Podcasts

Immunochromatographic testing for feline AB blood type | VETgirl Veterinary CE Podcasts

In today’s VETgirl online veterinary continuing education podcast, we review the accuracy of a newer test (using immunochromatography) for detecting feline blood types (AB) in a study titled “Evaluation of an immunochromatographic test for feline AB system blood typing.” AB blood typing is commonly performed in hospitalized cats to ensure blood compatibility and to prevent hemolytic transfusion reactions or potentially life-threatening reactions (e.g., B cats receiving A blood). Blood typing is a necessity for all feline transfusions because cats are born with antibodies against red blood cells of the opposite blood type. For this reason, there is no universal donor in cats, and cats must always be blood typed and/or cross-matched prior to administration! AB blood typing is also important in feline breeding programs in order to prevent neonatal isoerythrolysis. Several methods that allow AB blood typing have been previously validated and include gel column testing, which is no longer commercially available, as well as tube or plate testing, which are both cumbersome and difficult to standardize in practice. Agglutination cards are probably the most commonly used test kits in veterinary practice and can reliably identify type A and B cats, but traditionally have shown weak reactions with type AB blood, resulting in mistyping of AB cats.

A new immunochromatographic cartridge test, based on diffusion of RBCs passing through monoclonal antibody-containing strips, is commercially available for laboratory and in-hospital use. In this study, the authors wanted to assess the accuracy of this cartridge test for blood typing non-anemic and anemic samples obtained in the clinical and laboratory settings, to compare this test’s performance with the card agglutination method, and to evaluate the effects of hemolysis, autoagglutination, different anticoagulant and preservative solutions, temperature, and time of storage on the cartridge test performance.

This prospective observational study was performed at the University of Milan. The authors included surplus blood samples collected from cats presented to the Veterinary Transfusion Unit and Internal Medicine Clinic. The commercially available lab kits tested in this study include the RapidVet-H Feline card agglutination test, as well as the LabTEST A+B and QuickTEST A+B, which are two versions of the immunochromatographic cartridge test made by Alvedia. All samples were macroscopically evaluated for autoagglutination using a slide agglutination technique (with repeated washings of cells if agglutination was noted). The PCV or hematocrit was recorded, as well as the presence or absence of hemolysis based on the color of the plasma. All tests were performed at the same time using the sample blood sample, with the operator blinded to the results of the blood type test. Agreement of results of card agglutination and cartridge test was considered indicative of the true blood type. If a discordant result was found, the back-typing technique using a reference standard was used to identify the correct blood type.

A volume of 1 to 2.5 mL blood was drawn from 51 non-anemic and 19 anemic cats. The non-anemic cats were mostly blood donors, but the group also included breeding cats and cats undergoing pre-operative blood tests prior to spay or neuter procedures. The anemic cats had many different underlying diseases including neoplasia, hemorrhage, bone marrow aplasia, chronic kidney disease, and various viral and blood-borne infections. There was a mix of both domestic shorthairs and pure breed cats in each of the groups. The average PCV in the non-anemic cats was 31% compared to 14% in the anemic cats. Approximately 75% of all cats were type A, 20-25% were type B, and the remaining cats were type AB.

The majority of discordant results between the two testing methodologies occurred in the non-anemic cats, with 7 cats incorrectly typed by the agglutination card test as type B when they were, in fact, type AB cats. The majority of un-typed samples occurred in the anemic cats secondary to autoagglutination, which negated the use of the card agglutination test. That said, overall, the LabTEST cartridge test could determine the blood type in all of the anemic and non-anemic samples resulting in 100% sensitivity, specificity, and negative and positive predictive value in blood type A, B, and AB cats. The LabTEST cartridge test method also showed 100% repeatability, excellent reproducibility, and was able to correctly blood type samples stored at room temperature and refrigeration for up to 1 month. The type of anticoagulant used did not affect the blood typing results either.

Additionally, the 5 hemolyzed samples and 3 autoagglutinated samples were all correctly blood typed with the LabTEST cartridge test method. However, in 1 of the autoagglutinated anemic samples with a PCV of 4%, it was necessary to wash the RBCs before typing them, because the autoagglutinated sample did not migrate along the immunochromatographic strip to reach the 3 impregnated lines of the anti-A and anti-B antibodies and control. After washing, the sample correctly migrated along the strip and could be correctly typed.

Likewise, when the QuickTEST version of the cartridge test was used, it was able to accurately blood type all 15 samples tested, of which 13 were non-anemic and 2 were anemic samples. Therefore, the version of the test kit that most practices would use was also considered 100% accurate.

So, what can we take away from this VETgirl podcast? This study showed that the cartridge test is more accurate compared to the card agglutination test for providing AB type results in anemic and non-anemic cats. This is because of the cartridge test’s ability to provide test results in cats with autoagglutination, which prevents accurate interpretation of the card agglutination results. Also, the cartridge tests can more accurately type AB cats, probably because the kit uses different antibodies with different antigen-antibody affinities. Weaknesses of this study are that there were few ‘diseased cats’ that would have challenging blood type results, as well as the lack of a gold standard test. Back-typing was used in this study to confirm blood types and is an imperfect reference test because it cannot differentiate type AB from type A cats.

With that said, many practices are slowly moving away from using the card agglutination test kits and instead using the cartridge test kits for several reasons: First, the cartridge test requires a smaller volume of blood (Just 10 uL compared to the 150 uL required for the agglutination card test); and the time required to perform the cartridge test is just 2 minutes (compared to the 10 minutes required for the agglutination card test). Likewise, this study shows that samples stored for 1 month at room temperature or refrigeration and in various anticoagulants will still provide accurate results with the cartridge test. Keep in mind the next time you’re ordering new feline blood typing kits for your hospital!

References:
1. Spada E, Proverbio D, Baggiani L, et al. Evaluation of an immunochromatographic test for feline AB system blood typing. J Vet Emerg Crit Care 2015; online release.

Leave a Reply

Your email address will not be published. Required fields are marked *