Latex agglutination in the diagnosis of pneumococcal infection

A latex agglutination (LA) method for detection of pneumococcal antigens was evaluated and compared with counterimmunoelectrophoresis (CIE). LA was 2 to 10 times more sensitive than CIE for the detection of purified capsular polysaccharides in defined media, but only when a 1+ or 2+ agglutination reaction was interpreted as positive. LA was much less sensitive than CIE with clinical samples. In 50 cases of pneumococcal pneumonia, antigen was detected in the serum almost twice as often with CIE (40%) as with LA (22%). LA was positive in six cases of pneumonia where CIE was negative; however, in three of these cases, antigen was detected only in undiluted sera, which raised some question about the specificity of the result. With 18 samples of cerebrospinal fluid (CSF) from 11 patients with pneumococcal meningitis, the CIE test was positive more frequenlty (14 samples) than was LA (11 samples). Moreover, antigen was detected in CSF by LA in only one additional patient than was positive by CIE alone. There was one false-positive LA reaction among 45 samples of CSF from patients without pneumococcal infection. Although LA is a less complicated method than CIE, it is not a sensitive test for pneumococcal antigens and would be of little value as a routine diagnostic method.