Performance Characteristics of the Enferplex Test using individual milk samples from Cattle
Methodology
Test sensitivity was assessed on bovine milk samples (n = 305) from animals that were reactive in the SICCT test. In 199 of these animals a paired serum samples was also taken at the time of collection. Samples were obtained between 10 – 30 days post SICCT, making use of the anamnestic boosting effect of tuberculin on antibody levels. The specificity was determined using milk individual cows from herds that were deemed to be free of bTB on the basis tuberculin test history, long term freedom from bTB.
Relative sensitivity and diagnostic specificity of the Enferplex Bovine TB antibody test using individual milk samples from UK cattle
Kappa Analysis: A kappa value of 0.934 was obtained indicating almost perfect agreement between the Enferplex test results and SICCT test status in boosted samples.
Likelihood ratio (LR) for positive (LR+) and negative (LR-) individual paired milk and serum samples
Correlation between individual sera and milk samples
A correlation analysis was performed at the individual antigen and test results between the 199 paired serum and milk samples from SICCT positive animals.
McNemar test analysis
The McNemar test analysis results obtained were as follows: Boosted: High sensitivity setting: Chi squared – 1.500, P = 0.227, Boosted: High specificity setting: Chi squared – 0.444, P = 0.505. The results show that the differences in proportions between serum and milk were not statistically significant at either the high sensitivity setting or the high specificity setting of the test.
Correlation between serum and milk results obtained in the Enferplex Bovine TB antibody test following boosting
A strong non-parametric correlation was observed between the serum and milk results, with only a few outliers found with raw RLU results for individual antigens. The low P values indicate that the probability the relationship between serum and milk observed with each antigen occurred by chance is very low.
Results
Individual Milk
The results showed a relative sensitivity of 90.8% and 87.2% using the high sensitivity and high specificity settings of the test respectively.
The DSp, assessed in 1149 bTB free animals, was 99.7% and 99.8% using the high sensitivity and high specificity settings respectively. A kappa value of 0.934 was obtained indicating almost perfect agreement.
Likelihood ratio analysis showed a positive LR of 347.8, a negative LR of 0.092, and a diagnostic odds ratio of 3779.1 providing good evidence for infection being present or absent respectively (Caraguel & Colling, 2021).
Correlation of individual Milk with Serum
Analysis of 199 paired serum and milk samples from SICCT test positive animals showed correlation coefficients ranging between
0.76 – 0.96 for individual antigens in the test (P < 0.0001 for all antigens) indicating good correlation between serum and milk results.
Significance/Impact:
The Enferplex Bovine TB Antibody test shows high sensitivity and specificity using milk from individual cows and the relationship between serum and milk is highly correlated such that milk could be used as a test substrate in place of serum.