ASCP International (ASCPi) Practice Exam

This item is about detecting antibodies by binding prepared antigens to inert carriers and looking for clumping. In passive agglutination, antigens are artificially attached to particles such as latex beads. When the patient’s serum contains the specific antibody, those antibodies bind to the antigens on multiple particles and cross-link them, producing visible agglutination. This approach increases sensitivity and provides a straightforward visual readout because the antigen is presented on a solid phase, yet the antibody in the sample is what drives the clumping. Other methods differ in where the antigen is located or how the reaction is read. Direct agglutination uses antigens already present on particles like cells or bacteria, not treated to coat a separate carrier. Agglutination inhibition involves soluble antigen that blocks the clumping reaction, so a lack of agglutination indicates antibodies are present. Coagglutination uses a carrier such as bacteria to facilitate cross-linking. Thus, the technique described—antigens artificially coated on particles to detect antibodies—aligns with passive agglutination.