INTRODUCTION Platelets greatly contribute to initiation and progression of cardiovascular diseases. However, studies that have investigated the relationship of platelet count with clinical outcomes in patients with acute myocardial infarction are scarce. We sought to explore the association of platelet counts with infarct size and clinical outcome in patients presenting with acute STEMI treated with primary percutaneous coronary intervention (pPCI). The primary endpoint was 1-year all-cause mortality.

METHODS&RESULTS We grouped 1198 STEMI patients into tertiles according the blood platelet count on admission: tertile 1 (T1) = 102-206 [10⁹ platelets/l] (402 patients), tertile 2 (T2) = 207-259 [10⁹ platelets/l] (396 patients) and tertile 3 (T3) = 260-921 [10⁹ platelets/l] (400 patients). Using serial single-photon emission computerized tomography imaging we found that patients with highest platelet counts on admission (T3 group) showed an increased area at risk and infarct size: Myocardial area at risk or initial perfusion defect before pPCI (median) was 22.0% of the left ventricle (25th–75^th percentile, 12.0%–39.8%) in T1, 21.0% of the left ventricle (25th–75^th percentile, 11.0%–37.1%) in T2 and 26.0% of the left ventricle (25th–75^th percentile, 14.9–45.0%) in T3. The final infarct size in the 7 to 14 days scintigraphy (median) was 10.0% of the left ventricle (25th–75^th percentile, 2.0%–21.0%) in T1, 9.0% of the left ventricle (25th–75^th percentile, 2.0%–20.7%) in T2 and 12.0% of the left ventricle (25th–75^th percentile, 3.0%–27.3%) in T3. In line with these szintigraphic findings, peak creatine kinase myocardial band (CK-MB) and troponin T values, both enzymatic estimates of infarct sizes, were also highest in group T3. One-year follow-up for all-cause mortality - the primary endpoint - was available in 1198 (100%) patients. At 1 year, 16 patients in T1, 5 patients in T2 and 22 patients in T3 had died (Kaplan–Meier estimates of 1-year mortality: 4.2%, 1.3% and 5.6%, respectively; log-rank test, P=0.006). At 1-year, major adverse cardiovascular events (MACE) occurred in 89 patients in T1, in 82 patients T2 and in 120 patients in T3 (Kaplan–Meier estimates of 1-year MACE: 25.7%, 21.3% and 30.8%, respectively; log-rank test, P=0.012).

CONCLUSIONS In patients with STEMI undergoing primary percutaneous coronary intervention, blood platelet levels on admission were associated with increased infarct size and long-term mortality.