What does it mean if a urine sample is possibly contaminated?

What Does "Possibly Contaminated" Urine Mean?

A "possibly contaminated" urine sample means that bacteria or other organisms detected in the specimen likely originated from skin, genital, or perineal flora during collection rather than representing a true urinary tract infection, making the results unreliable for clinical decision-making. 1

Understanding Contamination

Contamination occurs when organisms from outside the urinary tract enter the specimen during collection. The most common sources include:

• Periurethral, vaginal, or perineal skin flora that contaminate the sample as urine passes through these areas 1
• Multiple bacterial species (mixed flora) growing in culture, which strongly suggests contamination rather than infection 2, 1
• Presence of epithelial cells alongside bacteria, which indicates skin cells were shed into the specimen during collection 1

How Laboratories Define Contamination

Contamination is typically identified by:

• Mixed growth of 2 or more organisms at any concentration 2
• Growth of organisms below 10^5 CFU/mL (100,000 colony-forming units per milliliter) that are not typical uropathogens 2
• Coagulase-negative staphylococci or other skin flora at low colony counts (<1,000 CFU/mL), which are not clinically relevant urine isolates 3
• Elevated epithelial cell counts (>few per high-powered field), indicating skin contamination 4

Why Contamination Rates Are High

Contamination is extremely common regardless of collection technique, with rates varying significantly by method:

• Clean-catch midstream collection: 14-32% contamination rate 2
• Sterile urine bag collection: 44-68% contamination rate 2
• Diaper collection: 29-61% contamination rate 2
• Catheterization: 12-15% contamination rate 2
• Suprapubic aspiration: 1-9% contamination rate 2

Even with proper midstream clean-catch technique, contamination occurs in approximately 30% of specimens from women, and studies show that cleansing and midstream collection do not significantly reduce contamination compared to simply voiding into a cup 5, 4. This is a critical pitfall—the traditional teaching about clean-catch technique may not be as effective as previously believed.

Clinical Implications

A contaminated specimen cannot be used to diagnose or exclude urinary tract infection. Here's what this means for patient care:

• 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, the American Academy of Pediatrics recommends confirming 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

When to Suspect True Infection Despite Contamination Concerns

Correlate culture results with urinalysis findings:

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

Practical Management Algorithm

If your patient has a "possibly contaminated" urine result:

1. Assess clinical symptoms: Dysuria, frequency, urgency, fever, flank pain 1
2. Review the collection method: Bag specimens have only 15% positive predictive value; catheterized specimens are far more reliable 1
3. Check urinalysis for pyuria and nitrites: Their presence increases likelihood of true infection 1
4. If symptomatic with high clinical suspicion: Recollect by catheterization (not another clean-catch) 1
5. If asymptomatic: No treatment or repeat testing needed 3

Common Pitfalls to Avoid

• Never treat mixed flora from bag specimens without confirmation by a more reliable collection method 1
• Don't assume clean-catch technique prevents contamination—studies show 30-50% contamination rates even with "ideal" technique 5, 4, 6
• Don't ignore collection method when interpreting colony counts—this leads to misdiagnosis 1
• Don't dismiss ongoing symptoms just because culture shows contamination—consider recollection with better technique 1

The bottom line: A contaminated urine specimen provides no useful diagnostic information and should prompt recollection using a more reliable method if clinical suspicion for UTI persists. 1