4.7. Limits of Quantification and Range of the Assay

The upper and lower limits of quantitation (ULOQ and LLOQ) of an analytical procedure are the lowest and highest amounts of the targeted substance in the sample that can be quantitatively determined under the prescribed experimental conditions. As a consequence, the range of an analytical procedure is the range between the lower and upper limits of quantitation for which the analytical procedure was demonstrated to have a suitable level of measurement error.

In practice, the information needed to establish the limits of quantitation and the associated range is already available in the measurement profile plot. As proposed by Hubert et al. (1999) and Hubert et al. (2004), the limits of quantitation are the most extreme (low, high) concentrations (quantities) at which the tolerance interval is still within the acceptance limits, should the tolerance limits cross the acceptance limits. If all tolerance limits lie within the acceptance limits, the limits of quantitation are defined as the most extreme quantities tested in the study.

Figure 4-8. Derivation of the limits of quantification and range of the assay using the measurement error profile

Figure 4.8 shows how the upper and lower limits of quantitation are defined. In this example, the lower limit is approximately 24 μM and the upper limit is the maximum concentration investigated in this study, i.e., 450 μM.