When evaluating the utility of a diagnostic test, test sensitivity and specificity must be considered. In addition, positive predictive value (PPV), which indicates the likelihood that an individual with a positive test result truly has the disease, should be considered. PPV is influenced by disease prevalence in the population being tested. For instance, if every dog that enters the clinic (a population with low HAC prevalence) is tested for HAC, the likelihood that a positive test result represents a true positive would be low (ie, a low PPV); however, if only dogs that have PU/PD, increased ALP activity, a stress leukogram, and alopecia (a population with a higher HAC prevalence) are tested, a positive test result is more likely accurate (ie, a high PPV).
To maximize the usefulness of HAC tests, only dogs for which there is a clinical suspicion of disease based on history, physical examination, and routine laboratory findings (Tables 1 and 2) should be tested. A diagnosis of HAC should never be made on the basis of adrenal function testing or imaging alone. Ideally, dogs should not be tested when other significant, concurrent diseases are present, as this increases the risk for false-positive results. However, finding an unexpected adrenal mass on imaging for a different presentation should prompt testing for HAC.