Testing for the DPYD gene is recommended for patients planning to start systemic treatment with with 5-fluorouracil-based chemotherapies (as advised by the UK MHRA on 22 October 2020).

5-fluorouracil (5-FU) is the active chemotherapy agent in a range of commonly used drugs to treat a range of cancers e.g. Capecitabine, Tegafur. 5-FU is metabolized by the dihydropyrimidine dehydrogenase enzyme (DPD) which is encoded by the DPYD gene. Several variants within DPYD have been described that lead to reduced or abolished DPD activity. Patients with these variants are at an increased risk of severe adverse reaction to 5-FU or fatal 5-FU toxicity. Testing for DPD deficiency by DPYD genotyping allows more accurate prediction of 5-FU toxicity and adverse events.