Table 3.
Diagnostic characteristics of different binary drug concentration thresholds to predict virological suppression (<50 copies/mL) using both established drug concentration cut-off values and alternative cut-off values (indicated by *) that maximised sensitivity and specificity in ROC analyses.
| Tenofovir-diphosphate (TFV-DP) in DBS | ||||||
|---|---|---|---|---|---|---|
| Select TFV-DP cut-off values | Probability of virologic suppression1 | Sensitivity2 (%) | Specificity3 (%) | PPV4 (%) | NPV5 (%) | Correctly classified6 (%) |
| >LLOQ | >0.190 | 97.7 | 73.5 | 86.9 | 94.7 | 89.1 |
| ≥350 | ≥0.464 | 92.1 | 81.6 | 90.0 | 85.1 | 88.3 |
| ≥399* | ≥0.515 | 92.1 | 83.7 | 91.0 | 85.4 | 89.1 |
| ≥700 | ≥0.787 | 73.9 | 87.8 | 91.5 | 65.2 | 78.8 |
| ≥1250 | ≥0.975 | 33.0 | 98.0 | 96.7 | 44.9 | 56.2 |
| ≥1850 | ≥0.998 | 6.8 | 100.0 | 100.0 | 37.4 | 40.2 |
| Plasma tenofovir (TFV) | ||||||
| Select plasma TFV cut-off values | Probability of virologic suppression1 | Sensitivity2 (%) | Specificity3 (%) | PPV4 (%) | NPV5 (%) | Correctly classified6 (%) |
| >LLOQ | ≥0.411 | 88.6 | 77.6 | 87.6 | 79.2 | 84.7 |
| ≥23.5* | ≥0.586 | 78.4 | 79.6 | 87.3 | 67.2 | 78.8 |
| ≥35.5 | ≥0.750 | 64.8 | 85.7 | 89.1 | 57.5 | 72.3 |
| Plasma efavirenz (EFV) | ||||||
| Select plasma EFV cut-off values | Probability of virologic suppression1 | Sensitivity2 (%) | Specificity3 (%) | PPV4 (%) | NPV5 (%) | Correctly classified6 (%) |
| >LLOQ | ≥0.253 | 97.7 | 75.5 | 87.8 | 94.9 | 89.8 |
| ≥0.70 | ≥0.483 | 89.8 | 79.6 | 88.8 | 81.3 | 86.1 |
| ≥1.00 | ≥0.605 | 85.2 | 81.6 | 89.3 | 75.5 | 83.9 |
| ≥1.13* | ≥0.636 | 81.8 | 83.7 | 90.0 | 71.9 | 82.5 |
| ≥4.00 | ≥0.995 | 14.8 | 95.9 | 86.7 | 38.5 | 43.8 |
Probability estimated from logistic regression models.
Proportion of subjects with a viral load <50 copies/mL who were detected using this cut-off
Proportion of subjects with a viral load ≥50 copies/mL who were detected using this cut-off
Positive Predictive Value: proportion of those who were detected with this drug concentration cut-off who had a viral load <50 copies/mL
Negative Predictive Value: proportion of those who were not detected with this drug concentration cut-off who had a viral load ≥50 copies/mL
Overall proportion of those above and below the cut-off who were correctly classified. Note that this proportion is always biased towards the larger group.