Use of diagnostic tests for biosecurity purposes for purchased cattle offers veterinarians a tool to reduce the risk of disease for our cow-calf clients, said Bob Larson, DVM, PhD, speaking at the 2010 Western Veterinary Conference. Veterinarians should use information about test sensitivity and specificity, disease prevalence, test cost, and the cost of disease to calculate the expected value of testing for biosecurity reasons.
The cost-effectiveness of alternate diagnostic testing strategies can be compared with a partial budget. In this partial budget, the test accuracy in a given popution values, test cost, cost of the negative condition, treatment cost, and cost of false positives are used to calculate the return for true positive, true negative, false positive, and false negative test results.
The value of a testing strategy, whereby all incoming animals are tested and the true-positive animals are isolated or euthanized, is the value of avoiding disease spread in the population. The cost of true negatives is essentially the cost of doing the diagnostic tests, including laboratory costs, veterinary labor and consulting costs for handling the tests, and labor for handling the animals. The cost for false positives is the cost of isolating or euthanizing an animal that was not truly infected. And, the cost of false negatives is the cost of leaving a positive animal in the herd. The economic benefit is simply the costs for true negatives, false positives, and false negatives subtracted from the return for true positives.
For conditions that are rare (low prevalence), even with an accurate test (but less than 100% specific), many of the positive test results will be false-positive, and the costs of finding true negatives (i.e., testing cost) and the cost of false-positives may be greater than the value of finding the few true positives. In this situation, a partial budget evaluation may indicate little or no economic benefit for a testing strategy unless the cost of disease is substantial.
Because some of the relatively infrequent negative conditions of interest to veterinarians can have significant production and economic costs when present, the cost of an infrequent but significant condition can be better evaluated as an assessment of risk and cost of risk avoidance. Once the cost of the risk is quantified, the producer and veterinarian can determine the effects such an event would have on a confined period’s cash flow, and can evaluate that effect with the cost of risk reduction.