If Linearity is performed for determining the minimum and possible maximum strength of formulated product, then is it necessary to perform Linearity separately for LOD and LOQ determination or LOD&LOQ can be calculated indirectly from the one time Linearity?
Many have shown that LOD and LOQ can be derived from a linear curve. Practically, you need to perform a separate test for LOD. You need to get some idea about the possible LOD value by running some low concentration standards. From there, you calculate the LOD (1:3) as well as the LOQ (1:10). Then, prepare standards with around the calculated LOD and LOQ and run a few times to find the true parctical values.
You can calculate LOD/LOQ in several ways, see ICH Q2(R1) for the 3 approaches (visual, signal to noise, or standard deviation of response and slope). That being said, you can only determine real LOD/LOQ with actual samples containing the analyte of interest by running experiments to show that the analyte can be detected or quantitated at that level.
Do you really need to calculate LOD and LOQ in method validation related to "Assay" test of formulated drug? Assay method do not require LOD and LOQ parameter as you are not going to get such a low result. As Kar-Weng Chan has written, that LOD value is determined by running low concentration standards. For Assay method, Linearity and Range at the concentration level of 10% to 200% of its initial concentration is ok. If, at all LOD and LOQ parameter are required in AMV of Assay, then it should be determined separately. Generally we do not consider LOD and LOQ parameter in AMV related to Assay of formulated drug.
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