What does a positive nitrate (NO3-) test result indicate?

Positive Nitrate Test Interpretation

A positive nitrate (NO3-) test on urinalysis strongly indicates the presence of gram-negative bacteria in the urinary tract, with a specificity of approximately 98%, and is highly suggestive of a urinary tract infection (UTI). 1

Diagnostic Significance

• Nitrates in urine are produced when bacteria with nitrate reductase enzyme (primarily gram-negative bacteria) convert nitrate to nitrite in the urinary tract
• The nitrite test has:
  • High specificity: 98% (90-100%)
  • Lower sensitivity: 53% (15-82%) 1
  • This means a positive result is very reliable, but a negative result doesn't rule out infection

Clinical Context and Interpretation

• Positive nitrite combined with positive leukocyte esterase provides the highest diagnostic accuracy for UTI:

  • Combined sensitivity: 93% (90-100%)
  • Combined specificity: 72% (58-91%) 1

• A positive nitrite test in infants under 90 days with fever without source is an independent risk factor for invasive bacterial infections (odds ratio 2.7) 2

• In pediatric patients, the prevalence of invasive bacterial infection is significantly higher with positive nitrite tests:

  • Normal urine dipstick: 2.2% IBI prevalence
  • Positive nitrite test: 8.3% IBI prevalence
  • Both nitrite and leukocyte esterase positive: 10.6% IBI prevalence 2

Important Considerations

• False negatives can occur with:

  • Insufficient bladder dwell time (< 4 hours)
  • UTIs caused by organisms that don't reduce nitrate (e.g., Enterococcus, Staphylococcus)
  • Low dietary nitrate intake
  • Dilute urine

• False positives are uncommon but can occur with:

  • Contaminated specimens
  • Improperly stored urine samples

Clinical Application

• In children under 2 years, a urine culture should always be obtained regardless of dipstick results 1

• In adults, a positive nitrite test should prompt:

  1. Collection of urine culture before starting antibiotics
  2. Assessment for UTI symptoms (dysuria, frequency, urgency, suprapubic pain)
  3. Consideration of antibiotic therapy based on clinical presentation

• For febrile infants, a positive nitrite test warrants particular attention as it increases the risk of invasive bacterial infection 2

Common Pitfalls to Avoid

• Relying solely on nitrite test without considering leukocyte esterase results or clinical symptoms
• Dismissing a negative nitrite test when clinical suspicion for UTI is high
• Failing to obtain urine culture before starting antibiotics
• Treating asymptomatic bacteriuria in non-pregnant adults based solely on positive dipstick results

Remember that approximately 10% of children with UTIs may have negative diagnostic tests, including nitrite testing, so these tests "cannot replace urine culture" when clinical suspicion is high 3.