Background To monitor accurately injury incidence trends, indicators should measure incidence independently of extraneous factors. Frequencies and rates of New Zealand's serious non-fatal self-harm indicators may be prone to fluctuations in reporting owing, for example, to changing social norms. Hence, they have been considered provisional.

Aim To validate empirically the serious non-fatal self-harm indicators.

Methods All serious non-fatal first admissions to hospital were identified and classified according to whether principal diagnosis (PDx) was injury or mental disorder, and conversely whether contributing diagnoses were mental disorder or injury. The proportion assigned self-harm external cause of injury code (E-code) was calculated for each year from 2001 to 2007. Subsequently, all cases with a self-harm E-code were identified, and the proportion with a PDx of injury and contributing diagnosis of mental disorder, or PDx of mental disorder and contributing diagnosis of injury over time, were determined.

Results No linear changes over time were detected in the proportion of cases assigned an injury PDx, or the proportion assigned a mental disorder PDx, or the proportion with a self-harm E-code. The estimated maximum observed increase in the frequency of serious non-fatal self-harm hospitalisation explained by changes in reporting was 19– 40%.

Conclusion Identification of serious non-fatal self-harm events using an operational definition of PDx of injury, a self-harm first listed E-code, and an appropriate severity cut-off point, is a valid method of monitoring incidence and rates in New Zealand.