Long-term storage stability of uracil in plasma

Area of Science:

• Biochemistry
• Pharmacogenomics
• Clinical Chemistry

Background:

• 5-fluorouracil (5-FU) is a cornerstone chemotherapy agent.
• Dihydropyrimidine dehydrogenase (DPD) metabolizes both 5-FU and uracil.
• DPD deficiency screening via plasma uracil (P-uracil) or DPYD genotyping is recommended by the EMA for safe 5-FU initiation.

Purpose of the Study:

• To assess the three-year stability of plasma uracil (P-uracil) concentrations at -80°C storage.
• To evaluate the impact of long-term storage on P-uracil measurements used for DPD deficiency assessment.

Main Methods:

• LC-MS/MS analysis of P-uracil in 78 patient samples collected before 5-FU treatment.
• Samples were stored at -80°C for three years (2022-2025) and reanalyzed.
• Statistical analysis compared initial and repeated measurements, including percentage difference (PD%) and clinical threshold crossing.

Main Results:

• No significant systematic change in median P-uracil levels was observed after three years of -80°C storage (p=0.241).
• A median percentage difference (PD%) of -2.2% was found, but individual samples showed wide variability (range -55.1% to +80.1%).
• Thirty-four samples exhibited PD% exceeding ±15%, and nine patients' results crossed the critical 16 µg/L DPD deficiency threshold.

Conclusions:

• Plasma uracil is systematically stable for three years when stored at -80°C.
• Significant inter-individual variability in repeated P-uracil measurements exists, potentially impacting clinical decisions.
• Careful consideration of measurement variability is essential when interpreting P-uracil results for DPD deficiency screening.