(13)C-uracil breath test to predict 5-fluorouracil toxicity in gastrointestinal cancer patients.

Purpose: Up to 30 % of patients undergoing 5-fluorouracil (5FU)-based chemotherapy experience severe toxicity. Dihydropyrimidine dehydrogenase (DPD) deficiency explains 36-61 % of cases. Predicting toxicity is an unmet challenge.