Investigating Herd Health Problems Series | Part 6

Serial and parallel testing

Practitioners are often faced with situations where diagnostic testing schemes that limit the probability of a false negative or a false positive test result are desired. Serial and parallel testing schemes can be used to maximize specificity/predictive value of a positive test result and sensitivity/predictive value of a negative test result, respectively.

Serial testing involves performing a diagnostic test and if the result is positive, following the initial test with a second test. The first test is typically non-invasive, simple, economical, etc., wile the second test may be more invasive, expensive or difficult to conduct. One example of a serial testing scheme is the screening tests performed in women for breast cancer. Mammograms and physical exams are typically the first test conducted, and if a mass is detected, more invasive biopsies are conducted. A classic example of serial testing in veterinary medicine testing cattle for bovine tuberculosis. The initial bovine tuberculosis test is the caudal fold test (CFT), and is performed by injecting 0.1 ml of tuberculin into the layers of skin in the caudal tail fold of the cow. This test is read between 66 and 78 hours after the injection. If a reaction (i.e. swelling in the skin) is palpated, the comparative cervical test (CCT) or the gamma interferon test is performed. If the results of the comparative cervical test are positive, the cow is considered positive and the appropriate procedures for bovine tuberculosis control are followed. If the cow is negative on the CCT, the animal is considered negative and no further action is required.

Parallel testing involves conducting two different diagnostic tests at the same time, and if either test is positive, the individual is considered infected. Parallel testing minimizes the likelihood of a false negative, because it is less likely that two tests would both yield false negatives than it is that either tests individually would yield a false negative. For example, when performing pregnancy examinations in a cow-calf herd, the practitioner might believe the prevalence of pregnancy in the herd is high (95% pregnant or greater) before beginning the testing. Cows that are open would be considered abnormal. In this case, the consequence of a false negative is great, as the cow may be culled and ultimately slaughtered. If the practitioner is using an extension probe that eliminates the need for manual palpation, any cow that is called open due to no evidence of pregnancy on ultrasound may also be manually palpated to ensure a pregnancy cannot be detected by palpation of the reproductive tract. This is an example of parallel testing where the risk of a false negative is lowered, and the predictive value of a negative test is increased.

When considering a herd health problem that may require diagnostic testing, the veterinary practitioner should consider which false outcome (i.e. false positive or false negative) is more detrimental to the overall production system, and design testing strategies accordingly. Improper use and interpretation of diagnostic tests will not only hinder a herd health investigation, but ultimately cost the veterinarian and producer both time and resources.