Testing for Failure of Passive Transfer

When a newborn foal, for whatever reason, fails to obtain the antibodies he needs from his mother in his first hours of life, this often results in a very sick or even dead foal from septicemia. Quick identification of the problem (failure of passive transfer, or FPT) is key to his survival, but the "gold standard" test for the problem takes 18-24 hours to yield results and is best handled in a laboratory. By the time these test results can be obtained and treatment initiated, it could be too late for the foal with FPT.

J.T. McClure, DVM, MS, of the University of Prince Edward Island, presented the results of a study comparing different types of stall-side tests for FPT at the 2003 American Association of Equine Practitioners convention. The tests included a glutaraldehyde coagulation test, the CITE ELISA, the SNAP ELISA 1 (2000), and SNAP ELISA 2 (revised version released in 2001).

McClure explained that a good test for FPT needs to be both highly sensitive and highly specific. A highly sensitive test is one that produces very few false negative results, and a highly specific one produces very few false positives.

"It is essential that screening tests for FPT have a high sensitivity to minimize the number of foals with FPT misclassified as having adequate passive transfer (false negative)," he explained.

Test sensitivity and specificity measures overall test performance.are quantified with the positive and negative predictive values (PPV and NPV), he said. "The PPV and NPV are measures of confidence a practitioner will have in a positive or negative test result, respectively," he explained. "For example, let us assume a test for FPT has a PPV of 90% and a NPV of 70%. If a foal tests positive for FPT, a practitioner could be confident nine times out of 10 that the test result is correct (PPV). In contrast, if the foal tests negative for FPT, the practitioner is only confident that result is correct seven of 10 times (NPV)."

What is more important for a FPT screening test, high sensitivity (PPV) or high specificity (NPV)? "By definition, screening tests should have high sensitivity so that very few foals with FPT go unidentified (false negative)," he said. "But we need to be careful not to choose a test with very low specificity because thenIf a FPT screening test has poor specificity, some foals will be misdiagnosed as having FPT and be subjected to unnecessary and expensive therapy."

Testing the Tests
A total of 201 foals were tested for FPT using the various tests. The CITE ELISA and SNAP ELISA 1 tests were not available during the entire study period (discontinued by manufacturers), and so were not used on all foal serum samples.

McClure reported that in this study the "gold standard" FPT test, the radial immunodiffusion test or RID, showed a FPT prevalenceincidence of 37.8% when detectingconsidering blood IgG (a type of immunoglobulin) levels  less than 800 mg/dl positive tests, andor an prevalenceincidence of 21.9% when detecting bloodif assuming that IgG levels below 400 mg/dl positive tests.

"When looking at the tests performance for detecting blood IgG concentrations <800 mg/dl, tThe CITE ELISA had a poor sensitivity (54-55%), but the best specificity (100%) of any of the screening tests," he said. "The glutaraldehyde coagulation test had the best sensitivity (95-100%), but the worst specificity (58-80%) of any of the screening tests. The SNAP ELISA 1 test had a better sensitivity (87-93%), but poorer specificity (65-92%) compared with SNAP ELISA 2 (sensitivity 76-88%, specificity 90-95%).

McClure noted that the highly sensitive, non-commercial glutaraldehyde coagulation test they used was an excellent screening test for FPT, but should be followed by a confirmatory test in the event of a positive result because of its poor sensitivity. While using RID for confirmation would be best, it would take longer; thus, he noted that the very specific SNAP ELISA 2 could be used as a quick confirmatory test. He said the sensitivity and specificity of theseis tests when used in combination were 92% PPV and 93%, respectively NPV.

"In conclusion, the CITE ELISA was unacceptable as a screening test for FPT because of its poor sensitivity and poor NPV," McClure stated. "The SNAP ELISA 2 and glutaraldehyde coagulation tests are more appropriate screening tests because they have a high sensitivity and NPV. The specificity and PPV of the SNAP ELISA 2 test is better than the glutaraldehyde coagulation test. A confirmatory test should be considered when using the glutaraldehyde coagulation test to screen for FPT in foals."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More