Pediatric Urine Concentration and UA Accuracy

July 13, 2021

Written by Clay Smith

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Children <24 months may still have a UTI even with as few as 3 WBCs/hpf on urine microscopy if the urine is very dilute. Small leukocyte esterase on urinalysis was a powerful positive discriminator at all urine concentrations.

Why does this matter?
We have previously covered the varying diagnostic accuracy of urinalysis (UA) in children based on urine concentration. In a very dilute urine, even a few white blood cells per high-power field (WBCs/hpf) are concerning, whereas it is the opposite in highly concentrated urine. This study takes this concept a step further.

This takes concentration…
This was a retrospective look at 24,171 children under age two years with paired UA and urine culture results; 2,003 (8.3%) had a positive culture. The ideal cut points for WBCs/hpf on microscopic exam varied based on urine concentration. For a specific gravity <1.011, 3 WBCs/hpf had a LR+ of 10.5 and LR- 0.12; specific gravity 1.011 to 1.020, 6 WBCs/hpf had a LR+ 12 and LR- 0.14; for specific gravity >1.020, 8 WBCs/hpf had a LR+ 11.1 and LR- 0.35. Any child who had small leukocyte esterase (LE) on UA had LR+ > 25 at all urine concentrations; LR- for no LE ranged from 0.12 at low specific gravity, 0.15 medium, to 0.41 high.

Source
Pyuria and Urine Concentration for Identifying Urinary Tract Infection in Young Children. Pediatrics. 2021 Feb;147(2):e2020014068. doi: 10.1542/peds.2020-014068.