What is the cause of nitrites in urine?

Causes of Nitrites in Urine

The presence of nitrites in urine is primarily caused by bacterial infection, specifically gram-negative bacteria that convert dietary nitrates to nitrites during their metabolism in the urinary tract. 1

Mechanism of Nitrite Formation

• Nitrites appear in urine when bacteria containing the enzyme nitrate reductase convert dietary nitrates to nitrites
• This conversion process requires approximately 4 hours of bacterial exposure to urinary nitrates 2
• Most gram-negative enteric bacteria (especially Enterobacterales) can perform this conversion 2, 3

Bacterial Species Associated with Positive Nitrite Tests

• Escherichia coli - most common cause (74.9% of UTI cases) 4
• Other common bacteria that produce nitrites:
  • Klebsiella species (7.9%)
  • Proteus species (5.6%)
  • Other gram-negative bacteria 4

Bacteria That Do NOT Produce Nitrites

• Enterococcus species (gram-positive) lack nitrate reductase enzyme 5
• Some strains of gram-negative bacteria may not produce detectable nitrites
• Only 62.5% of gram-negative organisms and 17.6% of gram-positive organisms yield positive nitrite results 6

Diagnostic Value of Nitrite Testing

• High specificity (98%, range 90-100%) but limited sensitivity (53%, range 15-82%) 2, 1
• False negative results are common due to:
  • Insufficient bladder incubation time (<4 hours) - particularly in children and infants who void frequently 2
  • Diluted urine specimens
  • Low dietary nitrate intake 3
  • Infection with non-nitrate-reducing organisms
  • Presence of ascorbic acid or other interfering substances 3
  • Low bacterial counts

Clinical Implications

• A positive nitrite test is highly specific for bacteriuria (few false positives) 2
• Negative nitrite results have little value in ruling out UTI due to poor sensitivity 2
• Nitrite testing should be combined with leukocyte esterase testing for improved diagnostic accuracy:
  • Combined sensitivity: 93-99.8%
  • Combined specificity: 70-72% 2, 1

Special Considerations in Different Populations

• Children/infants: Nitrite test has lower sensitivity due to frequent voiding patterns 2
• Elderly patients: Specificity of urine dipstick tests ranges from 20-70% 2
• Elderly patients with negative nitrite AND negative leukocyte esterase results are unlikely to have UTI 2

Important Caveats

• Asymptomatic bacteriuria can cause positive nitrite tests without clinical infection, especially in:
  • School-aged and older girls
  • Elderly patients (10-50% prevalence) 1
  • Catheterized patients
• Nitrite results should not guide antimicrobial selection, as studies show no consistent correlation between nitrite status and antibiotic resistance patterns 6, 5, 4
• Urine culture remains the gold standard for UTI diagnosis and should be performed when infection is suspected 1, 7

In summary, nitrites in urine primarily indicate the presence of nitrate-reducing bacteria, most commonly gram-negative enteric bacteria like E. coli. While a positive nitrite test strongly suggests bacteriuria, a negative test does not rule out UTI, and clinical symptoms along with other urinalysis findings should guide diagnosis and management.