FDA publishes Q&A on Internal Standard Responses in Chromatography Bioanalysis

In the context of chemical analyses, Internal Standards (IS) are used for the detection of sample losses of the analyte to be determined in certain methods. Internal standards are known substances with similar analytical behaviour to the substance to be evaluated, e.g. a stable or analogue isotope of the analyte. In contrast to external standards, these substances are located in the sample and are "also treated" - thus internal. In chromatographic bioanalysis, an IS is added to all samples, including calibration standards, as well as quality controls and samples before extraction.


Chromatographic analysis is often used in non-clinical and clinical studies to quantify substance concentrations. Their data are then often also important for the submission of marketing authorisations. However, the behaviour of such a standard can show a certain variability, e.g. when used in bioanalytics. The question then arises to what extent this influences the accuracy of the analytical data of the actual analyte.

This issue - the evaluation of the internal standard answers during chromatographic bioanalysis - is the subject of a new FDA catalogue of questions and answers. This guidance paper contains recommendations for sponsors, applicants and contracted research institutions regarding the variability of the internal standard (IS) response in chromatographic analytical data when new applications or biological licensing are submitted.


This paper answers fundamental questions such as

  • "What are internal standards in analytical chemistry and why are they used in bioanalytics?" or
  • "What are the sources of IS-answer variability?" - but also further questions such as
  • "When is it likely that IS-answer variability will not affect data accuracy?

The answers to the basic questions are brief and  provide a summary. For the more advanced questions, examples and graphs are also included to explain the answers.

For more details, please read the guide "Evaluation of Internal Standard Responses during Chromatographic Bioanalysis: Questions and Answers".

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