What constitutes a contaminated urine culture result?

What Constitutes a Contaminated Urine Culture Result

A urine culture is considered contaminated when it grows mixed flora (≥2 organisms), particularly when multiple commensal organisms are present at any concentration, or when ≥3 bacterial species are isolated. 1

Standard Contamination Definitions

The most widely accepted criteria for contamination include:

• Mixed growth of ≥2 organisms at concentrations ≥10,000 CFU/ml 1
• Growth of ≥3 bacterial species regardless of colony count 2
• Mixed commensal flora (skin, perineal, or vaginal organisms) at any concentration 3, 4
• Any growth <10^5 CFU/ml of multiple organisms in clean-catch specimens 1

Collection Method Matters Critically

Contamination rates vary dramatically by collection technique, which affects interpretation:

• Suprapubic aspiration: Essentially no contamination; any growth is significant 1, 5
• Catheterization: 4.7% contamination rate 5
• Midstream clean-catch (MSCC): 7.8-27% contamination rate 1, 5
• Sterile bag collection: 43.9-67.6% contamination rate 1, 3
• Diaper collection: 29-60.7% contamination rate 1

The median contamination rate across laboratories is 15%, but poorly performing laboratories can have rates exceeding 40%. 4

Clinical Significance of Contamination

Contaminated cultures should never be used to diagnose UTI or guide antimicrobial therapy. 3 Here's why:

• Even with colony counts >100,000 CFU/ml, mixed flora lacks diagnostic validity 3
• In properly collected specimens from febrile infants, bag specimens have a positive predictive value of only 15% due to contamination 1, 5
• Among positive bagged urine results, 85% are false positives 1

Important Exceptions: When Mixed Flora May Be Real

While mixed flora usually indicates contamination, there are specific clinical scenarios where polymicrobial bacteriuria represents true infection 6:

• Long-term catheterization: Polymicrobial bacteriuria is the norm and clinically significant 6
• Urosepsis: When the same combination of organisms grows from both blood and urine 6
• Reproducible mixed growth: Sequential cultures showing identical organism combinations 6

Factors That Increase Contamination Risk

Patient and collection factors associated with higher contamination rates include:

• Female sex: 15.89-fold increased odds of contamination 2
• Pregnancy: 14.34-fold increased odds 2
• Obesity: 1.93-fold increased odds 2
• Lack of perineal cleansing: 23.9% vs 7.8% contamination with cleansing 1, 5
• Delayed processing: Specimens held >1 hour at room temperature or >4 hours refrigerated 3

Common Pitfalls to Avoid

Do not rely on squamous cells to predict contamination. Despite conventional teaching, squamous cells are present in 94-96% of both contaminated and non-contaminated specimens from women, with only 21% predictive value for contamination. 7

Never treat asymptomatic patients based on contaminated cultures. 3 The only exceptions requiring treatment of asymptomatic bacteriuria are pregnant women, patients before urologic procedures with anticipated mucosal bleeding, and patients before transurethral prostate resection. 3

Bag-collected specimens in children should never be used to confirm UTI due to contamination rates of 60-67%. 3 A positive bag specimen requires confirmation by catheterization or suprapubic aspiration before treatment. 1, 5, 3

What To Do With a Contaminated Culture

If the patient is symptomatic and the culture shows contamination:

• Obtain a new specimen using proper collection technique 3
• For women: Use urethral catheterization (≥50,000 CFU/ml of single organism = significant) 3
• For men: Clean-catch midstream is acceptable (≥100,000 CFU/ml of single organism = significant) 3
• Suprapubic aspiration is most reliable but rarely necessary (≥1,000 CFU/ml = significant) 3

If the patient is asymptomatic: No further testing or treatment is indicated in most populations. 3

Reducing Contamination Rates

Evidence-based interventions to minimize contamination include:

• Refrigerate specimens immediately: This has the most substantial effect on reducing contamination 4
• Provide written patient instructions: Particularly effective in emergency room settings for both male and female patients 4
• Use catheterization in high-risk populations: Especially in infants, young children, and obese or pregnant women 1, 5, 2
• Process specimens promptly: Culture within 2 hours or refrigerate 1, 3