Are epithelial cells observed in a urine analysis likely to represent contamination?

Epithelial Cells in Urinalysis: Contamination Indicator

A urine sample with a significant number of epithelial cells (particularly squamous epithelial cells) suggests contamination, but this is NOT a reliable predictor of urine culture contamination or mixed growth. The presence of epithelial cells should prompt consideration of specimen quality, but clinical decisions should not rely solely on this finding.

Key Evidence-Based Findings

Guideline Recommendations

The most direct guideline statement comes from pediatric emergency medicine: A urine sample with more than 10 WBCs and a significant number of epithelial cells must be considered contaminated, requiring either an improved clean-catch method or catheterization 1. This guideline specifically requires both findings together, not epithelial cells alone.

Research Evidence Contradicts Traditional Teaching

Multiple high-quality studies demonstrate that epithelial cells are poor predictors of contamination:

• Squamous epithelial cells (SECs) were found in 94% of catheterized samples (which had 0% contamination) and 96% of clean-catch samples (only 21% contaminated) 2. This 1998 study definitively showed that SEC presence does not indicate contamination.

• ROC curve analysis showed SECs are a poor predictor of urine culture contamination (AUC = 0.680) 3. The study concluded that SECs cannot accurately predict contamination but may indicate reduced performance of other urinalysis markers.

• More recent 2020 data confirmed that SEC presence reduces diagnostic accuracy of urinalysis markers (AUCs ranging 0.557-0.796), with pyuria, bacteriuria, and leukocyte esterase showing reduced accuracy in contaminated samples 4.

Clinical Algorithm for Interpretation

When Epithelial Cells Are Present:

1. Do NOT automatically reject the specimen based on epithelial cells alone

2. Assess the clinical context:

   • Collection method (clean-catch vs. catheterized vs. bag collection)
   • Patient symptoms
   • Other urinalysis findings (WBCs, bacteria, nitrites)

3. Consider contamination MORE likely if:

   • High epithelial cell count (>21.35/µL in women, >5.05/µL in men per recent 2025 data) 5
   • AND elevated bacteria counts (>193.65/µL in women, >23.55/µL in men) 5
   • AND >10 WBCs present 1

4. Recollection is warranted when:

   • Multiple indicators suggest contamination (epithelial cells + high bacteria + WBCs)
   • Clinical suspicion for UTI remains high despite negative/equivocal results
   • Mixed growth on culture occurs

Important Caveats

Collection Method Matters More Than Epithelial Cells

• Bag collection has 12-83% false-positive rates 1
• Clean-catch contamination rates: 0-29% 1
• Catheterization: 95% sensitivity, 99% specificity 1

Sex Differences Are Significant

Women naturally shed more squamous epithelial cells from the genital tract during voiding. Different thresholds apply: women (>21.35 SEC/µL) vs. men (>5.05 SEC/µL) for predicting contamination 5.

The Real Impact of Epithelial Cells

Rather than predicting contamination directly, epithelial cells reduce the predictive performance of traditional urinalysis measures 3. When SECs are present:

• Positive likelihood ratio for bacteriuria drops from 4.98 to 2.35
• Sensitivity/specificity patterns change significantly

Bottom Line for Clinical Practice

Do not reflexively reject urine specimens based solely on epithelial cell presence. Instead, integrate epithelial cell findings with bacteria counts, WBC counts, clinical symptoms, and collection method. When multiple contamination indicators are present together, consider recollection using a more sterile technique (catheterization if clinically appropriate). The 2016 systematic review emphasizes that poorly collected specimens produce incorrect culture results and misdiagnosis 6, but epithelial cells alone are insufficient to make this determination.