Multi-level Interpretation of the New IDEXX M. paratuberculosis ELISA on Serum and Milk Based on Likelihood Ratio Analysis

The new IDEXX ELISA for para tuberculosis measures the cbncentration of antibody in clinical samples, serum, plasma or milk. ELISA reader results, measured as optical density (OD) units, are transformed to SIP (sample/positive) ratios. Conventional ELISA interpretations employ a single cutoff for interpretation of SIP values as either negative (below the cutoff) or positive (above the cutoff). Prior studies demonstrated a strong correlation of SIP values with the probability animals are shedding Mycobacterium avium subsp paratuberculosis (MAP) in fecal samples collected at the same time as serum samples by likelihood ratio (LR) analysis, highlighting the clinical value of knowing the magnitude of SIP or equivalent transformed ELISA OD values ( Collins et al, 2005).