Urine Nitrite — Urinary Tract Infection
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 65% (sensitivity 35%) |
| False-positive rate | 4% (specificity 96%) |
| Bayes factor (positive test) | 10× |
| Bayes factor (negative test) | 1/1.5× |
| Base rate | 50% of women presenting with urinary symptoms |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Woman with dysuria and frequency | 50% | 10 × 50% ≥ 100%50%× 10100% | 1/1.5 × 50% = 33% |
| Asymptomatic incidental finding | 5% | 10 × 5% = 50%5%× 1050% | 1/1.5 × 5% = 3.3% |
10 × 50% ≥ 100%50%× 10100%: exact posterior is 91%. Negative-test exact posterior at 50% prior is 40% (simple approximation gives 33%). Positive is strong, but the test misses 65% of UTIs. Negative is nearly useless (1/1.5×).
- + result: at a coin-flip prior (50%), risk pushes to 91%
- − result: barely budges it — still 33–40%
Sources:
- StatPearls. Urine nitrite test.