Since these values might still seem sub-optimal, a few points need to be taken into consideration. First of all, it should be noted that the acceptance criteria for endogenous compounds in a concentration series might be chosen less stringent than for usual calibration curves, as also proposed in the draft ICH guideline M10 on bioanalytical method validation advising a limit of 30% for the precision in a dilution series. The guideline describes dilution in blank matrix, but when measuring endogenous compounds, achieving these standards is even more difficult because of the different matrix for each sample in the calibration curve. Method 3 has some more disadvantages. The concentration series is limited in the minimum concentration factor (because of sensitivity to the endogenous analyte) and its maximum (because of solubility issues during reconstitution into a smaller volume). In the presented example, the minimum concentration factor (0.25) still gave a considerable peak for PEA, but was around the detection limit for DHEA, causing high variability (22%) and a suboptimal r2 (0.90). Finally, Method 3 does not allow to test the performance of the PCIS in correcting for absolute matrix effect (which is a requirement for absolute quantification) due to lack of a neat solution with the same concentration. Due to these limitations, this method is probably more useful to compare the performance of different PCISs rather than assessing the performance a single one.