ANTIBODY SCREEN BY FLOW CYTOMETRY: A TEN CELL PANEL VERSUS HLA COATED MICRO BEADS.
R. Parikh, JC Scornik. University of Florida College of Medicine, Gainesville, Florida.


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Flow cytometry (FC) has emerged as the most sensitive technique to detect HLA Class I antibodies (Abs). While this and other advantages make FC a nearly ideal crossmatch technique, it can not be efficiently applied for (Ab) screening. Recently, micro beads coated with HLA molecules have been commercially introduced for the purpose of Ab screening by FC (Flow PRA, One Lambda). The micro beads are coated with HLA from individual donors and pooled to represent a mixture containing HLA from multiple individuals. Although this technique has been found to be more sensitive than cytotoxicity based assays (Bray et al, Human Immunol 55[Sup 1]:36, 1997), little is known about its predictive value for a FC crossmatch. To study this issue, we evaluated sera for Class I IgG Abs by Flow PRA, FC with 10 individual panel cells, anti-human globulin (AHG) and enzyme immunoassay (EIA, Quickscreen). For the Flow PRA, the negative control peak was adjusted to read 10% ± 0.5 to allow for day to day comparisons. First, 20 sera that were negative by FC in a 10-cell panel had a Flow PRA of 6.4%±2.3 (mean±SD), with a range of 3.3-11.5. Next, we selected 20 patients' sera in whom the FC panel detected low-level Abs, since strongly positive specimens are usually reactive by most techniques. By definition, the reactivity by the FC panel was 100%, whereas the Flow PRA detected 16/20 reactions or 80% (p<0.053). The AHG and EIA tests both detected 12/20 or 60%. The 4 negative Flow PRA tests were very weak in the FC panel, whereas the positive Flow PRA results included both stronger and other very weak FC panel reactions. These data indicate that even though the FC cell panel remains as the most sensitive screening test, the Flow PRA is almost as sensitive, more practical and more amenable to standardization.