Squamous Epithelial Cells in Urinalysis and Contamination
More than 2 squamous epithelial cells (SECs) per high-power field (HPF) in a urinalysis suggests contamination, as demonstrated by recent evidence showing that properly collected specimens should have fewer than 2 SECs/HPF. 1
Understanding Squamous Epithelial Cells in Urinalysis
Squamous epithelial cells in urine samples come from the distal urethra, vagina (in women), or skin during collection. Their presence has traditionally been used as a marker for potential contamination, though the exact threshold has been debated.
Evidence-Based Thresholds
Recent research provides specific guidance on SEC thresholds:
- A 2022 study comparing catheterized vs. voided specimens found that no catheterized sample had >2 SECs/HPF, establishing this as a reasonable threshold for properly collected specimens 1
- When this threshold (<2 SECs/HPF) was applied to referral urinalyses, the positive predictive value for true microscopic hematuria increased from 46.1% to 68.8% 1
- Fewer than 2 SECs with elevated RBCs was a significant predictor for true microscopic hematuria (p=0.003) 1
Clinical Implications
Impact on Urinalysis Interpretation
The presence of SECs affects the reliability of other urinalysis parameters:
- In samples with >8 SECs/low-powered field, the positive likelihood ratio for predicting bacteriuria falls to 2.35, compared to 4.98 in samples without SECs 2
- Urinalysis samples with <8 SECs/low-powered field predict bacteriuria better (sensitivity=75%, specificity=84%) than samples with higher SEC counts (sensitivity=86%, specificity=70%) 2
Limitations of SECs as Contamination Markers
Despite their use as contamination markers, SECs alone have limitations:
- SECs are a poor predictor of urine culture contamination (area under ROC curve = 0.680) 2
- In one study, 94% of catheterized samples (which by definition are not contaminated) contained SECs, suggesting SECs alone cannot reliably indicate contamination 3
- The presence of SECs in mid-stream clean-catch samples had only a 21% predictive value for bacterial contamination 3
Proper Specimen Collection
To minimize contamination and ensure accurate results:
- Obtain a freshly voided, clean-catch, midstream urine specimen 4
- Process specimens within 1 hour if kept at room temperature or within 4 hours if refrigerated 4
- Consider catheterization for women with >2 SECs/HPF on initial urinalysis when evaluating for microscopic hematuria 1
Common Pitfalls and Caveats
Overreliance on SECs alone: While >2 SECs/HPF suggests contamination, this should be considered alongside other urinalysis findings.
Ignoring specimen quality: Written instructions alone for clean-catch collection do not significantly reduce contamination rates 5, suggesting that proper technique demonstration may be necessary.
False positives in women: Abnormal urinalysis results are common in disease-free women regardless of collection technique, with epithelial cells >few seen in 30% of samples even with ideal collection 6.
Repeat testing considerations: For patients with suspected contaminated specimens (>2 SECs/HPF), consider catheterization rather than repeated clean-catch attempts, particularly when evaluating for microscopic hematuria 1.
By using the threshold of >2 SECs/HPF to identify potentially contaminated specimens, clinicians can make more informed decisions about whether to repeat urinalysis or proceed with further evaluation based on the current specimen.