Question
Describe the etiology, specimen collection and laboratory diagnosis of urinary tract infection. (2+2+4)
Answer
Etiology of urinary tract infection (UTI): Escherichia coli is the most common causative organism (accounting for the majority of both community-acquired and uncomplicated UTIs), followed by Klebsiella pneumoniae, Proteus mirabilis, Enterococcus faecalis, and Staphylococcus saprophyticus (particularly in young sexually active women); in hospital-acquired/catheter-associated UTI, Pseudomonas aeruginosa and Candida species become more relevant.
Specimen collection: Clean-catch midstream urine is the standard, most commonly used specimen (after appropriate perineal cleaning to minimize contamination); catheter specimen of urine (aseptically aspirated from the catheter port, not from the collection bag) is used in catheterized patients; suprapubic aspiration is the most reliable method for avoiding contamination and is particularly used in infants, obtained via direct percutaneous bladder puncture. Samples must be transported promptly (within 1–2 hours) or refrigerated to prevent bacterial overgrowth that would invalidate quantitative culture results.
Laboratory diagnosis:
- Urine microscopy — presence of significant pyuria (pus cells) and bacteria supports the diagnosis; presence of red cells and casts may indicate additional pathology.
- Urine culture — the definitive test; a colony count of ≥10⁵ CFU/mL of a single predominant organism from a properly collected midstream sample is considered significant bacteriuria (lower thresholds, e.g., ≥10² CFU/mL, apply for suprapubic aspirate or in symptomatic patients).
- Antibiotic susceptibility testing — performed on significant isolates to guide targeted therapy.
- Dipstick screening tests (leukocyte esterase, nitrite) — rapid, useful screening tools, though less sensitive/specific than culture and not a substitute for it.
- Imaging (ultrasound/CT) — not part of routine microbiological diagnosis, but used to investigate underlying structural abnormalities in recurrent/complicated UTI.