The gel test: sensitivity and specificity for unexpected antibodies to blood group antigens

The recently FDA-licensed anti-IgG gel test for pretransfusion antibody detection requires crossover validation before implementation. Six hundred coded samples sent for routine pretransfusion tests were used to compare GEL (ID-MTS, Ortho Diagnostic Systems Inc., Raritan, NJ) with Löw and Messeter's low-ionic-strength saline (LISS). There were 456 GEL-LISS-, 97 GEL+LISS+, 45 GEL-LISS+, and 2 GEL+LISS- tests. The 144 positive tests involved 157 antibodies; 67 of these (cold auto, anti-M, -Le, etc.) were considered harmless with respect to transfusion management. GEL-LISS+ tests included seven samples containing potentially significant antibodies (assumed from specificity): anti-K(4), -Jka, -Fyb, and -S. Two potentially significant antibodies (anti- C and -D) were GEL+LISS-. Sensitivity and specificity for potentially significant antibodies were 92% and 96% for GEL, and 98% and 90% for LISS, respectively. The seven GEL-LISS+ samples associated with potentially significant antibodies were from six patients. Five of these antibodies, all detected in immune-suppressed patients, reacted predominantly as agglutinins in LISS. None of these seven antibodies were detected reliably by polyethylene glycol and LISS-additive tube methods. In light of the immune status of the patients with GEL-LISS+ agglutinins with specificity normally considered potentially significant, and because other valid methods did not detect these antibodies, their clinical importance is questionable. Excluding these questionable antibodies, GEL has the same sensitivity and better specificity than LISS. GEL is a valid method for pretransfusion antibody detection.