The Solution to Dilution is ….

Do we order a lot of urinalyses? Does the sun rise in the east? Does a bear ….

For a test we order with great frequency, there is actually quite a bit of complexity in its interpretation. The combination of symptoms, clinical context, the balance between sample contamination and presence of white blood cells, of nitrites and/or leukocyte esterase, and so on, can make it a relatively tricky test to interpret. The gold standard remains a urine culture.

Now – if you haven’t been already – you probably ought to be taking into account the urine specific gravity, as well.

This retrospective analysis of 14,971 children for whom paired urinalyses and urine cultures were available describes the test characteristics of WBCs/hpf, LE, and nitrites as stratified by urine specific gravity. There are a lot of numbers in this article – a “zillion” to be precise – across eighteen dense tables of +LR/-LR, sensitivity/specificity, and PPV/NPV, but the basic gist of the matter is: variations in urine concentration diminish the value of the test in different ways. As urine specific gravity increases, it becomes more likely a patient will not have a positive urine culture despite having typically diagnostic amounts of WBCs/hpf, +LE, and/or +nitrites. Likewise, with dilute urine, a lower threshold for WBCs/hpf may be needed to have adequate sensitivity.

Just one more layer to consider in this frequently used test of under-appreciated complexity.

“The Importance of Urine Concentration on the Diagnostic Performance of the Urinalysis for Pediatric Urinary Tract Infection”
https://www.ncbi.nlm.nih.gov/pubmed/28169050

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