Accuracy of urine output measurement with regular disposable nappies

Evaporation of urine from four types of disposable absorbent infant nappies (A, B, C, D) was assessed under a radiant warmer or in an incubator, with or without phototherapy. Each nappy was weighed dry and then 5, 10, 15 or 30 ml of urine were added. The nappy was immediately reweighed, placed in its study environment and then weighed 5, 15, 30, 60 and 120 min later. Under all conditions, the percentage evaporation from each type of nappy increased during the 120 min and was inversely correlated with urine volume. Without phototherapy, the maximum evaporation at 120 min with a 5-ml urine sample was 15.9% +/- 6.1% for nappy B in the incubator and 20.8% +/- 4.0% for nappy A in the radiant warmer. Phototherapy was associated with slight but significant changes in evaporation in the incubator and the radiant warmer. Nappy D allowed the lowest percentage evaporation in the incubator with or without phototherapy and in the radiant warmer with phototherapy (8.8% +/- 4.4%, 2.4% +/- 1.8% and 16.0% +/- 10%, respectively for a 5-ml urine sample).