Does a positive nitrite test indicate a urinary tract infection (UTI)?

Positive Nitrite Test as an Indicator of Urinary Tract Infection

A positive nitrite test strongly indicates a urinary tract infection (UTI) due to its high specificity (98%), though it cannot replace urine culture for definitive diagnosis. 1

Understanding the Nitrite Test

Mechanism and Performance

• The nitrite test detects the conversion of dietary nitrates to nitrites by bacteria in urine
• This conversion requires approximately 4 hours of bacterial exposure to urine 1
• Performance characteristics:
  • Specificity: 98% (90-100%) - very few false positives 1
  • Sensitivity: 53% (15-82%) - many false negatives 1

Clinical Interpretation

When interpreting a nitrite test:

• Positive result: Highly specific for UTI (98% specificity) 1

  • Indicates presence of nitrate-reducing bacteria (primarily gram-negative enteric bacteria)
  • Strongly supports UTI diagnosis when positive
  • Warrants empiric antibiotic treatment while awaiting culture results

• Negative result: Does not rule out UTI due to low sensitivity 1

  • Approximately 47% of UTIs will have negative nitrite tests
  • Cannot be used alone to exclude infection

Factors Affecting Nitrite Test Accuracy

Causes of False Negatives

• Frequent urination (insufficient bladder time for nitrate conversion) 1, 2
• Non-nitrate-reducing pathogens (e.g., Enterococcus, Staphylococcus) 3
• Low dietary nitrate intake
• Dilute urine
• Vitamin C consumption

Patient Populations with Lower Sensitivity

• Young children, especially infants who void frequently 1
• Symptomatic patients (52% sensitivity) versus asymptomatic patients (83% sensitivity) 2

Diagnostic Algorithm for Suspected UTI

1. Initial screening: Perform urinalysis including nitrite and leukocyte esterase tests

   • Positive nitrite: High probability of UTI (specificity 98%) 1
   • Negative nitrite + positive leukocyte esterase: Still high probability of UTI (PPV 79%) 4
   • Both negative: Cannot rule out UTI (approximately 50% still culture positive) 4

2. Confirmatory testing: Obtain urine culture regardless of urinalysis results

   • For children <2 years: Culture is mandatory even with negative urinalysis 1
   • For all patients: Culture confirms diagnosis and guides definitive therapy 1

3. Specimen collection (in order of decreasing contamination risk):

   • Suprapubic aspiration (1% contamination) 1
   • Catheterization (12% contamination) 1
   • Clean catch (26% contamination) 1
   • Bag collection (highest contamination rate) 1

Clinical Implications

Treatment Decisions

• Do not adjust antibiotic choice based solely on nitrite results 5
  • Even with negative nitrite tests, 95.6% of UTIs are still caused by gram-negative organisms 3
  • Only 3.2% of nitrite-negative UTIs are caused by Enterococcus 3

Diagnostic Value

• Combined testing improves diagnostic accuracy:
  • Leukocyte esterase OR nitrite positive: Sensitivity 93%, Specificity 72% 1
  • Leukocyte esterase AND nitrite positive: Higher specificity (>90%) 1

Common Pitfalls to Avoid

1. Relying solely on nitrite test to rule out UTI (misses up to 47% of infections)
2. Failing to obtain urine culture when initiating antibiotics for suspected UTI
3. Over-interpreting negative results in children who void frequently
4. Using bag specimens for culture (high false positive rate of 85%) 1
5. Testing urine that is not fresh (should be tested within 1 hour at room temperature or 4 hours if refrigerated) 1