Table 6.
Comparison of investigation criteria by cluster size and growth at first indicated investigation, Washington State 2018.
| Cluster Investigation Criteria | Number of Clusters Reaching Investigation Criteria | Prevalent Cases at First Indicated Investigation, Mean (95% CI) | Absolute 3 Month Cluster Growth at First Indicated Investigation, Mean (95% CI) a |
| 3 Cases at 0.015 | 17 | 20.5 (8.3–32.6) | 1.4 (0.8–2.0) |
| 5 Cases at 0.015 | 9 | 28.0 (4.9–50.2) | 1.4 (0.5–2.3) |
| 3 Cases at 0.005 | 10 | 24.6 (6.6–42.6) | 1.6 (0.8–2.4) |
| 5 Cases at 0.005 | 6 | 28.3 (−5.7–62.4) | 2.0 (0.5–3.4) |
| >0.9 Predicted Cases in 3 Months (Random Forest) b | 14 | 19.9 (16.8–23.1) | 1.3 (1.1–1.4) |
| >2.3 Predicted Cases in 3 Months (Random Forest) b | 4 | 31.8 (20.5–42.9) | 2.3 (1.3–3.2) |
| Cluster Investigation Criteria | Number of Clusters Reaching Investigation Criteria | Prevalent Cases at First Indicated Investigation, Mean (95% CI) | Cluster Growth Per 100 Person-Months at First Indicated Investigation, Mean (95% CI) a |
| 3 Cases at 0.015 | 17 | 20.5 (8.3–32.6) | 1.5 (1.3–7.7) |
| 5 Cases at 0.015 | 9 | 28.0 (4.9–50.2) | 2.6 (0.7–4.4) |
| 3 Cases at 0.005 | 10 | 24.6 (3.8–45.4) | 5.3 (−0.3–10.9) |
| 5 Cases at 0.005 | 6 | 28.3 (−5.7–62.4) | 3.1 (0.8–5.4) |
| >0.14 Predicted Cases Per 100 Person-Months (Random Forest) b | 15 | 7.5 (6.1–8.8) | 3.4 (2.8–4.0) |
| >0.42 Predicted Cases Per 100 Person-Months (Random Forest) b | 1 | 2 (NA) | 0 (NA) |
a Calculated from number of newly diagnosed cases joining the cluster in the subsequent 12-month period, 95% confidence intervals from normal distribution. b Random forest model with 500 trees and 6 variables selected at each branch fit to 2016 and 2017 cluster data. The models included the following variables: prevalent cases; number of cases diagnosed in the previous year; number of viremic cases; number of cases in past 1, 3, and 12 months (0.015 and 0.005 genetic distance); number of late diagnoses; number of white cases; number of female cases; and number of cases with injection drug transmission risk. Cutoffs were selected approximate the same number of investigations as the “3 Cases at 0.015” and “5 Cases at 0.005” criteria.