Nitrite vs Leukocyte Esterase for UTI Detection
Nitrite is more indicative of a urinary tract infection due to its high specificity (98%), though leukocyte esterase offers better sensitivity for screening purposes. 1
Diagnostic Performance Comparison
According to the American Academy of Pediatrics guidelines, the diagnostic performance of these tests differs significantly:
| Test | Sensitivity | Specificity |
|---|---|---|
| Leukocyte esterase | 83% (67-94%) | 78% (64-92%) |
| Nitrite | 53% (15-82%) | 98% (90-100%) |
| Both tests positive | Combined higher sensitivity | Combined higher specificity |
Clinical Interpretation
Nitrite Test
- Highly specific (98%) - very few false positives 1
- Indicates the presence of bacteria that convert dietary nitrates to nitrites
- Requires approximately 4 hours of bacterial exposure in the bladder 1
- Limited sensitivity (53%) due to:
- Frequent bladder emptying (especially in children)
- Not all uropathogens reduce nitrate to nitrite 1
Leukocyte Esterase Test
- Better sensitivity (83%) but lower specificity (78%) 1
- Detects white blood cell enzyme activity (pyuria)
- Helps distinguish true UTI from asymptomatic bacteriuria 1
- Sensitivity increases with higher WBC counts 2
Clinical Application Algorithm
Positive nitrite test: Highly suggestive of UTI (high specificity) - consider initiating treatment while awaiting culture results 1
Positive leukocyte esterase only: Suggests inflammation but less specific for UTI - requires clinical correlation 1
Both tests positive: Highest predictive value for UTI (specificity of 96%) 1
Both tests negative: Good negative predictive value (95%) but doesn't completely exclude UTI 2
Important Considerations
Nitrite test performance varies by patient population:
Leukocyte esterase may be absent in:
- UTIs caused by certain organisms (particularly Klebsiella and Enterococcus species) 4
- Early infections before significant pyuria develops
Spectrum bias affects test performance - sensitivity varies based on pre-test probability:
- Higher sensitivity (92%) in patients with typical UTI symptoms
- Lower sensitivity (56%) in patients with atypical presentation 3
Pitfalls to Avoid
- Relying solely on dipstick testing without culture confirmation 1
- Dismissing possible UTI when nitrite is negative but clinical suspicion is high 1
- Failing to consider that up to 20% of patients with culture-proven UTI may have negative urinalysis 1
- Overlooking that certain pathogens (particularly Klebsiella and Enterococcus) may cause UTI without significant pyuria 4
In summary, while nitrite has superior specificity making a positive result more reliable for UTI diagnosis, the combination of both tests provides the best diagnostic performance. Neither test alone is sufficient to rule out UTI when clinical suspicion is high, and urine culture remains the gold standard for diagnosis.