- Because Eniluracil changes the metabolism of 5-FU such that it is excreted primarily by the kidneys, an estimated creatinine clearance calculated using the Cockcroft and Gault formula must be obtained in patients with a serum creatinine > institutional normal limits; the estimated creatinine clearance must be >= 50 ml/min prior to starting treatment with Eniluracil/5-FU; if not, a measured creatinine clearance must be done (using a 24 hour urine collection); the measured creatinine clearance must be > 50 ml/min for the patient to be eligible