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Drainage Volume, Drainage Time, and Irrigation Volume in Chronic Subdural Hematoma. BACKGROUND AND OBJECTIVES: The benefit of postoperative drainage with intraoperative irrigation during chronic subdural hematoma surgery has been previously established. However, the relations between clinical outcomes and drainage volume, drainage time, and irrigation volume are not clear. METHODS: We conducted a post hoc analysis of a multicenter, randomized clinical trial (FINISH) conducted between 2020 and 2022 in Finland, examining the effect of subdural irrigation. We analyzed the impact of drainage volume, time, and irrigation volume on outcomes using logistic regression. The primary outcome was reoperation rate, and secondary outcomes were 6-month modified Rankin Scale, postoperative adverse events, postoperative hematoma width, and midline shift. RESULTS: A total of 546 patients were included, of whom 84 (15.4%) required reoperation. The median postoperative drainage volume was 70 mL (IQR 30-150 mL), and drainage time was 48 hours (IQR 42-50 hours). The median intraoperative irrigation volume was 400 mL (IQR 220-540 mL) in the irrigation group. A larger drainage volume was associated with an increased reoperation rate (odds ratio 1.15 for 100 mL increase, 95% CI 1.02-1.31, P = .03). Meanwhile, longer drainage times were associated with a decreased rate of reoperation (odds ratio 0.98, 95% CI 0.96-0.99, P = .02). Dichotomized, the reoperation rates were 13.9% and 18.2% for drainage volumes of <100 mL and ≥100 mL, and 18.2% and 12.1% for drainage times of <48 hours and ≥48 hours, respectively. Irrigation volume was not associated with the rate of reoperation. Observed variables did not associate with 6-month functional outcome. CONCLUSION: Based on our findings, patients with larger drainage volume and shorter drainage time were associated with an increased risk for hematoma recurrence. Optimal drainage after chronic subdural hematoma surgery might be more complex than just defining a standard drain time for all patients, and factors such as drainage volume might need to be considered.