The Independent Effect of Platelet Count On Mortality in a Large Inner City Elderly Outpatient Population

Platelets play a critical role in hemostasis and are also important in the development of pathologic processes including atherosclerosis and arterial thrombosis. Both thrombocytopenia and thrombophilia are common findings in several illnesses including liver diseases, infections, autoimmune disorders and malignancies. However, to date no study has directly investigated the effect of thrombocytopenia and thrombocytosis on the overall survival of the general population. The present study was thus undertaken to elucidate the independent association of abnormal platelet counts with subsequent mortality among a large inner city outpatient elderly cohort comprised of different ethnicities and comorbid conditions.

All 36,262 non-acutely ill individuals >65 years of age who were seen at an outpatient clinic within the Montefiore medical system from January 1st 1997 to May 1st 2008 and who underwent a complete blood count within 3 months of the visit were included in our cohort. Thrombocytopenia and thrombocytosis were defined as a platelet count of <150,000 per microliter or >450,000/microliter respectively. Data on 19 co-morbidities that could affect mortality and confound the analysis were extracted from the electronic medical records of each patient. These included the presence of anemia, neutropenia, congestive of heart failure, baseline anemia and/or neutropenia, prior myocardial infarction, peripheral vascular disorders, cerebrovascular disease, dementia, chronic pulmonary disease, rheumatologic disease, peptic ulcer disease, liver disease, diabetes, renal disease, malignancy, neurologic deficits and HIV infection. A Cox proportional hazard model was constructed to assess the independent association of abnormal platelet counts with subsequent mortality after controlling for all extracted co-morbidities, age, gender and race.

As shown in table 1, caucasians had higher thrombocytopenia rates compared to blacks, Hispanics and other ethnicities (p<0.001 for all comparisons). Ethnicities other than Caucasians, Blacks or Hispanics had significantly lower rates of thrombocytosis compared to Caucasians or Blacks (p values <0.01). The median platelet count was 237,000/microliter. The median follow-up period was 3.3 years with a total of 134,132 person-years of observation. Fully adjusted hazard ratios (HR) using normal platelet counts (≥150,000 and ≤450,000) as the reference group showed a significant association of thrombocytopenia (HR=1.43, 95% CI 1.34–1.53; p<0.001) and thrombocytosis (HR=1.73, 95% CI 1.54–1.94; p<0.001) with shorter overall survival. A statistically significant interaction term was also identified between race and thrombocytopenia (Table 1). More specifically, we found that Hispanics with thrombocytopenia are at a higher risk compared to Caucasians with thrombocytopenia (HR=1.34, 95% CI 1.13–1.59; p=0.001).

Thrombocytopenia and thrombocytosis are independently associated with increased mortality in our study population. The effect of thrombocytopenia on overall survival is more prominent on Hispanics. These findings suggest that abnormal platelet count may be a mediator of increased mortality risk in older patients and this effect may differ by ethnicity.

No relevant conflicts of interest to declare.