Automated particle-counting immunoassay of C-reactive protein and its application to serum, cord serum, and cerebrospinal fluid samples.

The assay for C-reactive protein has been fully automated as a particle-counting immunoassay. For cerebrospinal fluid, cord serum, and adults' serum its range of sensitivity extends from 1 microgram/L to 300 mg/L, with a minimal sample dilution of twofold and a maximal dilution of 50-fold being required. This range is so broad because free antibodies are added to the reaction medium. However, we have used Fab fragments rather than whole antibody to avoid too steep a standard curve and a decrease of agglutination at high concentrations of antigen. For 99 consecutive cord sera examined, the concentrations of C-reactive protein ranged from 7 micrograms/L to 1.750 mg/L. The geometric mean was 50 micrograms/L and the upper normal limit (geometric mean +/- 2 SD of the log values) was established at 525 micrograms/L.