Quantitative impact of pre‐analytical process on plasma uracil when testing for dihydropyrimidine dehydrogenase deficiency

Aims Determining dihydropyrimidine dehydrogenase (DPD) activity by measuring patient's uracil (U) plasma concentration is mandatory before fluoropyrimidine (FP) administration in France. In this study, we aimed to refine the pre‐analytical recommendations for determining U and dihydrouracil (UH2) concentrations, as they are essential in reliable DPD‐deficiency testing. Methods U and UH2 concentrations were collected from 14 hospital laboratories. Stability in whole blood and plasma after centrifugation, the type of anticoagulant and long‐term plasma storage were evaluated. The variation induced by time and temperature was calculated and compared to an acceptability range of ±20%. Inter‐occasion variability (IOV) of U and UH2 was assessed in 573 patients double sampled for DPD‐deficiency testing. Results Storage of blood samples before centrifugation at room temperature (RT) should not exceed 1 h, whereas cold (+4°C) storage maintains the stability of uracil after 5 hours. For patients correctly double sampled, IOV of U reached 22.4% for U (SD = 17.9%, range = 0–99%). Notably, 17% of them were assigned with a different phenotype (normal or DPD‐deficient) based on the analysis of their two samples. For those having at least one non‐compliant sample, this percentage increased up to 33.8%. The moment of blood collection did not affect the DPD phenotyping result. Conclusion Caution should be taken when interpreting U concentrations if the time before centrifugation exceeds 1 hour at RT, since it rises significantly afterwards. Not respecting the pre‐analytical conditions for DPD phenotyping increases the risk of DPD status misclassification.