What is the clinical significance of mixed flora on a urine culture?

Clinical Significance of Mixed Flora on Urine Culture

Mixed flora on urine culture typically represents contamination rather than true infection in most clinical scenarios, but interpretation must consider collection method, patient population, and clinical presentation.

Understanding Mixed Flora Results

• Mixed flora refers to the presence of multiple bacterial species in a urine culture, which most commonly indicates contamination with periurethral, vaginal, or perineal flora rather than true urinary tract infection (UTI) 1.
• Contamination rates vary significantly by collection method: clean-catch midstream specimens (27%), sterile urine bag specimens (65-68%), and catheterized specimens (4.7%) 1, 2.
• The presence of epithelial cells alongside multiple organisms strongly suggests contamination rather than infection 1.

Clinical Interpretation Based on Collection Method

Clean-Catch Specimens

• Contamination rates range from 0% to 29% for properly collected clean-catch specimens 1.
• A urine sample with >10 WBCs and significant epithelial cells should be considered contaminated and requires recollection 1.
• Cleansing the perineal area before collection significantly reduces contamination (7.8% with cleansing vs. 23.9% without) 1.

Catheterized Specimens

• Catheterization provides more reliable specimens with higher sensitivity (95%) and specificity (99%) 1.
• Mixed flora in catheterized specimens is less common (4.7%) and more likely to represent true polymicrobial infection 2.
• For indwelling catheters in place >few hours, mixed flora often represents biofilm colonization rather than infection 1.

Bag Collection

• Bag collection has the highest contamination rate (65-68%), similar to diaper collection 1.
• A negative culture from a bag specimen can help rule out UTI, but positive results (especially with mixed flora) should be confirmed with catheterization 1.

Clinical Significance in Different Populations

General Population

• In properly collected specimens from non-catheterized patients, mixed flora typically represents contamination in 67-77% of cases 3.
• Only 11-20% of mixed flora results from clean-catch specimens represent possible or probable true infection 3.

Catheterized Patients

• In long-term catheterized patients, polymicrobial bacteriuria is common and often clinically significant 4.
• For patients with indwelling catheters, mixed flora may represent true polymicrobial infection in 3-24% of cases 3.

Pediatric Considerations

• In children, clean-catch specimens show lower contamination rates (27%) compared to bag (68%) or pad (65%) collection methods 1.
• For febrile infants <24 months, a positive culture with mixed flora should be confirmed by catheterization or suprapubic aspiration before treatment 1.

Impact on Patient Management

• Sterile urine or urine with low mixed bacterial count (<10^5 CFU/ml) generally rules out UTI 1.
• Heavy mixed growth may mask a true infection, potentially requiring repeat testing 1.
• For urologic procedures like ureteroscopy, patients with preoperative mixed flora cultures don't show higher infection risk compared to those with negative cultures 5.

Recommendations for Practice

• For diagnostic accuracy, prioritize collection method based on clinical scenario: catheterization for ill patients or when accuracy is critical, clean-catch with proper cleansing for routine screening 1.
• When mixed flora is reported:
  • Consider recollection using a more reliable method if clinical suspicion for UTI remains high 1.
  • Evaluate for pyuria (>10 WBCs/HPF), which increases likelihood of true infection when present with bacteriuria 1.
  • In catheterized patients or those with urologic abnormalities, consider the possibility of true polymicrobial infection 4.
• For proper interpretation of urine cultures with mixed flora, consider:
  • Collection method and technique 1
  • Patient symptoms and risk factors 1
  • Presence of pyuria on urinalysis 1
  • Patient population (higher risk in catheterized patients) 4

Common Pitfalls to Avoid

• Don't automatically dismiss all mixed flora results as contamination, especially in catheterized patients or those with urologic abnormalities 4.
• Avoid treating mixed flora from bag specimens without confirmation by a more reliable collection method 1.
• Don't repeat antibiotics unnecessarily for mixed flora in preoperative patients, as evidence suggests this doesn't reduce postoperative infection risk 5.
• Remember that a heavy mixed growth may obscure a true pathogen, potentially requiring recollection 1.