Does Nitrite in Urine Indicate UTI?
A positive nitrite test strongly suggests UTI with excellent specificity (98%), but a negative nitrite test does NOT rule out UTI due to its limited sensitivity (53%). 1, 2
Understanding Nitrite Test Performance
The nitrite test detects bacterial conversion of urinary nitrates to nitrites, primarily by gram-negative enteric bacteria. Here's what the test characteristics mean in practice:
- High specificity (98%): When nitrite is positive, there is a very high likelihood of bacterial infection present 1, 2
- Low sensitivity (53%): Approximately half of true UTIs will have negative nitrite results 1, 2
- The combination of nitrite OR leukocyte esterase increases sensitivity to 93% while maintaining 96% specificity when both are positive 3, 1
Why Nitrite Can Be Falsely Negative in True UTIs
Several important clinical factors explain why nitrite fails to detect many real infections:
- Insufficient bladder dwell time: Bacterial conversion of nitrate to nitrite requires prolonged urine exposure to bacteria, which may not occur in young infants or patients who void frequently 3
- Non-nitrate-reducing organisms: Some uropathogens (including Pseudomonas aeruginosa, Enterococcus, and Staphylococcus saprophyticus) do not produce nitrite despite causing true infection 4
- Dietary factors: Lack of dietary nitrate intake prevents nitrite formation even when bacteria are present 5
- Dilute urine: High fluid intake can dilute nitrite below detectable levels 5
Clinical Decision-Making Algorithm
When nitrite is POSITIVE:
- Strongly consider empiric antibiotic treatment in symptomatic patients, as the false positive rate is less than 4% 3, 1
- Obtain urine culture to guide definitive therapy and antimicrobial susceptibility 1, 2
When nitrite is NEGATIVE:
- Do NOT rule out UTI - proceed with clinical assessment 1, 2
- Check leukocyte esterase: if positive (≥10 WBCs/HPF) with UTI symptoms (dysuria, frequency, urgency, fever, gross hematuria), obtain urine culture 2, 6
- In children <2 years with fever, urine culture is mandatory even with negative urinalysis, as 10-50% of culture-proven UTIs have false-negative urinalysis 3
Definitive Diagnosis Requirements
According to the American Academy of Pediatrics, UTI diagnosis requires BOTH:
- Urinalysis suggesting infection (pyuria and/or bacteriuria) 2
- Positive urine culture with ≥50,000 CFU/mL of a uropathogen 2
Critical Pitfalls to Avoid
- Never rule out UTI based solely on negative nitrite - this would miss approximately half of true infections 1, 2
- Do not treat asymptomatic bacteriuria - positive culture without symptoms does not warrant antibiotics except in pregnancy or before urologic procedures 1, 6
- Ensure proper specimen collection - contaminated specimens (especially bag specimens in children) have high false-positive rates; use catheterization or suprapubic aspiration for definitive diagnosis when needed 2, 6
- Do not adjust antibiotic selection based on nitrite presence - research shows nitrite-positive versus nitrite-negative groups have similar resistance patterns for most antibiotics 7, 8
Special Population Considerations
- Elderly patients: Do not treat based on positive nitrite alone without acute UTI symptoms (fever, dysuria, new incontinence); asymptomatic bacteriuria prevalence is 15-50% in long-term care facilities 1, 6
- Patients with indwelling catheters: Positive nitrite represents colonization unless accompanied by systemic symptoms warranting treatment 1, 6
- Young infants: Nitrite sensitivity is particularly poor due to frequent voiding; always obtain culture in febrile infants <2 years regardless of urinalysis results 3