. A total of 270 (4.6%) patients had thrombocytosis (platelets ≥450 × 10 9 /L). Sixty-three (1.1%) patients had platelets ≥600 × 10 9 /L. Patients' characteristics stratified by platelet count on admission are shown in Table 1. As shown, patients with thrombocytosis were significantly more likely to be younger, of female sex and to have lower comorbidity burden (CCI) than patients with thrombocytopenia (P < 0.05 for all comparisons) and of similar age, sex and comorbidity burden as patients with the normal number of platelets (differences not significant (n.s.)). Regarding specific comorbidities, arterial hypertension, diabetes mellitus, hyperlipoproteinemia, and active smoking were similarly distributed between subgroups of patients with different platelet count (n.s.) whereas patients with thrombocytosis were significantly less likely to be obese and more likely to have active malignancy (mostly gynecologic malignancy) than patients with normal platelet count, and less likely to have liver cirrhosis, chronic kidney disease or active anti-cancer therapy than patients with thrombocytopenia (P < 0.05 for all comparisons). COVID-19 severity on admission and presence of pneumonia did not significantly differ between subgroups of patients with different platelet count. However, the duration of COVID-19 symptoms significantly gradually increased in subgroups of patients presenting with increasing platelet count, with patients with Table 1. Patients' characteristics on admission stratified according to the baseline platelet count.
Above mentioned associations of platelets and MPV with clinical outcomes (with the exception of MPV and MV) remained statistically significant in the multivariate analyses adjusted for platelets, MPV, age, sex, COVID-19 severity, and CCI. Independent prognostic factors and associated adjusted hazard/odds ratios are shown in Supplementary Table S2.
There are several important points that we would like to emphasize. A much smaller proportion of COVID-19 patients presents with thrombocytosis (5%) than with thrombocytopenia (19%). Platelet count seems to be dominantly affected by specific comorbidities and duration of COVID-19 as opposed to its severity on presentation. Patients with higher platelets were more likely to have a longer duration of COVID-19 symptoms, stronger stimulation of myelopoiesis, and different, probably more mature inflammatory profile (higher WBC subsets, CRP and D-dimers but lower ferritin and IL-6). Elevated platelets were accompanied by a more favorable course of the disease, with less need for MV and less deaths. Even among patients with low, normal, and elevated platelets, further subgroups of patients could be stratified over whom a linear trend of improved survival with increasing platelets exists. Mechanisms behind these observations are elusive but could be attributable to the protective role of platelets for the lung parenchyma and more effective viral clearance by a higher number of platelets. Higher platelets were also associated with a lower rate of bacterial sepsis highlighting their important role in the immune system. It is of special interest that the frequency of VTE was significantly higher among COVID-19 patients with higher platelet count. These findings highly resemble phenomena reported in MPN patients with COVID-19 where patients with essential thrombocythemia (ET) who presented with higher platelet count in comparison to other MPN subsets [5] were reported to have a higher risk for VTE [5] but favorable disease course [4]. Hence, our data suggest that these observations might not be MPN-specific and could represent biological phenomena present in patients with non-clonally elevated platelet count as well. Arterial thromboses had no association with platelet count but were significantly associated with higher platelet volume. Similar to platelets, MPV reflected the need for MV and survival in univariate analyses. Associations of lower platelets and higher MPV with shorter survival seem to be independent of each other and of older age, male sex, more severe COVID-19, and higher comorbidity burden. Nevertheless, an association of MPV with MV was diminished after adjustments for platelet count and severity of COVID-19, probably due to baseline correlations with these parameters. Higher MPV might represent an influx of younger platelets due to their increased consumption and a higher degree of platelet activation due to inflammatory COVID-19 milieu.
Our findings are limited by single-center experience, retrospective study design, and no data on intensity and frequency of use of anticoagulant and antiplatelet therapies at the specific time of thrombotic events. Although we excluded patients with known or subsequently proven MPN, some patients could still have unrecognized MPN but their number is likely to be very low considering MPN incidence in an overall population. The main strengths of our study are a large sample size and adequate statistical power to perform presented analyses. These results provide new insights into the biology of thrombosis and hemostasis in COVID-19 patients where abnormalities in hematologic findings are common and difficult to interpret directly. The reactive thrombocytosis in COVID-19 patients seems to have important clinical correlations and deserves further research.