Question
Laboratory diagnosis of urinary tract infection
Answer
Laboratory diagnosis of urinary tract infection (UTI) aims to confirm significant bacteriuria and identify the causative organism.
Specimen collection: a properly collected clean-catch midstream urine (MSU) sample (or catheter/suprapubic aspirate when indicated), transported promptly (or refrigerated) to prevent bacterial overgrowth that would give a falsely high count.
Screening tests: urine microscopy (pyuria — >5–10 pus cells/hpf — and bacilluria on Gram-stained uncentrifuged urine correlate well with significant bacteriuria); dipstick tests for leukocyte esterase (detects pyuria) and nitrite (detects nitrate-reducing organisms, mainly Enterobacteriaceae).
Confirmatory test — culture: urine is cultured on CLED (cystine-lactose-electrolyte-deficient) agar or MacConkey agar, using a calibrated (standard) loop to inoculate a known volume, allowing accurate colony counting. Significant bacteriuria (Kass’s criterion) is defined as ≥10⁵ CFU/mL of a single organism from a properly collected MSU sample; lower counts may be significant in symptomatic patients or with suprapubic/catheter specimens.
Identification and susceptibility: the isolated organism is identified biochemically (commonest cause: E. coli; also Klebsiella, Proteus, Enterococcus, Staphylococcus saprophyticus, Pseudomonas) and antibiotic sensitivity testing performed to guide treatment.