Management of Mixed Urogenital Flora in Urine Cultures
Mixed urogenital flora in urine cultures typically indicates contamination rather than true urinary tract infection and should be interpreted in the context of collection method, patient symptoms, and urinalysis findings.
Understanding Mixed Flora Results
- Mixed flora refers to the presence of multiple bacterial species in a urine culture, most commonly indicating contamination with periurethral, vaginal, or perineal flora rather than true urinary tract infection (UTI) 1
- The presence of epithelial cells alongside multiple organisms strongly suggests contamination rather than infection 1
- In properly collected samples, mixed growth occasionally represents true mixed infection, particularly in patients with long-term catheterization 2
Interpretation Based on Collection Method
- Contamination rates vary significantly by collection method:
- Cleansing the perineal area before collection significantly reduces contamination (7.8% with cleansing vs. 23.9% without) 1
Clinical Approach to Mixed Flora Results
Step 1: Evaluate Pre-Test Probability of UTI
- Assess patient symptoms and risk factors for UTI 1
- Consider patient demographics (uncircumcised male infants and female infants have higher risk) 4
- Evaluate for fever and other potential sources of infection 4
Step 2: Review Urinalysis Results
- Pyuria (>10 WBCs/HPF) with bacteriuria increases likelihood of true infection 1
- Positive leukocyte esterase or nitrites on dipstick suggests infection despite mixed flora culture 4
- Absence of pyuria suggests contamination or asymptomatic bacteriuria rather than infection 4
Step 3: Consider Collection Method Reliability
- If specimen was collected by bag or non-clean catch method, results are highly unreliable 4
- For febrile infants <24 months, a positive culture with mixed flora should be confirmed by catheterization or suprapubic aspiration before treatment 1
- Catheterization provides more reliable specimens with higher sensitivity (95%) and specificity (99%) 1
Step 4: Management Decision
- For asymptomatic patients with mixed flora: No treatment needed; consider repeat testing only if high clinical suspicion 5
- For symptomatic patients with mixed flora and negative urinalysis: Consider recollection using a more reliable method 1
- For symptomatic patients with mixed flora and positive urinalysis: Obtain a new specimen via catheterization or suprapubic aspiration 4
Special Considerations
- In patients with indwelling catheters, mixed flora may represent true polymicrobial infection rather than contamination 2
- Heavy mixed growth may mask a true infection, potentially requiring repeat testing with a more reliable collection method 1
- In urologic procedures, mixed flora cultures have similar postoperative infection risk as negative cultures and don't necessarily require preoperative antibiotics 6
Common Pitfalls to Avoid
- Treating mixed flora from bag specimens without confirmation by a more reliable collection method 1
- Dismissing all mixed flora results as contamination without considering clinical context 7
- Failing to recognize that mixed flora in catheterized specimens is more likely to represent true infection than in voided specimens 2
- Not considering that pyuria is a key distinguishing factor between true UTI and asymptomatic bacteriuria or contamination 4
Recommendations for Recollection
- Use catheterization for the most reliable results, especially in infants and young children 4
- For adults, proper clean-catch technique with perineal cleansing significantly reduces contamination 1
- Process specimens promptly or refrigerate to prevent overgrowth of contaminants 4
- For definitive diagnosis in febrile infants, both pyuria/bacteriuria on urinalysis and ≥50,000 CFU/mL of a single uropathogen from a catheterized specimen are required 4