🔬 Urine Culture and Sensitivity

A laboratory test where urine is cultured to identify bacterial or fungal pathogens and determine their susceptibility to various antibiotic agents.

Indications include suspected UTI in children, men, or pregnant women (where asymptomatic bacteriuria requires treatment). It is indicated in cases of suspected upper UTI (pyelonephritis), recurrent UTI (>=2 in 6 months or >=3 in 12 months), and when symptoms persist despite empirical antibiotics. It is also required for patients with indwelling catheters who are systemically unwell (catheter-associated UTI).

A mid-stream urine (MSU) sample is collected into a sterile boric acid container (boric acid acts as a preservative to prevent bacterial overgrowth during transport). The laboratory inoculates the urine onto agar plates (e.g., CLED or MacConkey agar) and incubates them at 37°C for 24-48 hours. If growth occurs, automated or manual methods (e.g., disc diffusion) are used to determine which antibiotics inhibit the growth of the isolated bacteria.

A normal (negative) result is reported as 'No growth after 48 hours' or 'Insignificant growth' (typically <10^3 or 10^4 CFU/mL). This suggests that no pathogenic bacteria are present in the urine at a high enough concentration to suggest infection. The urine should ideally be sterile, though minimal contamination from urethral flora is common.

Growth is interpreted alongside clinical symptoms. Significant bacteriuria (10^5 CFU/mL) in a symptomatic patient confirms the diagnosis. Mixed growth of multiple organisms usually suggests skin/fecal contamination rather than true infection. The sensitivity results guide the choice of antibiotic; for example, if an isolate is resistant to trimethoprim but sensitive to nitrofurantoin, the treatment plan must be adjusted accordingly. Resistance patterns (e.g., ESBL production) have high clinical significance.

A 'positive' culture is typically defined as a single organism growth of >=10^5 colony-forming units (CFU)/mL, though lower counts (10^3) may be significant in symptomatic patients or men. Common pathogens identified include E. coli (most common), Proteus mirabilis, Klebsiella, and Enterococcus. Sensitivity testing (antibiogram) reports whether the organism is 'Sensitive', 'Intermediate', or 'Resistant' to specific antibiotics like Nitrofurantoin, Trimethoprim, or Amoxicillin.

Urine culture provides definitive identification of the causative organism in a UTI and, crucially, its antibiotic sensitivity profile. This allows for 'narrowing' or 'switching' of empirical antibiotic therapy to a targeted agent, promoting antimicrobial stewardship and ensuring effective treatment. It is mandatory in cases where empirical therapy has failed, in complicated UTIs (e.g., pregnancy, male patients, pyelonephritis), and in recurrent infections.

The most frequent pitfall is a contaminated sample (e.g., 'mixed growth' of skin flora like Coagulase-negative Staphylococci), which can lead to unnecessary antibiotic use. Delay in transporting the sample to the lab without a preservative (like boric acid) can lead to false-positive high colony counts due to bacteria multiplying at room temperature. Providing a 'first-catch' rather than 'mid-stream' sample often results in contamination.

The primary limitation is the 24-48 hour turnaround time for results, necessitating empirical treatment in the interim. The test is highly susceptible to contamination if the 'mid-stream' technique is not followed. It cannot distinguish between infection (symptoms) and asymptomatic bacteriuria (no symptoms), which should generally not be treated in non-pregnant adults. Simple cystitis in non-pregnant women is often treated without culture.

Students must know which patient groups require a culture (pregnant women, children, men, and complicated cases) rather than just a dipstick. Understanding the concept of 'asymptomatic bacteriuria' and why we only treat it in pregnancy is a frequent exam topic. Recognising 'mixed growth' as a sign of contamination is also vital.