What does the presence of nitrite in the urine indicate?

What Does Nitrite in the Urine Mean?

The presence of nitrite in urine indicates bacterial conversion of dietary nitrates to nitrites, which is highly specific (98-100%) for bacterial urinary tract infection, particularly with gram-negative organisms like E. coli, Proteus, and Klebsiella. 1

Biochemical Basis

• Nitrite is produced when bacteria in the bladder convert dietary nitrates to nitrites through bacterial metabolism, a process that requires approximately 4 hours of bladder dwell time 1
• Most gram-negative enteric bacteria (the most common uropathogens) possess the enzymes necessary for this nitrate-to-nitrite conversion 1
• Approximately 62.5% of gram-negative organisms and only 17.6% of gram-positive organisms produce positive nitrite results 2

Diagnostic Performance Characteristics

The nitrite test has excellent specificity but poor sensitivity for detecting UTI:

• Specificity: 98% (range 90-100%) - meaning very few false-positive results 1
• Sensitivity: 53% (range 15-82%) - meaning many true infections are missed 1
• When combined with positive leukocyte esterase, specificity increases to 96% with combined sensitivity of 93% 1, 3

Clinical Interpretation Algorithm

When nitrite is POSITIVE:

• This strongly suggests bacterial UTI and warrants treatment if the patient has accompanying urinary symptoms (dysuria, frequency, urgency, fever, or gross hematuria) 3, 4
• Obtain urine culture before initiating antibiotics to guide definitive therapy 3, 4
• Start empiric antibiotics immediately after culture collection in symptomatic patients 4
• First-line options include nitrofurantoin, trimethoprim-sulfamethoxazole (if local resistance <20%), or fosfomycin 3, 4

When nitrite is NEGATIVE:

• This does NOT rule out UTI - negative nitrite results have little value in excluding infection, particularly in infants and children who void frequently 1
• The test requires 4 hours of bladder dwell time, so frequent voiding prevents adequate nitrite accumulation 1, 5
• Not all urinary pathogens reduce nitrate to nitrite (particularly gram-positive organisms like Enterococcus) 1
• In febrile infants <2 years, urine culture is mandatory even with negative urinalysis, as 10-50% of culture-proven UTIs have false-negative urinalysis 3, 6

Critical Pitfalls to Avoid

Common causes of false-negative nitrite results:

• Insufficient bladder dwell time (<4 hours) - especially problematic in young infants who void frequently 1, 6, 5
• Infection with organisms that don't produce nitrite (gram-positive bacteria, Pseudomonas, Acinetobacter) 1
• Dilute urine from high fluid intake 5
• Ascorbic acid (vitamin C) interference 1

Important clinical considerations:

• The presence of nitrite does NOT predict antibiotic resistance patterns - do not adjust empiric therapy based solely on nitrite positivity 2, 7
• Asymptomatic bacteriuria with positive nitrite should NOT be treated in most populations (except pregnant women and patients undergoing urologic procedures with anticipated mucosal bleeding) 3, 6
• Always correlate nitrite results with clinical symptoms - never treat based on laboratory findings alone without specific urinary symptoms 3, 4

Special Population Considerations

Pediatric patients (especially <2 years):

• Nitrite sensitivity is particularly poor (as low as 15-25%) due to frequent voiding 1, 6
• Always obtain urine culture in febrile infants regardless of urinalysis results 3, 6
• Use catheterization or suprapubic aspiration for specimen collection to ensure accuracy 1, 3

Elderly and long-term care residents:

• High prevalence (15-50%) of asymptomatic bacteriuria with positive nitrite 3
• Only treat if acute onset of specific urinary symptoms (dysuria, fever >38.3°C, gross hematuria) or systemic signs (hemodynamic instability, rigors) are present 3, 6
• Do not screen for or treat asymptomatic bacteriuria even with positive nitrite 3, 6

Catheterized patients:

• Bacteriuria and positive nitrite are nearly universal in chronic catheterization 3
• Only treat if symptomatic with fever, hemodynamic instability, or specific urinary symptoms 4, 6
• Change catheter before collecting culture specimen if treatment is indicated 4, 6

Optimal Specimen Collection

• Use first morning urine specimen when possible, as overnight bladder dwell time (6-8 hours) maximizes nitrite detection sensitivity 5
• Process specimen within 1 hour at room temperature or 4 hours if refrigerated to maintain test accuracy 1
• Ensure proper collection technique (midstream clean-catch or catheterization) to avoid contamination 3