Table 4. Test characteristics of Accumast to identify Gram-positive bacteria associated with clinical mastitis determined by standard laboratory culture.
| Plate results | ||||
|---|---|---|---|---|
| Parameter | Overall Gram-positive Streptococcus | Overall Gram-positive Staphylococcus | S. aureus | Staphylococcus sp. |
| True Prevalence, % (CI1) n/n | 47.1 (46.9–47.3) 249/529 | 7.0 (6.9–7.1) 37/529 | 1.3 (1.3–1.4) 7/529 | 5.7 (5.6–5.8) 30/529 |
| Sensitivity, % (CI) | 90.0 (89.7–92.7) | 78.4 (76.2–80.6) | 100.0 (100.0–100.0) | 70.0 (67.0–73.0) |
| Specificity, % (CI) | 92.9 (92.7–93.0) | 94.9 (94.8–95.0) | 99.8 (99.8–99.8) | 95.0 (94.9–95.1) |
| PPV2, % (CI) | 91.8 (91.6–92.0) | 53.7 (51.9–55.5) | 87.5 (79.4–95.6) | 45.7 (43.5–47.8) |
| NPV3, % (CI) | 91.2 (91.0–91.4) | 98.3 (98.3–98.4) | 100.0 (100.0–100.0) | 98.1 (98.1–98.2) |
| Accuracy, % (CI) | 91.5 (91.4–91.6) | 93.8 (93.7–93.9) | 99.8 (99.8–99.8) | 93.6 (93.5–93.7) |
| κ4, % (CI) | 0.82 (0.78–0.87) | 0.60 (0.48–0.72) | 0.93 (0.80–1.00) | 0.52 (0.37–0.66) |
| κ P-value | <0.0001 | <0.0001 | <0.0001 | <0.0001 |
Each milk sample was cultured for identification of bacteria associated with clinical mastitis and culture results from the reference laboratory were considered the gold standard. Only sections with more than five colonies were considered positive. Samples considered contaminated in either standard culture (n = 3) or on-farm culture (n = 6) were not included in the analysis (n = 9). Plates with no bacterial growth (n = 168) were considered in all columns. Pure and mixed cultures with the species of interest were combined. Growth on Gram-positive section regardless of color of colonies was considered positive for overall calculations. Only correct identification of bacterial group based on color was considered positive for within group calculations.
1 95% confidence interval.
2 Positive predictive value.
3 Negative predictive value.
4 Cohen’s kappa coefficient. κ ≤ 0 denotes poor agreement; 0.01 to 0.20 denotes slight agreement; 0.21 to 0.40 denotes fair agreement; 0.41 to 0.60 denotes moderate agreement; 0.61 to 0.80 denotes substantial agreement and 0.81 to 1.00 denotes almost perfect agreement.