Interpretation of an enzyme-linked immunosorbent test using different cut-offs between positive and negative samples for diagnosis of paratuberculosis

Abstract

The purpose of this study was to compare the serologic responses of cows infected and non-infected with Mycobacterium paratuberculosis in infected and non-infected herds using different cut-offs between positive and negative samples. Infection status of cattle was determined by fecal culture. The serological test was an enzyme-linked immunosorbent assay (ELISA) using an affinity-purified antigen. Cows differed in their level of exposure (high- vs. low-prevalence and negative herds) and disease manifestation (clinical vs. subclinical infection). The ELISA optical density (OD) for cow samples from fecal culture-positive herds (no clinical cases) was higher than that for samples from the fecal culture-negative herd (0.49 vs. 0.26, respectively). The mean OD value (0.61) for clinically-affected cows was not different from that of fecal culture-positive, clinically-normal cows (0.49), nor were differences found between the OD values from fecal culture-negative cows from the herd with no history of paratuberculosis (0.21) and cattle with negative cultures from paratuberculosis-infected herds (0.26). The likelihood-ratio method was used to calculate the optimum ELISA discrimination value between test-positives and -negatives at a given prevalence in a herd, a d a receiver-operator characteristic (ROC) curve was constructed.