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Immediate hypersensitivity to platelet concentrate: allergic or not?

The frequency of hypersensitivity reactions following transfusion of labile blood products has been estimated as 5·4 cases per 10 000 units delivered in France and apheresis platelet concentrates (APC) account for 61% of the reactions (France annual haemovigilance report 2009, ANSM 2012). The mechanisms (allergic or non-allergic) as well as the specific agent involved remain unknown. We report two patients where allergic hypersensitivity to platelet concentrate (PC) was clearly demonstrated using specific mediator (histamine and tryptase) measurements and flow cytometry basophil activation tests. Plasma histamine concentrations were measured by a radioimmunoassay (Beckman Coulter, Immunotech, Marseille, France) and total mast cell tryptase by an automated fluoroimmunoassay (Thermofisher scientific, Phadia SAS, Uppsala, Sweden). The basophil activation test was performed by challenging the patient’s whole blood with the suspected agent and measuring marker CD63, a specific marker of basophil activation (Flow cytometry test, Flow2 Cast, Bühlmann). A male patient, 72 years old, experienced haemorrhage after femoral stent. He was transfused three red blood cell concentrates, one platelet concentrate mix (PCM), three methylene blue-inactivated fresh frozen plasmas within 4 h, and one APC 1 h later. Fifteen minutes after the beginning of the APC transfusion, he presented hypotension (systolic blood pressure < 50 mmHg), peripheral oxygen desaturation (72%), bronchoconstriction and urticaria. He was treated with adrenaline 0·025 mg intravenously, followed by a continuous infusion at a 0·04 μg kg−1 min−1 rate. He recovered. Plasma histamine concentration was increased (66 nmol L−1, 55 min after the reaction; normal values < 6 nmol L−1), as well as was tryptase (18 μg L−1, basal value 2 days later: 6·4 μg L−1; normal values <12·5 μg L−1). The left-over APC and the APC supernatant obtained after centrifugation within 4 h of reaction at 1000 × g, and 15 ◦C, were kept frozen at −20 ◦C. A basophil activation test was performed five weeks after the reaction, with the APC and the APC supernatant, at four different concentrations, and showed positivity for both at dilutions 1:4, 1:20 and 1:100 (CD63 expression: 36%, 29% and 10%, respectively; normal values < 5%) (data not shown). A female patient, 84 years old, presented with severe haemorrhage after cervical arthrodesis. She received six red