Assessment of urine specific gravity by reagent strip test in newborn infants

Specific gravity was measured with a strip test (N-Labstix SG, Ames Division, Miles, Puteaux, France) in 98 urine specimens obtained from 57 newborn infants; osmolality was measured with an osmometer. The strip test did not accurately predict urine osmolalities; a very weak correlation was found between the specific gravity and the osmolality (r2 = 0.598, P < 0.01). Specific gravity values up to 1.015 always indicated hypotonic urine with osmolality less than 211 mosmol/kg H2O, whereas higher values could be associated with either hypotonic or hypertonic urine. Therefore, the strip test cannot be recommended in the neonatal period because its clinical usefulness is strictly restricted to urine samples with low specific gravities (< or = 1.015) and without confounding variables (urine pH > or = 6.5, glucosuria, proteinuria, haematuria).