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Introduction: Mean platelet volume (MPV) is a common platelet index that estimates average platelet size. In retrospective cohorts, MPV is reported to be higher, on average, in patients who have experienced acute cardiovascular or cerebrovascular events. Aneurysmal subarachnoid hemorrhage (aSAH) is often associated with increased thrombogenic state due to reactive platelets resulting in delayed cerebral ischemia (DCI).Hypothesis: We assessed whether trends in MPV over the first 21 days after aSAH predicts occurrence of DCI.Methods: The present study enrolled a prospective cohort of 47 patients with aSAH. Clinical (H&H, WFNS) grades and daily MPV was recorded. Patients surviving atleast 7 days following aSAH were included for analysis to determine temporal trend of MPV. Mixed linear regression model was used to compare MPV trends between groups defined by clinical grades of varying severity. Segmented linear regression analysis was performed to identify transition point for patients developing DCI in comparison to those without DCIResults: Average age of cohort was 53.3±13.2 years with 68% women. Thirty-five of the 47 (74.5%) were H&H I-III as compared to 21/47 (44.7%) with WFNS 4-5. Thirty-two patients developed DCI. Baseline MPV was no different (~10.5fL) among patients with different clinical grades. In 32 patients who developed DCI, a transition point at 3 days was identified prior to which MPV increased by 0.13fL/day (95% CI 0.04, 0.23; p=0.0047) followed by a decline of 0.03fL/day (95% CI 0.02, 0.04; p=0.0006). Similar trend was not seen in those without DCI. Rate of decline in MPV were steeper among patients with H&H I-III and WFNS 1-3 as compared to those with H&H IV-V and 4-5 [0.26fL/day; 90% CI 0.004, 0.048, p=0.051 and 0.23fL/day; 90% CI 0.003, 0.042, p=0.06 respectively].Conclusion: Rise in MPV during the first 3 days after aSAH with DCI followed by a decline indicates release of larger platelets after aSAH and possible association with DCI. Absence of declining trend MPV among higher clinical grade aSAH patients suggests persistence of reactive platelets in circulation.