What causes a positive nitrite test in urinalysis?

Causes of Positive Nitrites in Urinalysis

Positive nitrites in urinalysis are primarily caused by gram-negative bacteria that convert dietary nitrates to nitrites through the action of the enzyme nitrate reductase. 1, 2

Bacterial Causes

• Gram-negative enteric bacteria are the main cause of positive nitrite tests, as they possess the nitrate reductase enzyme necessary for converting urinary nitrates to nitrites 1
• Common gram-negative bacteria causing positive nitrites include Escherichia coli, Klebsiella, Proteus, Pseudomonas, and other Enterobacteriaceae 2, 3
• Gram-positive bacteria generally do not produce positive nitrite tests, as many lack nitrate reductase activity 3, 4
• Enterococcus species specifically cannot reduce nitrates and will typically yield negative nitrite results despite causing true UTIs 4

Physiological Requirements

• The conversion of dietary nitrates to nitrites requires approximately 4 hours of bacterial exposure to urine in the bladder 1
• Sufficient bladder dwell time is necessary for the nitrite test to become positive, making it less reliable in:
  • Infants and young children who empty their bladders frequently 1, 5
  • Patients with urinary frequency symptoms 6
  • Patients with dilute urine from high fluid intake 2

Diagnostic Performance

• Nitrite testing has high specificity (98%, range 90-100%) but limited sensitivity (53%, range 15-82%) for detecting UTIs 1, 2
• A positive nitrite test is highly reliable for confirming bacteriuria (positive predictive value of 90-96%) 7, 3
• The high specificity means there are very few false-positive results - when nitrites are positive, a UTI is very likely present 1, 2
• The low sensitivity means negative nitrite tests cannot rule out UTI - many true infections will have negative nitrite results 1

Factors Affecting Nitrite Test Results

• False-negative nitrite results may occur due to:

  • Insufficient bladder incubation time (less than 4 hours) 1, 6
  • Infection with non-nitrate-reducing organisms (e.g., Enterococcus, Staphylococcus) 3, 4
  • Low dietary nitrate intake 2
  • Vitamin C consumption (ascorbic acid can interfere with the test) 5
  • High urinary glucose or protein levels 5

• False-positive nitrite results are uncommon but may occur with:

  • Contaminated specimens 5
  • Certain medications that discolor urine 5
  • Improperly stored urine specimens (bacterial overgrowth in specimen) 1

Clinical Implications

• A positive nitrite test strongly suggests the presence of bacteriuria and should prompt urine culture 2, 8
• When combined with leukocyte esterase testing, diagnostic accuracy improves significantly:
  • Positive nitrite AND positive leukocyte esterase: 96% specificity for UTI 5
  • Positive nitrite OR positive leukocyte esterase: 93% sensitivity for UTI 2
• First morning urine specimens may improve nitrite test sensitivity by allowing longer bladder incubation time 6
• Urine specimens must be fresh (processed within 1 hour at room temperature or 4 hours if refrigerated) to ensure accurate nitrite results 1

Important Considerations

• Nitrite testing alone is insufficient for diagnosing UTI, especially in children under 2 years 1
• The combination of urinalysis findings (nitrites, leukocyte esterase, microscopy) with clinical symptoms provides the most accurate assessment 2, 5
• A positive nitrite test in asymptomatic patients may represent asymptomatic bacteriuria rather than true infection requiring treatment 1, 5
• The American Academy of Pediatrics recommends using both urinalysis and urine culture for definitive diagnosis of UTI in young children 1, 2