TABLE 3.
Sensitivity and specificity estimates with corresponding PPV for direct streak methodsa
| Direct streak method and no. of positive plates | E. coli O157 concn (log10 CFU/g) | % Sensitivity (95% CI) | % Specificity (95% CI) | PPV
|
||
|---|---|---|---|---|---|---|
| At study prevalence | At 10% prevalence | At 20% prevalence | ||||
| Preenrichment | ||||||
| ≥1 | 3 | 89.2 (83.0-93.7) | 89.8 (86.0-92.8) | 0.80 | 0.49 | 0.69 |
| 4 | 96.3 (89.7-99.2) | 78.1 (73.8-82.1) | 0.48 | 0.33 | 0.52 | |
| ≥2 | 3 | 79.7 (72.3-86.0) | 96.7 (94.2-98.3) | 0.92 | 0.73 | 0.86 |
| 4 | 87.8 (78.7-94.0) | 85.7 (81.9-89.0) | 0.56 | 0.41 | 0.61 | |
| 3 | 3 | 62.2 (53.8-70.0) | 98.5 (96.5-99.5) | 0.95 | 0.82 | 0.91 |
| 4 | 68.3 (57.1-78.1) | 89.7 (86.3-92.5) | 0.58 | 0.42 | 0.62 | |
| Postenrichment | ||||||
| ≥1 | 3 | 96.6 (92.3-98.9) | 61.4 (56.0-66.7) | 0.53 | 0.22 | 0.39 |
| 4 | 97.6 (91.5-99.7) | 52.0 (47.0-57.0) | 0.30 | 0.18 | 0.34 | |
| ≥2 | 3 | 91.9 (86.3-95.7) | 68.7 (63.4-73.6) | 0.57 | 0.25 | 0.42 |
| 4 | 93.9 (86.3-98.0) | 59.1 (54.0-63.9) | 0.32 | 0.20 | 0.36 | |
| 3 | 3 | 77.0 (69.4-83.5) | 75.6 (70.6-80.1) | 0.59 | 0.26 | 0.44 |
| 4 | 79.3 (68.9-87.4) | 67.3 (62.5-71.9) | 0.33 | 0.21 | 0.38 | |
a
Direct streak methods used one or more, two or more, or three positive plates to classify samples as positive at concentrations of greater than 3 or 4 log10 E. coli O157 CFU per g of feces. Samples were considered positive if there was at least one confirmed E. coli O157 colony when two colonies per plate were evaluated. Corresponding PPV are the proportions of direct-streak-positive samples that were positive at the given concentration by the gold standard. These values were calculated at the observed prevalence and 10 and 20% prevalences by use of sensitivity and specificity estimates from the study.