How do you determine if a Urine Analysis (UA) is contaminated?

How to Determine if a Urine Analysis is Contaminated

A urine culture is contaminated when it shows mixed growth of 2 or more organisms at any concentration, or when skin flora (like coagulase-negative staphylococci) grow at low colony counts (<1,000 CFU/mL), making the results unreliable for clinical decision-making. 1

Laboratory Indicators of Contamination

Culture-Based Criteria

• Mixed bacterial growth (≥2 organisms) at any concentration is the primary indicator of contamination 1
• Low colony counts of non-typical uropathogens (<10^5 CFU/mL) suggest contamination rather than infection 1
• Skin flora organisms (coagulase-negative staphylococci, diphtheroids) at <1,000 CFU/mL are not clinically relevant and indicate contamination 1

Microscopic Indicators

• Presence of epithelial cells alongside bacteria indicates skin cells were shed into the specimen during collection, suggesting contamination 1
• High epithelial cell counts (>few per high-power field) strongly correlate with contaminated specimens 2

Collection Method Predicts Contamination Risk

Understanding which collection method was used helps you anticipate contamination likelihood:

Highest Contamination Risk

• Sterile urine bag collection: 44-68% contamination rate 1
• Diaper collection: 29-61% contamination rate 1
• Clean-catch urine (pediatrics): 26% contamination rate 3

Moderate Contamination Risk

• Clean-catch midstream (adults): 14-32% contamination rate 1
• Midstream without cleansing: Similar rates to clean-catch 3

Lowest Contamination Risk

• Catheterization: 12-15% contamination rate 3, 1
• Suprapubic aspiration: 1-9% contamination rate 3, 1

Clinical Context Clues

When Contamination is Likely Despite Positive Culture

• Absence of pyuria (<10 WBCs/mm³) with bacteriuria suggests contamination or asymptomatic bacteriuria rather than active infection 1
• Negative leukocyte esterase AND negative nitrites with positive culture suggests contamination 1
• Discordance between urinalysis and culture: If the urinalysis shows no inflammatory markers but culture is positive, suspect contamination 2

When True Infection is Likely Despite Mixed Flora

• Pyuria (≥10 WBCs/mm³) with bacteriuria increases likelihood of true infection even when some contamination is present 1
• Positive leukocyte esterase (84% sensitivity) or positive nitrites (99% specificity) suggest infection despite mixed flora 1

Specimen Handling Issues That Cause False Contamination

Time-Related Problems

• Specimens held >2 hours at room temperature show bacterial overgrowth in 32% of cases, causing false-positive results 4
• Specimens held >1 hour without refrigeration may show bacterial overgrowth leading to false contamination diagnosis 4
• Proper refrigeration (4°C) within 1 hour preserves specimen integrity for up to 24 hours 4

Common Pitfall

Even with "ideal" clean-catch technique in disease-free women, urinalysis indices are often abnormal: leukocyte esterase positive in 35%, WBCs >5/HPF in 27.5%, bacteria present in 62.5%, and culture contamination in 63% 2. This means contamination is common even with perfect technique, so clinical context is essential.

What to Do With a Contaminated Specimen

• Do not treat based on contaminated cultures as this leads to inappropriate antibiotic use 1
• Repeat collection using a more reliable method (catheterization or suprapubic aspiration) if clinical suspicion for UTI remains high 1
• For febrile infants <24 months, confirm any positive culture showing mixed flora by catheterization or suprapubic aspiration before initiating treatment 1
• Heavy mixed growth may mask a true pathogen, potentially requiring recollection 1

Key Takeaway

Only nitrites show high specificity for true infection 2. When you see mixed growth, high epithelial cells, or discordance between urinalysis and culture, the specimen is contaminated and cannot be used to diagnose or exclude UTI 1.