Nieuw ontwerp externe kwaliteitscontrole voor analyse van thiopurines maakt grote verschillen tussen laboratoria zichtbaar

Objective The aim of the study was to compare analytical results of 6-TGN and 6-MMPR blood level estimations between laboratories. Design and methods Participating laboratories were asked to select patient samples from their routine analysis and exchange these with another assigned laboratory. Because of large differences in results between laboratories, all standard operating procedures were reviewed, revealing that the origin of these differences could lie in the method of hydrolysis and the preparation of calibrator samples. To investigate the contribution of the calibrators to these differences, one participating laboratory was asked to prepare a batch of calibrators to be shipped to the participating laboratories for analysis. Results For 6-TGN and 6-MMPR 43% and 24% of the results were outside of the 80-120% range. When correcting the results from the exchange of the patient samples with the results of the calibrators, the mean absolute difference for 6-TGN improved from 24.8% to 16.3% (P < 0.001), whereas the mean absolute difference for 6-MMPR slightly increased from 17.3% to 20% (P = 0.02). Conclusion This first EQAS for thiopurine drugs shows that there is a difference between laboratories in the analysis of 6-TGN, and to a lesser extent of 6-MMPR. This difference for 6-TGN can partially be explained by the use of in-house prepared calibrators that differ among the participants.