Urine Leukocyte Esterase — Urinary Tract Infection
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 23% (sensitivity 77%) |
| False-positive rate | 30% (specificity 70%) |
| Bayes factor | 3× |
| Base rate | 50% of women presenting with urinary symptoms |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Woman with dysuria and frequency | 50% | 3 × 50% ≥ 100%50%× 3100% | 1/3 × 50% = 17%50%÷ 317% |
3 × 50% ≥ 100%50%× 3100%: exact posterior is 75%. Negative-test exact posterior at 50% prior is 25% (simple approximation gives 17%). A positive LE is only weakly confirmatory. Combined dipstick (LE + nitrite both positive) has a substantially higher Bayes factor.
- + result: at a coin-flip prior (50%), only pushes to 75%
- − result: weak — barely changes the prior
Sources:
- Primary care validation study, PMC, 2007. PMC 1874525.