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. 2002 Oct;8(10):1103–1110. doi: 10.3201/eid0810.020380

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Graphic representation of minimum detectable concentration (MDC), reliable detection limit (RDL), and reactivity threshold. The MDC is the concentration of anti-protective antigen antibody (anti-PA) corresponding to the interpolated intersection of the lower asymptote of the upper 95% confidence limit of the 4-parameter logistic log fit of the standard curve data. The RDL is the concentration of anti-PA antibody corresponding to the interpolated intersection of the lower asymptote of the upper 95% confidence limit with the lower 95% confidence limit of the standard’s data. The reactivity threshold was determined as the upper 95% confidence limit of the frequency distribution from log_₁₀-transformed optical density (OD) values of control human sera tested at 1/50 dilution. This OD value was converted to an anti-PA immunoglobulin (Ig) G concentration by using the standard curve calibration factor. Where this calculated value is below the MDC of the assay, the MDC was selected as the default reactivity threshold.