Looking Beyond the Imperfect Sensitivity of Pyuria for the Detection of Positive Urine Cultures

In their recent publication, Shaikh et al1 demonstrate that ∼1 in 5 children with positive urine cultures do not have pyuria on the urinalysis (UA). This apparent imperfect sensitivity of pyuria, when using positive urine cultures (rather than UTI) as a gold standard, has previously been well-documented, and has driven numerous investigators to try to better interpret this finding.Hoberman et al demonstrated that nearly all (14/15) patients in one study who had positive urine cultures without pyuria lacked evidence of pyelonephritis on DMSA scan.2 Three investigations have demonstrated perfect or near-perfect sensitivity of pyuria in infants with bacteremic UTI,3–5 a condition that – unlike a positive urine culture alone - is irrefutably a true infection. One investigation revealed that febrile infants with positive cultures and no pyuria were more likely to have respiratory symptoms and less likely to have elevated inflammatory markers than infants with positive urine cultures and pyuria.6In their Discussion, Shaikh et al query what might happen to patients who have positive urine cultures without pyuria who go untreated. We recently studied this exact question and found that infants in this scenario are generally improved at the time the culture results. Though many were treated based on the positive culture, none the 36 who remained untreated were later diagnosed with UTI.7 Finally, the “two-step” approach to UTI diagnosis (bag UA followed by catheterized culture only if UA is positive) resulted in substantially fewer catheterizations without an increase in delayed UTI diagnoses.8 While none of these studies individually proves that a UTI diagnosis should require pyuria, collectively they support the most recent AAP recommendation to include a positive UA in the diagnostic criteria.9Shaikh et al suggest that contamination and asymptomatic bacteriuria (AB) contribute to only a small fraction of positive cultures without pyuria. They estimate a urine culture contamination rate of ∼5/1000, citing two studies from 1959 and 1961. One compared urine cultures obtained by catheterization vs suprapubic aspiration (SPA) from 42 anesthetized children, many of whom had culture growth on the catheterized but not the SPA specimen, including one (2.4% of sample) that grew 87 000 cfu/mL of Proteus.10 The second study revealed no significant growth from catheterized specimens in 50 healthy infants.11 Even if more clarity is provided around the 5/1000 estimate for these 92 patients, the generalizability of contamination estimates from healthy and/or anesthetized patients to sicker children is limited. As Shaikh et al acknowledge, contamination is challenging to measure, but a more recent publication estimated contamination rates that are much higher.12We must continue to identify creative ways to investigate the clinical implications of patients who have positive urine cultures without pyuria. Hopefully we can move away from asking whether it occurs to determining why it occurs.