Stability of lyophilized pooled sera as quality control materials for tumor marker assays in external quality assessment

Background: External quality assessment(EQA) requires stable quality control (QC) materials. We evaluated the stability of QC materials made of lyophilized and liquid pooled sera for the tumor markers a-fetoprotein, carcinoembryonic antigen, carbohydrate antigen 125, and carbohydrate antigen 19-9. Methods: Specimens of the 4 tumor markers were collected from the sera of patients and stored at -20 degrees C. After sera collection and pooling, liquid or lyophilized samples were stored at -20 degrees C, 5 degrees C, or room temperature. Tumor markers were quantified on days 0, 1, 4, 7, 14, 30, and 90 of storage. Internal QC results were analyzed, and the effects of heat inactivation and sucrose addition were assessed. Results: Heat inactivation lowered tumor marker levels in lyophilized pooled sera, whereas sucrose addition had no effect. The coefficients of variation of the internal QC results were stable, whereas those of lyophilized samples were higher than those of liquid samples. Tumor marker levels were significantly lower in lyophilized samples (p < 0.05) and did not significantly differ according to storage temperature. A declining trend over time was observed for all tumor markers. Conclusions: Lyophilized QC materials are insufficiently