OneFineAcre
Herd Master
We had a false positive for Johnnes on an ELISA once. Out of 20 animals, only one tested positive on the ELISA. Did a second test on that animal an "acid fast stain" test. More accurate and more expensive than the ELISA. You get false positives on an ELISA, but not false negatives. If you have a herd of animals you do the cheapest test on the whole herd, If you get positives you do the second test. Just being in heat can cause a false positive.