Comparison of methods for quantitating antigen-specific immunoglobulin M antibody with a reverse enzyme-linked immunosorbent assay.

We compared two methods for quantitating antigen-specific antibody by a reverse enzyme-linked immunosorbent assay. In the serial-dilution method, the result was determined to be the highest dilution of a serum yielding an absorbance above an established threshold. In the single-dilution method, the result was determined by comparing the absorbance yielded by the test serum at a standard dilution to that yielded by positive and negative reference sera at the same dilution. The results in the single-dilution method reflected the antigen-specific immunoglobulin M antibody activity as a proportion of total immunoglobulin M antibody in a serum, i.e., the immune load, whereas the results in the serial-dilution method reflected the absolute concentration of antigen-specific immunoglobulin M antibody activity. Compared with results in the serial-dilution method, results in the single-dilution method had considerably greater reproducibility on a day-to-day basis and under various test conditions. The single-dilution method was more useful in discriminating between sera from patients in an early stage of clinical infection due to Toxoplasma gondii and sera from patients in a later stage of infection.