Table 3.
Performance of Kaplan Criteria and CART-Derived Characteristics Among Outbreaks Reported Through the NORS, 2009–2012
| Clinical and Epidemiologic Profiles | Confirmed Norovirus (%)a | Suspected Norovirus (%)b | Confirmed Nonviral (%)c | Unknown (%)d | Cohen’s Kappa (95% CI)g | Likelihood Ratiog | Sensitivity (95% CI), %g | Specificity (95% CI), %g | Positive Predictive Value (95% CI), %g | Negative Predictive Value (95% CI), %g |
|---|---|---|---|---|---|---|---|---|---|---|
| Kaplan et al.e | ||||||||||
| Outbreaks with all criteria | 108 (3.7) | 29 (2.2) | 19 (1.2) | 121 (3.3) | ||||||
| Outbreaks that fit the criteria | 69 (63.9) | 12 (41.4) | 0 (0.0) | 35 (28.9) | 0.34 (0.22–0.48) | Undefined | 63.9 (54.5–72.3) | 100 (83.2–100) | 100 (94.7–100) | 32.8 (22.1–45.6) |
| Outbreaks that did not fit the criteria | 39 (36.1) | 17 (58.6) | 19 (100.0) | 86 (71.1) | ||||||
| CART-derived characteristicsf | ||||||||||
| Outbreaks with all criteria | 706 (24.0) | 324 (24.5) | 607 (38.6) | 762 (19.2) | ||||||
| Outbreaks that fit the criteria | 605 (85.7) | 261 (80.6) | 46 (7.6) | 536 (70.3) | 0.78 (0.72–0.83) | 11.3 | 85.7 (82.9–88.1) | 92.4 (90.0–94.3) | 92.9 (90.7–94.7) | 84.7 (81.8–87.3) |
| Outbreaks that did not fit the criteria | 101 (14.3) | 63 (19.4) | 561 (92.4) | 226 (29.7) | ||||||
Abbreviations: CART, classification and regression tree; CI, confidence interval; NORS, National Outbreak Reporting System.
aSingle-etiology outbreaks with norovirus reported as confirmed (laboratory confirmation of 2 or more cases, n = 2939).
bSingle-etiology outbreaks with norovirus reported as suspected (laboratory confirmation in fewer than 2 cases, n = 1321).
cSingle-etiology outbreaks with a confirmed nonviral etiology (n = 1573) (Supplementary Table 2).
dOutbreaks reported without any suspected or confirmed etiology (n = 3694).
eKaplan criteria includes vomiting ≥0.5 affected persons, median incubation period of 24–48 hours, and median duration of illness of 12–60 hours.
fCART-characteristics include fever-to-vomiting ratio <1, proportion with bloody stools <0.1, and proportion with vomiting ≥0.26.
gValues determined by comparing confirmed norovirus with confirmed nonviral outbreaks.