Aim: Monitoring the internal standard (IS) response is common practice in bioanalysis by LC-MS/MS. IS response variation may raise questions on assay quality and should trigger investigations into the root cause. Results: In two case studies with IS variability, re-analysis of diluted samples and spiking predose study samples revealed no effect of IS variability on results. The D17-labeled IS in a third case proved not to be suitable during method development and was replaced by a differently labeled IS. Conclusion: Determining the exact root cause for varying IS response is not always feasible; however, assay accuracy and reliability of results should be demonstrated. In some cases, assay re-development is needed to solve the problem.
Keywords: IS variability; LC–MS/MS; biological samples; internal standard response; sporadic; stable isotope-labeled IS; trend.