Interpreting microbiological investigations for acute cystitis in adults

Urinalysis (a ‘dipstick’ test) demonstrating an absence of both leukocyte esterase and nitrite has a high negative predictive value for the diagnosis of UTI in symptomatic patientsNelson, 2024Simerville, 2005. A positive urinalysis result (positive for either leukocyte esterase and/or nitrites) has a low positive predictive value for UTI in the absence of symptoms and is not an indication for antibioticsAdvani, 2024Nelson, 2024Simerville, 2005.

Microscopy and culture of a midstream urine sample is the accepted investigation to confirm if a symptomatic patient has a UTI, if indicated. The diagnosis of acute cystitis is unlikely if pyuria is absent on microscopy, regardless of whether bacteriuria is present.

The established definition of significant bacteriuria is 10⁸ colony forming units (CFU)/L or more (equivalent to 10⁵ CFU/mL or more) from a midstream urine sample. Lower bacterial counts of 10⁵ CFU/L or more (equivalent to 10² CFU/mL or more) are considered acceptable to make the diagnosis of UTI in people with symptomsNelson, 2024.

The growth of mixed bacterial types on urine culture usually indicates contamination with normal genital tract flora.