Slope-Ratio Assays

This chapter discusses slope-ratio assays. The response could be plotted as a straight line against the logarithm of the dose over an adequate dosage range. Parallel lines were fitted to concurrent tests with the standard and the unknown and the relative potency of the unknown determined on the log dose scale from the horizontal distance between them for a log ratio assay. Sometimes, the response is a linear function of the dose rather than of the log dose. The most important applications are in the microbiological assays where in many cases the response can be plotted linearly against arithmetic dosage units at the lower vitamin concentrations. Concurrent tests on two preparations that differ only in their content of a single active ingredient are fitted by two straight lines that converge at 0 dose. The potency of an unknown relative to the standard can then be determined from the ratio of the slopes of the fitted lines. Assays meeting these conditions have been designated as slope-ratio assays. The term has since been extended to assays where the response can be plotted linearly against some power of the dose. One of the first requirements for an efficient microbiological assay is a linear relation between some function of dosage and response over an effective range.