Ascertainment of self-harm at general hospitals in Hong Kong.

•Overall ascertainment rate of self-harm in ED is significantly lower.•Admission during 07:00–23:00 was associated with a lower capture rate in ED.•The length of stay at ED and in-hospital was associated with a higher capture rate.•Overall underestimated by 20% was due to missing records during the clinical process.•Low ascertainment rate at ED (23%) & ward (66%) indicates to improve self-harm coding.