Nitrite is More Specific than Leukocyte Esterase for UTI Diagnosis
Nitrite testing has significantly higher specificity (98%) compared to leukocyte esterase (78%) for diagnosing urinary tract infections, making nitrite the more specific marker for UTI. 1
Diagnostic Accuracy Comparison
The diagnostic accuracy of both tests varies considerably:
| Test | Sensitivity | Specificity |
|---|---|---|
| Leukocyte esterase | 83% (67-94%) | 78% (64-92%) |
| Nitrite | 53% (15-82%) | 98% (90-100%) |
| Combined (either positive) | 93% (90-100%) | 72% (58-91%) |
This pattern is consistently reported across multiple studies:
- The Pediatric Research in Office Settings study found nitrites had better specificity (99%) compared to leukocyte esterase (91%) 2
- Leukocyte esterase demonstrates higher sensitivity (84%) compared to nitrites (39%) 2
- A 2004 study confirmed nitrite test was more specific (87%) than leukocyte esterase test (54%) 3
Clinical Implications
When to Use Each Test
Nitrite test: Best used when you need to confirm a UTI with high confidence (rule in)
- Positive nitrite strongly suggests bacteriuria due to its high specificity
- Particularly useful for organisms that reduce nitrates to nitrites (most Enterobacteriaceae)
- False positives are rare (specificity 98%)
Leukocyte esterase: Best used to screen for possible UTI (rule out)
- More sensitive but less specific than nitrite
- Detects pyuria, which may be present in conditions other than UTI
- Higher false positive rate (specificity 78%)
Combined Testing Approach
The combination of both tests provides the highest diagnostic utility:
- Combined sensitivity: 93% (using either test positive as criterion)
- Combined negative predictive value: 95% 3
Potential Pitfalls
False negative nitrite results may occur with:
- Non-nitrate-reducing organisms
- Insufficient bladder incubation time (< 4 hours)
- Dilute urine or vitamin C consumption
False positive leukocyte esterase results may occur with:
- Non-infectious causes of pyuria (interstitial cystitis, urethritis)
- Contamination from vaginal secretions
- Certain medications
Recommendations for Practice
- Use nitrite test when specificity is critical (confirming UTI diagnosis)
- Use leukocyte esterase when sensitivity is needed (screening)
- For optimal diagnostic accuracy, use both tests in combination
- Remember that a positive nitrite result is more reliable for confirming UTI than a positive leukocyte esterase result
- Neither test alone is sufficient for diagnosis - clinical symptoms and urine culture remain essential components of UTI diagnosis
Special Considerations
- In pediatric populations, catheterized specimens show higher specificity for both tests compared to bag specimens 2
- The American Academy of Pediatrics recommends laboratory confirmation with both tests in children 1
- In elderly patients, who may present with atypical symptoms, laboratory confirmation using both tests is particularly important 1