Schmutz in the urine contaminates culture results

Reference

Maher PJ, Jablonowski KD, Richardson LD. Squamous epithelial cell presence reduces accuracy of urinalysis for prediction of positive urine cultures. Am J Emerg Med 2020;38(7):1384-1388.

Synopsis

These authors examined the records of 6490 patients who had a urine culture and a contemporaneous urinalysis performed in a single emergency department over a 26-month period. The authors included all samples, regardless of collection method. They classified a urine culture as positive when they detected at least 10^4 colony-forming units (CFUs) of pathogenic bacteria, negative when they detected no growth, or contaminated for all other results (including a growth of fewer than 10^4 CFUs pathogenic organisms, mixed growth of multiple bacterial species in any concentration, or any growth of nonpathogenic organisms). Although these classifications are well-grounded in the literature, there is a bit of noise as to the number of CFUs and method of collection. Women composed approximately 48% of the total panel, but 74% of the samples with more than 5 squamous epithelial cells (SECs) came from women. Additionally, patients with more than 5 SECs were younger (50.2 vs 54.6 years of age) and had a higher body mass index (29.2 vs 27.9) than those with 5 or fewer SECs. Overall, 19.2% of all samples grew more than 10^4 pathogens and 21.6% were contaminated. When more than 5 SECs were present, 21.4% of the cultures were positive and 44.5% were contaminated. The authors then performed a bunch of statistical jujitsu that basically indicated that urine chemistry and microscopy all performed better when SECs were absent. The "good" news is that leukocyte esterase activity didn't perform all that well regardless of the presence or absence of SECs and the presence of nitrites remained fairly specific independent of SECs.