The evaluation of diagnostic tests for Johne's disease in cattle presents special problems because of the pathogenesis of the disease, in particular the long incubation period, and the lack of an efficient diagnostic standard. The development of an antibody response and detectable lesions may only occur many years after an animal is initially infected. The definition of the diseased state in clinically affected cattle is relatively easy since the presence of clinical signs is highly correlated to the existence of gastrointestinal lesions. In animals with subclinical infection, particularly younger animals, the diseased state is less easy to determine. The only way to determine whether an animal is infected may be to retest the animal for an extended period to allow an opportunity for the disease to progress to a stage where a definitive diagnosis can be made. A lengthy period of testing using existing diagnostic tests may therefore be required to determine whether an animal which is positive to a new test is truly infected. In younger animals even necropsy with histopathology and cultural examination, may not detect infected animals. The use of an age correlated sensitivity is suggested as a method of more accurately portraying the efficiency of diagnostic tests for Johne's disease. Similarly, it is difficult to define animals as non-infected when they are derived from populations where Johne's disease is present. Examples of the effects of these factors on the determination of sensitivity and specificity will be presented.