If a urinalysis shows 16–25 squamous epithelial cells, indicating contamination, is an elevated urine bilirubin result reliable?

Elevated Urine Bilirubin in Contaminated Specimens: Reliability Assessment

An elevated urine bilirubin result remains clinically unreliable when 16-25 squamous epithelial cells are present, as specimen contamination compromises the overall diagnostic accuracy of urinalysis parameters, though bilirubin itself is less directly affected by squamous cells than other markers.

Understanding the Impact of Contamination

The presence of 16-25 squamous epithelial cells (SECs) per low-powered field indicates significant specimen contamination from perineal, vaginal, or periurethral sources 1. However, the relationship between SECs and urine bilirubin reliability is nuanced:

Direct vs. Indirect Effects

• SECs primarily affect cellular and bacterial markers: Research demonstrates that squamous epithelial cells significantly reduce the diagnostic accuracy of urinalysis markers like pyuria, bacteriuria, and leukocyte esterase 2, 3. The presence of SECs above 8/lpf reduces the positive likelihood ratio for predicting bacteriuria from 4.98 to 2.35 2.

• Bilirubin is a chemical marker, not cellular: Unlike white blood cells or bacteria that can be introduced through contamination, bilirubin is a soluble chemical compound. The dipstick detects conjugated bilirubin in urine, which originates from the bloodstream and is filtered by the kidneys—not from external contamination sources.

The Real Problem with Urine Bilirubin Testing

The fundamental issue is not contamination-related but rather the inherently poor clinical utility of urine bilirubin testing itself 4. In a large retrospective study of 241,929 urine bilirubin tests:

• Only 0.3% yielded positive results
• Of "unexpected positives" (those without recent abnormal liver function tests), only 0.13% of all tests were clinically meaningful
• The test has a high false-positive rate regardless of specimen quality 4

Clinical Recommendation Algorithm

When faced with elevated urine bilirubin in a contaminated specimen:

1. Do not rely on the urine bilirubin result for clinical decision-making—not because of SEC contamination per se, but because urine bilirubin testing lacks clinical utility even in clean specimens 4

2. If hepatobiliary disease is suspected clinically:

   • Order serum liver function tests (AST, ALT, GGT, total and direct bilirubin) directly
   • Do not use urine bilirubin as a screening tool
   • 85% of unexpected positive urine bilirubin results that prompted subsequent LFT testing showed abnormalities, but this represents an inefficient diagnostic pathway 4

3. If other urinalysis parameters are needed:

   • Recollect the specimen using proper midstream clean-catch technique with cleansing 1
   • For men: midstream collection with cleansing is recommended
   • For women: midstream collection with cleansing is recommended
   • Avoid relying on any urinalysis parameters when SECs exceed 8/lpf 2

Critical Caveats

• Contamination affects specimen quality globally: While bilirubin may not be directly contaminated by squamous cells, a contaminated specimen suggests poor collection technique, raising questions about the entire sample's reliability 1

• Gender-specific thresholds matter: Recent data suggests SEC cutoffs of 21.35/µL for women and 5.05/µL for men better predict contamination 5, though your specimen shows contamination by any standard

• Hyperbilirubinuria can cause tubular injury: In patients with true hyperbilirubinemia, urine sediment may show increased granular casts and renal tubular epithelial cells 6, which could be obscured by contamination

The bottom line: Obtain serum bilirubin and liver function tests directly rather than relying on urine bilirubin from any specimen, contaminated or not 4.