Establishing assay cutoffs for HLA antibody screening of apheresis donors

Abstract

Background: Transfusion-related acute lung injury (TRALI) is the leading cause of transfusion-related deaths. Donor HLA antibodies have been implicated in TRALI cases. Blood centers are implementing TRALI risk reduction strategies based on HLA antibody screening of some subpopulations of ever-pregnant apheresis platelet (PLT) donors. However, if screening assay cutoffs are too sensitive, donation loss may adversely impact blood availability.

Study design and methods: Pregnancy history and HLA antibody screening and single-antigen bead data from blood donors in the Retrovirus Epidemiology Donor Study-II Leukocyte Antibody Prevalence Study were evaluated for correlations between assay screening values, HLA antibody titer, and number of HLA antigen specificities. The probabilities of matching a cognate antigen in a recipient were calculated and examined in association with total number of specificities observed and screening values. The relative impact of imposing various screening assay cutoffs or pregnancy stratification was examined in relation to detection of HLA antibody-reactive donations and loss of donors and donations.

Results: We provide evidence that higher HLA antibody screening assay values are associated with maintaining higher screening signals upon dilution and an increased breadth of specificities compared with lower screening values; the latter correlated with an increased risk of a cognate antigen match in potential recipients. Depending on the TRALI risk reduction strategy used, the potential loss of donations ranged between 0.9 and 6.0%.

Conclusion: This analysis should enable blood centers to decide upon a TRALI risk reduction strategy for apheresis PLTs that is consistent with how much donation loss the blood center can tolerate.

Figure 1. HLA Ab screen reactive rates in LAPS donations at >3SD, >4SD, or >5SD cutoffs by pregnancy

The percent reactive donations at the various HLA Ab screen assay cutoffs was determined. 3SD cutoffs = 10.8 for Class I, 6.9 for Class II. 4SD cutoffs = 25.4 for Class I, 13.8 for Class II. 5SD cutoffs = 59.3 for Class I, 27.5 for Class II. Denominators used in the calculations of reactivity rates were: n=1138 non-transfused males, 5834 all females, 1816 never pregnant females, 3992 females with 1 or more pregnancies, 3358 females with 2 or more pregnancies, 2051 females with 3 or more pregnancies, and 993 females with 4 or more pregnancies.

Figure 2. HLA Ab screening NBG values at various dilutions

A) Class I HLA Ab results; B) Class II HLA Ab results. The lines connect the results for each individual sample at various dilutions.

Figure 3. Number of SAB serological specificities in donors who screened positive with different screen cutoffs

Class I data (A) and Class II data (B) were categorized by screening result and SAB reactivity. See text for more information regarding these categories.

Figure 4. HLA Ab screening reactivity rates and projected donation loss under various potential TRALI reduction strategies

The potential loss of donations at the various cutoffs if blood centers implemented a policy of testing ever pregnant females (triangles), females with 2 or more pregnancies (open circles), 3 more pregnancies (boxes), or 4 more pregnancies (closed circles).