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A recent study suggests that using a lower cut off value of white blood cells in dilute urine, may have a higher likelihood of detecting a urinary tract infection in children.
In dilute urine (specific gravity < 1.015), the optimal white blood cell cut off point was 3 WBC/hpf (Positive LR 9.9). With higher specific gravities, the optimal cut off was 6 WBC/hpf (Positive LR 10). Positive leukocyte esterase has a high likelihood ratio regardless of the urine concentration.
Additional Information
This was a retrospective study of 2700 infants < 3 months old who were evaluated for urinary tract infections (UTI). The UTI prevalence in this group was 7.8%. A UTI was defined as at least 50,000 colony forming units/mL from a catheterized specimen. Test characteristics looked at white blood cell and leukocyte esterase cut-offs, dichotomized into specific gravities: dilute (<1.015) and concentrated (>/=1.015).