Unprevented or prevented dispensing incidents: which outcome to use in dispensing error research?

To compare the rate, error type, causes and clinical significance of unprevented and prevented dispensing incidents reported by Welsh National Health Service (NHS) hospital pharmacies.

Details of all unprevented and prevented dispensing incidents occurring over 3 months (September-December 2005) at five district general hospitals across Wales were reported and analysed using a validated method. Rates of unprevented and prevented dispensing incidents were compared using Mann-Whitney U test. Reported error types, contributory factors and clinical significance of unprevented and prevented incidents were compared using Fisher's exact test.

Thirty-five unprevented and 291 prevented dispensing incidents were reported amongst 221,670 items. The rate of unprevented (16/100,000 items) and prevented dispensing incidents (131/100,000 items; P = 0.04) was significantly different. There was a significant difference in the proportions of prevented and unprevented dispensing incidents involving the wrong directions/warnings on the label (prevented, n = 100, 29%; unprevented, n = 4, 10%; P = 0.02) and the wrong drug details on the label (prevented, n = 15, 4%; unprevented, n = 6, 14%; P = 0.01). There was a significant difference in the proportions of prevented and unprevented dispensing incidents involving supply of the wrong strength (prevented, n = 46, 14%; unprevented, n = 2, 5%; P = 0.02) and issue of expired medicines (prevented, n = 3, 1%; unprevented, n = 5, 12%; P = 0.002).

The use of prevented dispensing incidents as a surrogate marker for unprevented incidents is questionable. There were significant differences between unprevented and prevented dispensing incidents in terms of rate and error types. This is consistent with the medication error iceberg. Care must be exercised when extrapolating prevented dispensing incident data on error types to unprevented dispensing incidents.