Use of flow cytometry (Sysmex^® UF-100) to screen for positive urine cultures: in search for the ideal cut-off

Abstract

Background: Cultures for urinary tract infections (UTI) constitute a large workload in the clinical microbiology laboratory, although up to 80% are usually negative. Several automated methods are available to screen urines for UTI, one being the flow cytometry-based Sysmex ᭨ UF-100. Methods: The performance of the UF-100 was evaluated over a 16-month period using urine culture as the reference method. Results: During this period, a total of 5356 urine samples were studied (469 children; 3229 women and 1658 men), of which 706 were culture positive (593 grew Gram negative bacilli). Receiver operating characteristics (ROC) curve analysis showed an area under the curve (AUC) of 0.83 for leukocytes and 0.85 for bacterial count. Applying cut-off values reported in the literature gave sensitivities ranging from 75% to 90%, resulting in 73-174 false negatives (FN). Using a logical combination (leukocytes G15=10 6 /L OR bacteria G500=10 6 /L) gave a sensitivity of 98%. However, the specificity dropped to 25%, resulting in 15 FN. Conclusions: Screening urine samples for UTI detects a large number of culture positive samples. However, the rather large number of FN observed precludes the use of the UF-100 as a routine screening method to exclude urine samples from culture. Clin Chem Lab Med 2010;48:289-92.

Screening for Urinary Tract Infection with the Sysmex UF-1000i Urine Flow Cytometer

“…Although these numbers are comparable to another recent study evaluating the UF-1000i (11), most studies have reported a percentage of negative culture samples around 70 to 80% (2,4,15,22). The organisms identified in our study correspond well with earlier published results (2,4,9).…”

“…Considering these facts, it is no surprise that urine samples are the most frequently received specimens in many microbiology laboratories. Nevertheless, a high number of these urine cultures will not yield any bacteria at all, and percentages for negative urine cultures up to 80% have been reported (2,15). Since urine culture is a laborious procedure, a more cost-effective approach would have a large impact on laboratory economics.…”

“…However, the results of studies that compare the bacterial and leukocyte counts with the UF-100 with urine culture for the diagnosis of UTI vary widely. While some studies have reported an adequate performance in relation to urine culture (6,9,10,13), others have found a relatively large number of false negatives, which decreases sensitivity and makes the UF-100 unsuitable as a screening method to detect negative culture samples (2,15,22). Comparing studies is difficult though, since reported sensitivities and specificities depend on the definition used for gold standard positive and negative urines, and these definitions vary among laboratories.…”

Screening urine samples for the absence of urinary tract infection using the sediMAX automated microscopy analyser

“…Some authors believe a culture to be negative until a count of i10 5 c.f.u. ml 21 is reached (Manoni et al, 2002;Falbo et al, 2012), whilst others deem samples positive at 10 3 c.f.u.ml 21 (Brilha et al, 2010;Kim et al, 2007). Culture classification has previously been suggested to have a profound effect on the calculated performance of microscopy analysers.…”

Standardization of isothermal microcalorimetry in urinary tract infection detection by using artificial urine

“…Although generally not required in cases of acute, uncomplicated cystitis [4], urine culture is still considered as standard of care in pyelonephritis, pregnant women, and men. Not surprisingly, urine cultures are among the most frequent culture analyses performed in the clinical microbiology laboratory [5]. However, up to 80 % of urine samples are negative, resulting in considerable workload and consumption of resources [6].…”