The role of pretreatment urine culture in the evaluation of suspected UTI is to confirm the presence of bacteriuria and to identify and provide antibiotic susceptibility information on the causative organism (often retrospectively, if treatment is empirically given) (Hooten & Gupta, 2021; Meyrien, 2021).
A urine Gram stain limits potential causative organisms and helps select antimicrobial treatment. Culture of a clean-catch voided urine specimen is warranted for most patients with suspected UTI, except for healthy, nonpregnant, young females with typical symptoms of acute simple cystitis (i.e., without fever or concern for ascending infection) who have no risk factors for resistant infection. In general, urine culture should not be performed in nonpregnant patients without symptoms consistent with a UTI, as bacteriuria does not indicate a UTI in an asymptomatic patient.
In asymptomatic patients, the standard threshold for bacterial growth on a midstream voided urine reflective of bladder bacteriuria as opposed to contamination is ≥105 colony forming units (CFU)/mL (Hooten & Gupta, 2021; Meyrien, 2021). However, in symptomatic females with pyuria, lower midstream urine counts (i.e., ≥102/mL) have been associated with the presence of bladder bacteriuria. Thus, in such instances, the findings of a colony count <105 but ≥102/mL may still indicate a UTI. Lower bacterial counts still representative of infection are also seen in males, in patients already on antimicrobials, and with organisms other than E. coli and Proteus species.
Normal values in a noninfected midstream, clean-catch sample is <105 CFU/mL, with bacterial growth primarily due to E. coli from fecal contamination. In early studies, >95% of patients with bacterial counts ≥105 CFU/mL in voided urine, but only a minority of those with lower counts had concomitant bacteriuria on a catheterized specimen (Hooten & Gupta, 2021; Meyrien, 2021).
Subsequent studies have identified many females with symptoms and pyuria consistent with a UTI, but colony counts <105 CFU/mL in voided urine (Hooten & Gupta, 2021; Meyrien, 2021). This was demonstrated in a study of 202 premenopausal, nonpregnant females who presented with at least two symptoms of acute cystitis, a midstream, clean-catch urine collected, and who subsequently underwent urethral catheterization to collect a bladder urine specimen (Hooten & Gupta, 2021). Of the 121 females who had E. coli grow in the catheterized specimen, 49 (40%) had counts of <105 CFU/mL in their voided urine. A ≥102 CFU/mL threshold in voided specimens had a 93% positive predictive value for bladder bacteriuria with E. coli. Another study estimated that 88% of females with symptoms and a CFU count ≥102/mL on voided urine have a UTI (Hooten & Gupta, 2021; Meyrien, 2021).
It is not well understood why some infected females have low colony counts. One possibility is that low counts reflect insufficient sensitivity of conventional urine cultures.
In a study that included 220 females with symptoms of acute cystitis and 86 asymptomatic females, urine was tested with both standard culture and quantitative polymerase chain reaction (qPCR) for E. coli, S. saprophyticus, and sexually transmitted pathogens (Hooten & Gupta, 2021; Meyrien, 2021). Among symptomatic females, 81% of urine cultures were positive for any uropathogen, while 95% of samples were qPCR positive for E. coli (19 were positive for S. saprophyticus qPCR, 1 for Mycoplasma genitalium, and 1 for Trichomonas vaginalis). In contrast, urine culture and qPCR were positive for E. coli in only 11% and 12%, respectively. These findings suggest that almost all females with typical urinary complaints and a negative culture still have an infection with E. coli. Management of these patients is similar to those with higher colony counts (Hooten & Gupta, 2021; Meyrien, 2021).
There are also several other settings in which a colony count of ≤105/mL represents true infection rather than contamination (Ali & Wesonga, 2021; Hooten & Gupta, 2021; Meyrien, 2021):
- Among patients already being treated with antimicrobials.
- Among males, in whom contamination is a much lesser problem.
- When organisms other than E. coli and Proteus are present (Pseudomonas, Klebsiella, Enterobacter, Serratia, and Moraxella species).