COMPARISON OF 5 CALCULATION MODES FOR ANTIBODY ELISA PROCEDURES USINGPERTUSSIS SEROLOGY AS A MODEL

During a phase III pertussis vaccine trial, serum antibody responses w ere measured by two enzyme-linked immunosorbent assays (ELISA) for per tussis toxin and filamentous haemagglutinin. These were used both for studies of antibody levels after vaccination and for diagnostic purpos es. Since the absorbance values obtained were not directly proportiona l to the amount of antibody in the samples, ELISA optical densities we re transformed to units by calibration to a reference serum. Five diff erent calculation modes were compared. In four of these modes unit cal culations were based on the relationship between dose response curves of the serum sample and a reference serum. In addition, traditional en dpoint titres were included in the comparison. The calculation mode us ing reference line units showed the highest reproducibility, with intr aassay coefficients of variation (CV) within the same test plate of 4- 7% and interassay CVs of 12-14%. The CVs among the other methods range d from 6 to 31% for intra-assay comparisons and from 12 to 47% for int erassay comparisons. Furthermore, the CV values for intra-assay variat ions were used to calculate standardized differences between 79 pairs of acute and convalescent sera from cases confirmed by culture. These differences were then used to estimate the 'diagnostic sensitivity' fo r the different calculation modes. The results indicated that use of t he reference line units was the most sensitive, whereas use of the end point titers was the least sensitive of these calculation modes.