Cautionary tales from the neonatal intensive care unit: diapers may mislead urinary output estimation in extremely low birthweight infants
Objective: To examine the validity of the weighing of modern polyacrylate diapers to determine urine output in the extremely low birth weight infant nursed in a humidified incubator. Design: Observational study with repeated measures using high-absorbency cellulose/polyacrylate diapers and a humidified incubator in a modeled clinical scenario. Setting: A tertiary referral neonatal intensive care unit. Interventions: Six dry diapers were placed into eight levels of humidity, between 55% and 90% inclusive, in an incubator set at 37°C. Hourly weight increments were recorded. The study was repeated with 5 mL of normal saline added to the diaper to mimic prior urine output. Results were compared by Kruskal-Wallis nonparametric analysis of variance. Measurements and Main Results: Dry diapers increased in weight for each humidity level after 1 hr (p < .05). This was significantly greater at higher humidity. The maximum increase was a median of 3.06 g (range, 2.47-3.31 g) at 85% humidity after 6 hrs. When 5 mL of 0.9% saline had been added, the diaper weight changes depended on environmental relative humidity. At >=80% humidity, the diapers continued to gain weight; at 70% or 75% humidity, they did not change weight; and at <=65% humidity, the diapers lost weight. Maximal evaporative loss was median 1.11 g (range, 0.8-1.24 g) at 6 hrs in 55% relative humidity. Conclusions: The use of diaper weighing in high-humidity infant environments may not be an accurate measure of actual urine output in the extremely low birth weight infant. This study demonstrates the need for caution when interpreting this measure, and we discuss some possible clinical approaches to ameliorate this difficulty.