Does a Urinalysis Need Nitrites to Indicate UTI?
No, a urinalysis does not require nitrites to indicate a urinary tract infection—nitrites have only 53% sensitivity, meaning nearly half of true UTIs will have negative nitrites, though when present they are highly specific (98%) for infection. 1
Understanding Nitrite Test Limitations
The nitrite test detects only gram-negative bacteria that convert dietary nitrates to nitrites, and this conversion requires approximately 4 hours of bladder dwell time. 2 This creates several critical scenarios where nitrites will be falsely negative despite true infection:
- Frequent voiding (especially in young infants, pregnant women, or patients with urgency symptoms) prevents adequate bladder dwell time for nitrite formation 3, 4
- Non-nitrite-producing organisms such as Enterococcus, Staphylococcus saprophyticus, and some gram-positive bacteria will never produce positive nitrites regardless of infection severity 5
- Early infection may not allow sufficient time for bacterial metabolism to generate detectable nitrites 3
Diagnostic Approach: Multiple Parameters Required
A positive urinalysis for UTI can be indicated by ANY of the following: positive leukocyte esterase, positive nitrites, microscopy showing WBCs, or microscopy showing bacteria. 1 The most sensitive screening approach combines leukocyte esterase OR nitrite positivity, achieving 93% sensitivity with 96% specificity when both are positive. 1, 4
Algorithmic Decision-Making:
For symptomatic patients:
- Positive leukocyte esterase alone (without nitrites) has 83% sensitivity and 78% specificity—sufficient to support empiric treatment in symptomatic patients 1
- Microscopy showing >5 WBC/HPF has 90-96% sensitivity even without nitrites 1
- Bacteriuria on microscopy has 81% sensitivity and 83% specificity independent of nitrite results 1
For children <2 years with fever:
- Urine culture is mandatory regardless of urinalysis results (including negative nitrites), as 10-50% of culture-proven UTIs have false-negative urinalysis 4
- Pyuria is absent in approximately 20% of febrile infants with culture-proven pyelonephritis 3, 1
Critical Pitfall to Avoid
Do not rule out UTI based solely on negative nitrite results—this would miss nearly half of true infections. 1, 4 The combination of negative leukocyte esterase AND negative nitrites makes UTI unlikely (<0.3% probability), but negative nitrites alone are insufficient to exclude infection. 1
When Culture Is Required
Definitive diagnosis requires urine culture with ≥50,000 CFU/mL of a uropathogen combined with urinalysis evidence of infection (pyuria and/or bacteriuria). 1 Culture is mandatory in:
- All children <2 years with suspected UTI, even with negative urinalysis 4
- All pregnant women with suspected UTI 2
- Any patient being started on antibiotics for suspected UTI 1
- Cases where urinalysis results are equivocal or discordant with clinical presentation 1
Clinical Context Matters
In patients with high pretest probability based on symptoms (dysuria, frequency, urgency), negative nitrites should not deter diagnosis or treatment. 5 Conversely, positive nitrites in asymptomatic patients (particularly elderly or catheterized patients) likely represent asymptomatic bacteriuria and should not be treated. 4