According to this study:

* Weighing diapers of extremely-low-birth-weight babies is an inaccurate measure of urine output.

Extremely-low-birth-weight infants are usually kept in high-humidity incubators. Various methods are used to monitor their fluid and electrolyte balances. This study examines the validity of calculating the urine output of extremely-low-birth-weight infants by weighing diapers.

The study was conducted in a neonatal ICU with conditions set to mimic those of a typical unit. Polyacrylate diapers, both wet (with saline to simulate urine) and dry, were placed in incubators with humidity levels of 55% to 85%. Fluid-filled bags were placed in the diapers to simulate baby weight, and the diapers were weighed every hour for six hours.

Dry diapers increased in weight with greater humidity; those in 85% humidity were the heaviest after six hours. Wet diapers lost weight in 60% humidity or less. Wet diapers in 65% to 75% humidity did not significantly change weight, but those in 80% humidity or more progressively gained weight. Most extremely-low-birth-weight babies are initially kept in 80% to 85% humidity, at which level both wet and dry diapers had the greatest increases in weight.

The authors conclude that weighing diapers of extremely- low-birth-weight babies in high-humidity incubators is an ineffective way of measuring urine output. The increase in diaper weight caused by humidity may be enough to mask oliguria or anuria in extremely-low- birth-weight babies. The authors suggest possible solutions such as prehumidifying the diaper or using the study's data to estimate weight gain caused by the environment and subtracting this from the measured diaper weight. The authors are investigating these methods.

SDSJ

Amey M, et al. Pediatr Crit Care Med 2008; 9(1):76-9.