Comparison of true prevalence and percentage of patients above a cut-off threshold for screening tests
| True prevalence, % | Sensitivity | Specificity | Percentage of patients above cut-off | Percentage of patients with false-positive screens among those above cut-off | Percentage of patients with false-negative screens among those below cut-off | Percentage of patients above cut-off − true prevalence | Percentage of patients above cut-off/true prevalence |
|---|---|---|---|---|---|---|---|
| Basic scenario:* sensitivity and specificity were constant across levels of true prevalence | |||||||
| 0.0 | 78.0 | 87.0 | 13.0 | 100.0 | 0.0 | 13.0 | – |
| 5.0 | 78.0 | 87.0 | 16.3 | 76.0 | 1.3 | 11.3 | 3.3 |
| 10.0 | 78.0 | 87.0 | 19.5 | 60.0 | 2.7 | 9.5 | 2.0 |
| 15.0 | 78.0 | 87.0 | 22.8 | 48.6 | 4.3 | 7.8 | 1.5 |
| 20.0 | 78.0 | 87.0 | 26.0 | 40.0 | 5.9 | 6.0 | 1.3 |
| 25.0 | 78.0 | 87.0 | 29.3 | 33.3 | 7.8 | 4.3 | 1.2 |
| 30.0 | 78.0 | 87.0 | 32.5 | 28.0 | 9.8 | 2.5 | 1.1 |
| Adjusted for varying sensitivity and specificity:† sensitivity changed by 1% and specificity by 2% per 10% change in prevalence | |||||||
| 0.0 | 77.0 | 89.0 | 11.0 | 100.0 | 0.0 | 11.0 | – |
| 5.0 | 77.5 | 88.0 | 15.3 | 74.6 | 1.3 | 10.3 | 3.1 |
| 10.0 | 78.0 | 87.0 | 19.5 | 60.0 | 2.7 | 9.5 | 2.0 |
| 15.0 | 78.5 | 86.0 | 23.7 | 50.3 | 4.2 | 8.7 | 1.6 |
| 20.0 | 79.0 | 85.0 | 27.8 | 43.2 | 5.8 | 7.8 | 1.4 |
| 25.0 | 79.5 | 84.0 | 31.9 | 37.6 | 7.5 | 6.9 | 1.3 |
| 30.0 | 80.0 | 83.0 | 35.9 | 33.1 | 9.4 | 5.9 | 1.2 |
↵* Based on sensitivity = 78% and specificity = 87%, which are estimates for the standard cut-off threshold of 10 or greater for the Patient Health Questionnaire-9 from a meta-analysis of published results from 21 292 patients (2573 cases, 12%). (17)
↵† Sensitivity and specificity may vary with disease prevalence. (18), (19) Thus, estimates of sensitivity and specificity were adjusted upward or downward from a prevalence of 10% based on a meta-analysis (19) that found that sensitivity may decrease 1% and specificity may increase 2% per 10% reduction in prevalence.