Detection limit estimated from slope of calibration curve: An application to competitive ELISA

This paper theoretically derives a general rule that while the slope of the semi-logarithmic plot (Y vs. log X) of a calibration curve varies depending on analyte concentration, X, the slope takes a specific value at the detection limit (L-D)This rule holds good irrespective of the shape of the calibration curve (linear or non-linear) and in this paper, is applied to competitive ELISA (enzyme linked immunosorbent assay). The following, relationship is deduced: slope of log-dose BIBO at L-D = [relative standard deviation (RSD) of blank responses] divided by 0.13. The L-D obtained from the above-mentioned slope corresponds to the dose at which the RSD of dose estimates is 0.3 (= 30%). A commercial kit for 17alpha-hydroxyprogesterone is taken as an example.