Quantitative assay of soluble beta-hemolytic streptococcal antigens via an immunochemical turbidimetric method with a spectrophotometer.

Soluble, group A, B, C, and G beta-hemolytic streptococcal antigens were successfully identified in a prototype spectrophotometric system by an immunochemical turbidimetric assay. Any spectrophotometic system which can take a zero reading followed by a second reading 2 or more min later can be used for the assay. Maximum absorbance was obtained near a wavelength of 340 nm. A wide range of linearity between antigen concentration and absorbance was observed at some antibody dilutions, resulting in a simple assay which can be used to quantitate amounts of antigen in solution. Minimal cross-reactions that present no problem in interpretation was observed. Simulated emergency samples were solubilized and assayed for group A and B bacteria within 3 h of colony recognition. Reproducibility of the absorbance resulting from the antibody-antigen reaction was great, with low coefficients of variation over a period of 50 days. The simplicity of the assay solutions, requiring only antisera and a buffer, and the accessibility to high levels of quality control are among the greatest assets of the technique to clinical laboratories.