It is commonly acknowledged that random and systematic analytical errors contribute to poor data quality, and moreover, to imprecise and inaccurate pharmacokinetic parameters. To investigate the random errors in GLP bioanalysis, common ground has been found in today's bioanalysis to assess the reproducibility of the method by reanalyzing part of the incurred samples. The undesired systematic errors in bioanalysis affecting the trueness of the method and leading to inaccurate data remain relatively unattended so far. In order to obtain both precise and accurate data it is suggested in this paper to apply standard addition experiments to calculate the relative systematic errors as an estimate for the incurred sample accuracy. This approach, which can be seen as an important extension to current guidelines in GLP bioanalysis, is illustrated by assessing the accuracy of the bioanalytical results for a bioequivalence study for alendronate.