What is the threshold for squamous epithelial cells per high-power field that determines a clean catch in a urinalysis?

Threshold for Squamous Epithelial Cells in Clean Catch Urinalysis

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 Specimen Contamination

Squamous epithelial cells (SECs) in urine samples are often used as markers of potential contamination, though their presence alone does not automatically invalidate results. The relationship between SECs and specimen quality requires proper understanding:

Significance of Squamous Epithelial Cell Count

• According to clinical guidelines, a urine sample with more than 10 WBCs and a significant number of epithelial cells should be considered contaminated 1
• When this threshold is exceeded, either an improved clean-catch method or catheterization should be attempted

Collection Methods and Contamination Rates

• Clean-catch midstream void specimens have contamination rates ranging from 0% to 29% 1
• Bag collection methods have much higher contamination rates (12% to 83%) 1
• Catheterization provides specimens with higher sensitivity (95%) and specificity (99%) 1

Evidence on SECs as Contamination Markers

Research findings on SECs as contamination indicators show mixed results:

• A 1998 study found that 96% of midstream clean-catch samples contained squamous cells, but only 21% showed bacterial contamination, suggesting SECs alone are not a good predictor of contamination 2

• A 2016 study demonstrated that SEC count was a poor predictor of urine culture contamination (area under ROC curve = 0.680) 3

• However, another study found that specimens with <10 SEC/mm³ had fewer isolates (0.9 per culture) compared to specimens with ≥10 SEC/mm³ (2 isolates per culture) 4

Improving Clean Catch Collection

To reduce contamination rates:

• Illustrated instructions for patients can reduce contamination rates from 40% to 25% 5
• Proper collection technique includes:
  • Hand washing
  • Cleaning with a towelette
  • Beginning to void, then stopping and collecting the midstream portion

Clinical Implications

While SECs may not perfectly predict contamination, they do affect the diagnostic performance of urinalysis:

• Urinalysis samples with fewer than 8 SECs/low-power field predict bacteriuria better (sensitivity=75%, specificity=84%) than samples with more than 8 SECs/lpf (sensitivity=86%, specificity=70%) 3

• The positive likelihood ratio for predicting bacteriuria decreases from 4.98 in the absence of SECs to 2.35 for samples with more than 8 SECs/lpf 3

Practical Approach to Urinalysis Interpretation

1. Consider a threshold of >10 SECs per high-power field as indicating potential contamination
2. For contaminated specimens, consider recollection with better technique or catheterization if clinically warranted
3. Remember that even properly collected specimens may contain some SECs
4. Interpret urinalysis results in conjunction with clinical presentation and other laboratory findings

Common Pitfalls

• Assuming all specimens with SECs are contaminated (most specimens contain some SECs)
• Relying solely on SEC count without considering other urinalysis parameters
• Failing to provide proper collection instructions to patients
• Not recognizing that a negative urine culture from a bag specimen can still be helpful to rule out UTI, while positive results require confirmation 1