May 2026

Interpreting Bovine ELISA Antibody Test Results

By Gregg A. Hanzlicek, DVM, PhD, PAS

KVDL reports ELISA antibody tests as test result (antibody present or not present) and a numerical value. Although the final report will identify the positive and negative animals, it is also important to look at the numerical values.

The numerical values are ratios and can include: 1) Percent inhibition (%I), 2) Sample optical density:Positive control optical density ratio (S/P), 3) Sample optical density:Positive control optical density ratio X 100 (S/P%) or 4) Sample optical density:Negative control optical density ratio X 100 (S/N%).

How can these numerical values be used in both individual and herd management programs?

Maybe we should first discuss what these numbers should not be used for. They should not be used to offer a prognosis or to identify the stage of infection (except for Johne’s ELISA).

The numerical values can help identify animals that are true negatives and positives. The animals closest to the cutoff may be misidentified. For example, the cutoff of the Anaplasma marginale ELISA is 30% I. Animals just under 30%, although designated as negative, could actually be positive. The opposite is true for those just over 30%; they may not be true positives.

These distinctions are important when managing herd diseases either through culling or segregation. In the above example, if animals just under the cutoff were assumed to be negative, but were actually positive and either remained in the herd or placed in the non-infected group, the Anaplasma control program could be adversely affected.

To help determine the true infection status of animals near the cutoff, a follow-up PCR is recommended.

For true positives, there are four facts known about the animal. The animal has been exposed to the pathogen, has become infected, has had time to develop antibodies, and for many bovine diseases is a life-long carrier. The life-long carrier designation would hold true for BLV, Anaplasmosis, and Johne’s disease.

We will discuss the utility of the S/P ratio when interpreting Johne’s ELISA results.

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