Indirect sandwich enzyme-linked immunosorbent assay for rapid detection of Streptococcus pneumoniae type 3 antigen

An indirect sandwich enzyme-linked immunosorbent assay for the detection of the polysaccharide antigen of type 3 pneumococcus (SSS-III) is reported. Polystyrene balls with attached anti-SSS-III antibody serve as the solid phase, and antigen is detected using an alkaline phosphatase-labeled antiglobulin conjugate. The reaction is quantitated by spectrophotometry. Concentrations of antigen are estimated by comparison with standard curves prepared with purified SSS-III. For this assay, the practical lower limit of detection of SSS-III is approximately 2 to 3 ng/ml, thus making the test sensitivity about 25 times that reported for counterimmunoelectrophoresis. In preliminary tests with simulated human clinical specimens, none of the body fluids tested (spinal fluid, serum, urine, and sputum) interfered with detection of antigen, nor did they produce false-positive or false-negative results. Streptococcus pneumoniae type 3 whole organisms were readily detected in simulated clinically positive sputum specimens. Cross-reactions were not observed with Haemophilus influenzae type b, group B Streptococcus, Klebsiella pneumoniae, Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa.