Evaluation of the Directigen and Phadebact agglutination tests

Comparison testing of the Directigen latex agglutination (LA) kit, the Phadebact coagglutination (COA) kit, and counter-immunoelectrophoresis (CIE) demonstrated that the commercial LA reagents were slightly more sensitive than the COA reagents for the detection of pneumococcal polysaccharide types 2, 4, 8, 9, 12, 19, 23, 25, 51, and 56, and meningococcal polysaccharide from Group C. The COA reagents were slightly more sensitive than the LA reagents for the detection of pneumococcal polysaccharide type 6A. The sensitivity of LA and COA reagents for the detection of Hemophilus influenzae type b capsular polysaccharide, pneumococcal polysaccharide types 1, 3, 14, and meningococcal Group A were equivalent. Purified meningococcal polysaccharides of Groups B, C, and W135 were detected uniformly by CIE but not with the COA reagent. The COA reagent reacted with antigen of Groups B, C, and W135 from broth culture but with less sensitivity than CIE. In general, CIE was the least sensitive method for detecting bacterial antigens. In addition, the commercial CIE antisera for H. influenzae type b, or meningococcal polysaccharides were susceptible to false-negative results due to antigen excess.