4.8. Limit of Detection

The limit of detection (LOD) of an analytical procedure is the lowest amount of the targeted substance in the sample that can be detected reliably, but not necessarily quantified as an accurate value using the experimental conditions prescribed. A variety of methods to estimate the LOD have been proposed in the literature, generally based on the calibration information and estimates. None of them are really satisfactory and are sensitive to various assumptions related to the model, the design of experiment and modeling of heterogeneity of variances. Based on our experience, the "best" and most consistent estimate proposed for bioanalytical methods aiming at covering a large range of concentrations is based on dividing the lower limit of quantitation by 3 or 3.33. This is justified by the fact that it is largely accepted that the LOD is three times the noise of the signal and the LLOQ is ten times the same noise.

So for the example used in Program 4.8, the LOD can be established around 8 μM. This is consistent with Figure 4.2. The 8 μM level appears to be the lowest concentration level producing a signal that is clearly above the signals in the lower asymptote.