Because DPD deficiency affects 5%–7% of the Caucasian population, the European Medicines Agency (EMA) recommended in 2020 that prior to commencing fluoropyrimidine-containing therapy, patients should be tested either by genotyping the corresponding DPYD gene, or by phenotyping which is done by measuring plasma uracil concentrations or the dihydrouracil:uracil (UH2:U) ratio (Etienne-Grimaldi et al., 2023; European Medicines Agency, 2020). This evidence concerns the gene DPYD and dihydropyrimidine dehydrogenase deficiency.