False reactivity in GTI Pak Plus ELISA kits due to the presence of anti-mouse antibody in patients' samples

The development of commercially available ELISA kits (GTI, Inc., Waukesha, WI) that use antigens adhered to microtiter plate wells by the use of mouse monoclonal antibodies made it possible for hospital transfusion service laboratories to test for platelet- and/or HLA-specific antibodies without reliance on reference laboratories. However, human anti-mouse antibodies (HAMAs) may cause false reactions in ELISAs. We designed a study to determine the impact of HAMAs on these ELISAs. Samples from 210 patients were evaluated from January 1995 to April 2002; 79 (38%) were found to be positive for HLA- and/or platelet-specific antibodies. Thirty (38%) of these positive samples,as well as ten negative samples that served as controls, underwent HAMA neutralization/inhibition procedures before being retested by ELISA. One (10%) of the control samples was reactive after treatment. When the samples that were positive in routine testing were treated to neutralize/inhibit HAMAs, reactivity was unchanged in 20 (67%); reactivity was eliminated in eight (27%) of the samples tested. All of the specimens that showed a reduction or elimination of their reactivity after neutralization/inhibition had an initial optical density (OD) ratio < 3.0 whereas those that remained unchanged in reactivity had an OD ratio > 7.0 (p < 0.05). Reactivity present only in the treated samples was observed in three (10%) of the positive samples tested;one was additionally reactive with HLA antigen only and two with glycoprotein Ia/IIa. The presence of HAMAs should be considered when antibodies against more than one platelet-specific glycoprotein are detected and if the optical density ratio is < 3.0.