Objective: To assess the impact and outcomes of a novel program for routine preemptive DPYD testing in fluoropyrimidine (FP)-naïve patients.

Methods: This single-center, retrospective cohort study included adult patients who either received a systemic FP or had a DPYD test result between July 1, 2022, and June 30, 2023. Patients were categorized into preemptive or standard cohorts on the basis of the timing of their DPYD test relative to their initial FP dose. Primary outcomes measured were 90-day all-cause mortality, and FP-related hospitalizations and emergency department (ED) visits after the first FP dose. Secondary outcomes included the incidence of empiric dose reductions, FP avoidance, and dose escalation tolerability among patients with dihydropyrimidine dehydrogenase (DPD) deficiency.

Results: Among 1,281 patients, 90-day all-cause mortality was 5.78% in the preemptive cohort versus 8.23% in the standard cohort (adjusted hazard ratio [HR], 0.69 [95% CI, 0.43 to 1.10]; P = .12), with a notable overrepresentation of patients treated with curative intent in the preemptive group (53.0% v 39.4%, P < .0001). Deaths attributed to DPD deficiency were one (0.18%) in the preemptive cohort and four (0.72%) in the standard cohort (not statistically significant with limited power). Hospitalizations and ED visits related to FP toxicity were paradoxically higher in the preemptive cohort (13.99% v 8.69%, adjusted HR, 1.67 [95% CI, 1.15 to 2.43]; P = .007). Among patients with DPD deficiency in the preemptive cohort, 84.6% received an empiric FP dose reduction, and dose escalation was attempted in 52.2% of these cases.

Conclusions: Preemptive DPYD testing did not significantly reduce treatment-related mortality, although a numerical decrease suggests potential benefits that may be substantiated with greater statistical power. Nearly half of the patients managed with a dose reduction did not undergo dose escalation.