Urine culture and colony count

Principle

• A calibrated loop delivers a fixed volume of urine so colonies can be converted to colony-forming units per mL (CFU/mL).
• 1 µL (0.001 mL) loop: colonies × 1,000 = CFU/mL; 10 µL (0.01 mL) loop: colonies × 100 = CFU/mL.

Specimen

• Clean-catch midstream urine, straight catheter, suprapubic aspirate or catheter port sample (never from the drainage bag).
• Plate within 2 hours of collection, or refrigerate at 2–8 °C (up to 24 h) or use boric acid preservative tubes.

Reagents and equipment

• Sheep blood agar plus MacConkey agar, or CLED agar; chromogenic urine agars are also used.
• Calibrated loops (plastic disposable or checked platinum).

Procedure

• Mix urine gently; dip the loop vertically just below the surface.
• Streak a single line down the plate, then cross-streak perpendicular across it for an even spread.
• Incubate at 35 ± 2 °C for 18–24 h (longer if clinically indicated).
• Count colonies of each morphotype, multiply by the loop factor, then identify and test susceptibility of significant isolates.

Interpretation

• Classic threshold for significant bacteriuria in midstream urine: ≥10^5 CFU/mL of a single uropathogen.
• Lower counts (for example ≥10^2–10^4 CFU/mL) may be significant in symptomatic women, men, children and catheter specimens; follow local policy.
• Any growth from a suprapubic aspirate is potentially significant.
• Three or more organisms with no predominant one usually indicates contamination; request repeat.

Quality control and pitfalls

• Delay at room temperature lets bacteria multiply, falsely raising counts.
• Tilting the loop or a wet loop changes the volume delivered.
• Recent antibiotics, very dilute urine or fastidious organisms can give falsely low counts.

Clinical use

• Diagnosis of urinary tract infection, pyelonephritis and asymptomatic bacteriuria in pregnancy; guides therapy.