A good article written by Chris at "Chris Beat Cancer" discusses the toxicity induce by 5-FU that can be lethal for those missing a liver enzyme called Dihydropyrimidine Dehydrogenase (DPD). 

It seems that DPD deficiency affects 5% of the population which could be at risk of death from only 1 dose of 5-FU. An additional 3% to 5% of the population has a partial DPD deficiency.

There is a blood test that costs around $100 to check for the DPD enzyme deficiency. 

More details you can find at https://www.chrisbeatcancer.com/one-dose-of-5-fu-chemo-can-kill-you/?fbclid=IwAR1XyPDmr_HJNl9m3TiPMALZfDlPPoPj0INoC76OeeUWSc1B_mlCFmwIHZw