Alloimmunisation in transfused patients: serial cross-matching in a population of hospitalised cats.

Objectives Cross-matching is currently recommended as part of pre-transfusion testing for repeat transfusions in cats 4 days after having received an initial transfusion. This prospective study determined when and if cats developed positive cross-match (CM) results after having been transfused with AB-compatible blood. Methods Donors were selected according to standard transfusion safety protocols. Twenty-one hospitalised anaemic recipients (blood type A: n = 20; blood type B: n = 1) received 1-4 (median 2) whole blood transfusions (WBTs) over 1-6 days (median 2) in 33 transfusion instances. The tube CM method, including major, minor and recipient control, was employed. Macroscopic and microscopic agglutination reactions were evaluated according to a predetermined scale. CM tests with a positive recipient control could not be evaluated. Results No signs of an acute transfusion reaction were observed. A total of 63 CMs were performed. In one cat with immune-mediated haemolytic anaemia the CM could not be evaluated (positive recipient control). The minor CM was negative in all cases. Fifteen of 20 cats had a negative major CM (MCM) 1-12 days (median 5) after their first transfusion. A positive MCM was observed in five cases after 2-10 days (median 5) post-first WBT. These five cats had received a total of 1-4 (median 2) WBTs. Cats with a negative MCM had received 1-3 (median 2) WBTs. In 51.5% (17/33) of transfusion instances, the cat's haematocrit increased as expected, with cats with a positive MCM at 40% (4/10) vs 56.5% (13/23) if MCM was negative. Conclusions and relevance Twenty-five percent (5/20) of the feline recipients likely developed alloantibodies against erythrocyte antigens outside of the AB system as early as 2 days post-first WBT. This adds data to the recommendation to include cross-matching in pre-transfusion screening tests.