For example, if you test an animal in the preclinical stages of M. paratuberculosis infection, before diarrhea develops, the animal will likely test negative - but it's a false negative. The measures of a test's accuracy are its sensitivity and its specificity. Sensitivity refers to the ability of a test to detect infected animals. The lower the sensitivity, the more false-negatives will occur. Tests with low sensitivity underestimate disease in an infected herd.
Specificity refers to the test's ability to detect noninfected animals.
It's the probability or likelihood of a negative test result in animals known to be free of infection. A test with low specificity will produce more false positives than a highly specific test. The ability of a test to accurately distinguish an animal's Johne's status will depend on the test, the lab conducting the test and the stage of infection. Which tests are best to use and in which combination will be unique to your herd's situation. You should thoroughly discuss your options and the ramifications with your herd veterinarian.