Infectious diseases remain the major constraint to achieving the genetic potential of pigs in modern swine production. The accurate diagnosis of disease and/or surveillance for infections usually relies on diagnostic tests. Unfortunately, there are no perfect diagnostic tests. No test will always correctly identify all truly positive or truly negative samples.
Inaccurate test results can be a frustrating experience for producers and veterinarians. A good example is the PRRS (porcine reproductive and respiratory syndrome) ELISA (enzyme-linked immunosorbent assay) test from IDEXX Laboratories commonly used to detect PRRS antibody. Some herds have experienced a false positive rate of up to 3%, although 1% is considered normal.
A false positive reported from a population that is expected to be negative often leads to undesirable and expensive outcomes, including: stopping the shipment of animals or semen; the expense and delay of additional testing of those false positive samples with PCR (polymerase chain reaction) or other serology tests; the expense of additional sampling and testing to assure the population is negative; and the heartburn, concern, and consternation on the part of producers, owners, veterinarians and laboratory personnel in dealing with the problem.
Defining Accuracy of a Test
The accuracy of a test is usually defined in terms of sensitivity and specificity. The sensitivity estimates how often the diagnostic test can find its target when the target is present in an optimized specimen. Said another way, sensitivity is the likelihood that the test will identify a truly positive sample as positive by the test. A test that is 99% sensitive will miss one true positive (have one false negative) in every 100 positive samples. This problem is commonly minimized by testing a larger number of animals.
The other measure of accuracy is specificity, which estimates how often a false positive test will occur by measuring how often a negative test is truly negative. A test that is 99% specific will have one false positive test (sample was actually negative) for every 100 tests of truly negative samples. So in the case of PRRS ELISA, 98% specificity means that one could expect two false positive animals in every group of 100 truly negative samples (or one false positive for every 50 truly negative samples).
New PRRS Test Kit
There is good news – the testing technology is improving. A new antibody test for PRRS has been introduced by IDEXX: the HerdChek PRRSX3 Antibody ELISA Kit. This updated test method detects PRRS antibodies in plasma or serum with 98.8% sensitivity and 99.9% specificity. The improved specificity means there are fewer false positive results to add confusion and urgency to monitoring and surveillance programs.
This means the likelihood of getting a false positive with this new test kit is reduced by 90% compared to current and previous test kits. Said another way, with the new system a false positive test will occur about one time per 1,000 samples rather than the current two times per 100 samples. The advantage is obvious to those who have to deal with false positive test results.
The HerdChek PRRSX3 Antibody ELISA Kit is currently licensed in Europe and is being imported into the United States. Validation studies at the Iowa State University Veterinary Diagnostic Laboratory have affirmed the claim of increased specificity. The test kit is available now and is likely to be implemented as a routine test in the near future.
Multitude of Tests Available
The previous discussion applies to one common test, but diagnostic laboratories offer literally hundreds of tests. There are overwhelming quantities of information on diseases and diagnosis. The methods and data generated are often complicated and confusing. Not all laboratories use the same test methods or naming procedures. An important question to answer before submitting samples is: “What decision(s) will be influenced by the results?”
The most valuable tool a swine practitioner has for gathering information on diagnostic testing and interpretation is a telephone call to your veterinary diagnostic laboratory. Diagnosticians can help formulate your diagnostic question(s) and offer guidance for specimen selection and appropriate tests with a common goal to optimize the value of the diagnostic investigation.
In general, the accuracy of diagnostic tests is not just a function of the test itself. Individual test sensitivity and specificity are important, but the variation in test results and inaccurate results are often introduced by other factors. Accuracy of tests heavily depends on animals sampled, sample selection, sample preservation, sample transport, the test selected and the proper execution of the test within the laboratory.
Interpretation of Test Results
One more pitfall is the interpretation of the results. One of the most frequently asked questions of diagnosticians in laboratories is: “What does this result mean?” A test result is what it is. Interpretation and application requires case/herd context which is usually unique for every case. Each herd is different. The purpose for submission is different.
Hence the application of the results must be in context to the herd, the intervention strategies that are available to be employed, and the goals that the producer wishes to accomplish.
Ultimately, a diagnostic laboratory and diagnosticians are trying to provide the needed information to help the veterinarian and producer make the right decision for the herd. Perhaps the real question should be “How do I apply this information to a decision for the case at hand?” That is the job of the veterinarian and producer. A diagnostician can assist by providing nuances of agents, tests, methods and expected results.
Test capability and performance within laboratories are constantly improving. To optimize the value of a diagnostic laboratory submission, the submitter should have clarity of purpose (“What is the question that diagnostic testing can answer?”) and a good idea of how the results will be applied to impact a decision (“What decision or intervention will be made based on diagnostic laboratory results?”).
Each herd is different, so the answers to these questions are not the same for all farms. Once there is clarity of purpose and utility, the diagnostic process requires careful selection and presentation of samples for the appropriate testing methods.
Kent Schwartz, DVM
Iowa State University Veterinary Diagnostic Laboratory