Squamous Epithelial Cells in Clean Catch Urine Specimens
More than 10 squamous epithelial cells per high-power field in a urinalysis sample indicates contamination and suggests the need for recollection of a clean catch specimen. 1
Understanding Urine Contamination
Squamous epithelial cells (SECs) in urine samples are commonly used as markers of specimen contamination. Their presence in significant numbers suggests that the sample may contain contaminants from the external genitalia, potentially affecting the reliability of the urinalysis results.
Key Thresholds:
- >10 SECs per high-power field (HPF): Widely accepted threshold indicating contamination 1
- >8 SECs per low-power field: Associated with reduced diagnostic performance of urinalysis for detecting bacteriuria 2
Evidence on Contamination Assessment
The relationship between SECs and urine contamination is complex:
- A 2016 study found that SEC count was actually a poor predictor of urine culture contamination (area under ROC curve = 0.680) 2
- However, samples with fewer than 8 SECs/LPF predicted bacteriuria better (sensitivity=75%, specificity=84%) than samples with more than 8 SECs/LPF (sensitivity=86%, specificity=70%) 2
- A 1998 study showed that squamous cells were present in 94% of catheterized samples (which had no bacterial contamination) and 96% of midstream samples (of which only 21% had bacterial contamination) 3
Impact on Clinical Decision-Making
When evaluating urinalysis results:
- Samples with >10 SECs/HPF should be considered potentially contaminated 1
- The presence of both >10 WBCs and significant epithelial cells strongly suggests contamination 1
- Urinalysis indices are often abnormal in disease-free women even with ideal collection technique, with only nitrite showing high specificity 4
Collection Techniques and Contamination Rates
Collection method significantly impacts contamination rates:
- Clean-catch midstream void specimens: 0-29% contamination rate 1
- Bag collection methods: 12-83% contamination rate 1
- Catheterization: Provides highest sensitivity (95%) and specificity (99%) 1
Improving Collection Quality
Illustrated instructions have been shown to reduce contamination rates from 40% to 25% by improving proper collection technique 5. Key elements of proper technique include:
- Hand washing
- Cleaning with a towelette
- Beginning to void, then stopping and collecting the midstream portion 1
Clinical Implications
A 2005 study highlighted that nephrologist-performed urinalysis is superior to laboratory-performed urinalysis in determining correct diagnoses, with nephrologists more likely to recognize renal tubular epithelial cells, granular casts, and dysmorphic red blood cells 6. Laboratory technicians may incorrectly identify renal tubular epithelial cells as squamous epithelial cells.
Bottom Line
When evaluating urine specimens, consider samples with more than 10 squamous epithelial cells per high-power field as potentially contaminated and consider recollection with proper clean catch technique. However, recognize that the presence of squamous cells alone is not a perfect predictor of bacterial contamination, and clinical context remains important in interpretation.