Patients with pre-treatment screening based on [U] value according to INCa/HAS recommendations.

Eastern Cooperative Oncology Group performance status (ECOG PS) ≤2

Fluoropyrimidine-naïve patients with gastrointestinal cancer starting chemotherapy combining fluoropyrimidine (5-FU or capecitabine) and oxaliplatin whatever the context (adjuvant, neoadjuvant, palliative) including the following regimens (the most frequently prescribed in gastrointestinal cancers):

• biweekly 5-FU and oxaliplatin (FOLFOX) +/- targeted therapy (TT)
• three-weekly capecitabine and oxaliplatin (CAPOX) +/- TT

Age ≥ 18 years

Patients eligible for full standard fluoropyrimidine and oxaliplatin doses regardless of DPD deficiency

Adequate bone marrow function (cell blood count (CBC)), estimated glomerular filtration rate (DFG) ≥ 50 ml/min, alkaline phosphatase (ALP) / aspartate aminotransferase (ASAT) / alanine aminotransferase (ALAT) ≤ 5 upper limit of normal (ULN), and bilirubin ≤ 50 micromol/L

Patient must have signed and dated a written informed consent form prior to any trial specific procedures. When the patient is physically unable to give their written consent, a trusted person of their choice, independent from the investigator or the sponsor, can confirm in writing the patient's consent.

Women of childbearing potential must have a negative serum or urine pregnancy test.

Patients must agree to remain abstinent or use contraceptive methods with a failure rate of < 1% per year for the duration of study treatment and within 6 months after completing treatment.

Patients must be affiliated to a Social Security System (or equivalent).

Patient is willing and able to comply with the protocol for the duration of the trial including undergoing treatment and scheduled visits, and examinations including follow-up.