What is the significance of epithelial cells in the urine?

Significance of Epithelial Cells in Urine

The presence of epithelial cells in urine primarily serves as a marker of specimen contamination rather than disease, with squamous epithelial cells indicating contamination from the distal urethra or genitourinary tract, while renal tubular epithelial cells (RTECs) may indicate upper urinary tract pathology. 1, 2

Origin and Types of Epithelial Cells

Squamous Epithelial Cells

• Originate from the distal urethra, bladder trigone, or cervicovaginal contamination during collection 1, 2
• The distal third of the urethra is dominated by squamous epithelium 2
• These cells are the most common type seen in routine urinalysis and typically indicate contamination rather than pathology 1

Transitional (Urothelial) Epithelial Cells

• Arise from the urinary tract lining (renal pelvis, ureters, bladder) 3
• When atypical, may indicate urothelial neoplasia and require cytologic evaluation using standardized reporting systems 4

Renal Tubular Epithelial Cells (RTECs)

• Contain nephron-specific proteins and indicate upper urinary tract pathology 3, 5
• Demonstrate acceptable diagnostic performance for upper urinary tract infections 5
• May help discriminate between upper and lower UTIs 5

Clinical Significance: Contamination vs. Pathology

Contamination Indicators

• A urine sample with >10 WBCs and significant squamous epithelial cells should be considered potentially contaminated 1, 2
• Squamous epithelial cells are a poor predictor of urine culture contamination (area under ROC curve = 0.680), despite widespread belief otherwise 6
• Specimens with ≥10 squamous epithelial cells/mm³ had twice as many bacterial isolates (2 isolates per culture) compared to specimens with <10 cells/mm³ (0.9 isolate per culture) 7

Impact on Urinalysis Performance

• The presence of >8 squamous epithelial cells per low-powered field significantly reduces the predictive value of urinalysis for bacteriuria 6
• Urinalysis with <8 SECs/lpf: sensitivity 75%, specificity 84% for bacteriuria 6
• Urinalysis with >8 SECs/lpf: sensitivity 86%, specificity 70% for bacteriuria (lower diagnostic odds ratio: 8.7 vs. 17.5) 6

Collection Method and Contamination Rates

The collection method dramatically affects contamination rates and specimen reliability:

• Catheterized specimens: 4.7% contamination rate with 95% sensitivity and 99% specificity 1, 2
• Clean-catch midstream: 27% contamination rate 1, 2
• Bag specimens: 65-68% contamination rate 1, 2
• Proper perineal cleansing reduces contamination from 23.9% to 7.8% 1, 2

Practical Clinical Recommendations

When to Recollect Specimens

When significant squamous epithelial cells are present alongside mixed bacterial flora, recollect using catheterization if clinical suspicion for UTI remains high 1, 2

Pediatric Considerations

• In children with fever and suspected UTI, confirm any positive culture showing mixed flora and squamous cells by catheterization or suprapubic aspiration before initiating treatment 1

Interpretation Algorithm

1. Assess collection method first - catheterized specimens are most reliable 1, 2
2. Quantify squamous epithelial cells - significant numbers (>10/mm³ or >8/lpf) suggest contamination 1, 6, 7
3. Evaluate in context - consider patient symptoms, presence of pyuria, and bacterial growth pattern 1, 2
4. If RTECs are present - consider upper urinary tract pathology including pyelonephritis 5

Special Considerations for Cytology

Atypical Urothelial Cells

• The International Consultation on Urologic Disease (ICUD) recommends standardized reporting using terminology similar to the Bethesda System 4
• Atypical urothelial cells should be substratified into two classes with different follow-up implications 4:
  • Atypical urothelial cells of undetermined significance: follow with repeat urine cytology 4
  • Atypical urothelial cells, cannot rule out high-grade carcinoma or favor neoplasm: requires endoscopic evaluation 4

Common Pitfalls to Avoid

• Do not assume squamous epithelial cells reliably predict culture contamination - they are poor predictors despite common practice 6
• Do not dismiss specimens with squamous cells if clinical suspicion is high - recollect with better technique rather than ignoring potential infection 1, 2
• Do not overlook RTECs - unlike squamous cells, these indicate actual upper tract pathology 5
• Do not interpret urinalysis results in isolation - always consider collection method, cell counts, and clinical context together 1, 2