P075 : VARIABLE ANTIGEN EXPRESSION ON PLATELETS: IMPLICATIONS FOR ANTIGEN-NEGATIVE PLATELET SELECTION

Abstract

Aim Our center has a homogeneous platelet donor population, yet heterogeneous platelet recipient population. Therefore, for ongoing support of our refractory patients, we would prefer to use the Single Antigen Bead (SAB; Luminex) results to select platelets equivalent to a crossmatch negative platelet. Our preliminary step for establishing a ‘virtual’ crossmatch, similar to what is done for the solid organ transplant population, is to better understand the relationship between Mean Florescence Intensity (MFI) and the platelet crossmatch. Methods Three patient samples were identified with relative mono-specificity (A2, A3, and B27), and selected based on single antigen bead study (Luminex). These sera were diluted and the MFI was determined on each dilution. Twenty-four antigen-positive platelet units were crossmatched (CaptureP, Immucor) with multiple dilutions of the relevant sera, targeted for approximate MFI. The strength of the platelet crossmatch was graded by two technologists, blinded to the dilution, with 4+ being strong positive to 1+ being weak positive. Results All 24 antigen-positive platelet units were positive at MFI 10,000 or greater, yet negative at MFI 4000 (see table). There was platelet-specific variability between MFI 8000 and MFI 6000. Conclusion These results are consistent with variability of antigen expression by platelets. Therefore, a single MFI cut-off for selecting antigen-negative platelets may not be ideal for patient care. Depending on platelet availability, one may choose to utilize a more restrictive or liberal MFI threshold when deciding which antigens to select against. J. Kreuter: Other (Identify); Company/Organization; Immucor. P. Duellman: Other (Identify); Company/Organization; Immucor. S. De Goey: Other (Identify); Company/Organization; Immucor.