5-Fluorouracil (5-FU) is rapidly degraded by dihyropyrimidine dehydrogenase (DPD). Therefore, DPD deficiency can lead to severe toxicity or even death following treatment with 5-FU or capecitabine. Different tests based on assessing DPD enzyme activity, genetic variants in DPYD and mRNA variants have been studied for screening for DPD deficiency, but none of these are implemented broadly into clinical practice. We give an overview of the tests that can be used to detect DPD deficiency and discuss the advantages and disadvantages of these tests.