Which is more specific for diagnosing Urinary Tract Infections (UTIs), nitrate or leukocyte esterase?

Nitrite Testing is More Specific for UTI Than Leukocyte Esterase

Nitrite testing has significantly higher specificity (98%) compared to leukocyte esterase (78%) for diagnosing urinary tract infections, making it a more reliable positive indicator of UTI when present. 1

Diagnostic Accuracy Comparison

The American Academy of Pediatrics and Infectious Diseases Society of America report the following diagnostic accuracy metrics:

Test	Sensitivity	Specificity
Nitrite	53% (15-82%)	98% (90-100%)
Leukocyte esterase	83% (67-94%)	78% (64-92%)
Combined (either positive)	93% (90-100%)	72% (58-91%)

This data clearly demonstrates that while nitrite has lower sensitivity, it has superior specificity for UTI diagnosis 1. The 2003 clinical policy for children with fever similarly confirms that nitrite testing has high specificity (true-positive results) but lower sensitivity, while leukocyte esterase has higher sensitivity but lower specificity 2.

Clinical Implications

When to Trust Each Test

• Positive nitrite: Highly suggestive of UTI due to 98% specificity 1
• Negative nitrite: Does not rule out UTI (poor sensitivity of 53%)
• Positive leukocyte esterase: Moderately suggestive of UTI (78% specificity)
• Negative leukocyte esterase: Good for ruling out UTI (83% sensitivity)

Combined Testing Approach

When both tests are positive, the specificity increases to 96%, meaning the false positive rate is less than 4% 2. This makes the combination particularly powerful for confirming UTI diagnosis.

Mechanism and Limitations

• Nitrite test: Detects nitrites formed by bacterial metabolism of urinary nitrates, particularly by gram-negative enteric bacteria 2

  • Limitation: Requires sufficient bladder time for bacteria to convert nitrates to nitrites
  • Limitation: May be falsely negative in frequent voiders (e.g., young infants)

• Leukocyte esterase: Detects white blood cells in urine indirectly 2

  • Limitation: Can be positive in inflammatory conditions without infection
  • Limitation: Lower specificity means more false positives

Optimal Diagnostic Strategy

1. Use both tests together for optimal diagnostic accuracy
2. A positive nitrite test should be given more weight when considering UTI diagnosis
3. If nitrite is positive, UTI is highly likely regardless of leukocyte esterase result
4. If only leukocyte esterase is positive, consider other clinical factors before diagnosing UTI
5. If both tests are negative, UTI is unlikely but not completely ruled out

Common Pitfalls to Avoid

• Don't rely solely on leukocyte esterase: Despite better sensitivity, its lower specificity means more false positives 1
• Don't rule out UTI based on negative nitrite alone: Its low sensitivity (53%) means many UTIs will be missed 1
• Don't ignore specimen collection method: Proper collection (clean-catch or catheterization) is essential for accurate results 1
• Don't forget timing: Process specimens within 1 hour at room temperature or 4 hours if refrigerated 1
• Don't overlook contamination signs: High squamous epithelial cell count (10-20/HPF) suggests contamination rather than infection 1

In conclusion, while leukocyte esterase is more sensitive, nitrite testing is significantly more specific for UTI diagnosis, making it a more reliable positive indicator when present.