Long‐term stability of pleural fluid carcinoembryonic antigen and its effect on the diagnostic accuracy for malignant pleural effusion

BACKGROUND: The in vitro stability assessment is essential for investigating the diagnostic accuracy of pleural biomarkers. This study aimed to investigate the long‐term stability of pleural fluid carcinoembryonic antigen (CEA) at −80°C to −70°C. In addition, we analyzed the effects of frozen storage on the diagnostic accuracy of CEA for malignant pleural effusion (MPE). METHODS: Pleural fluid CEA of participants in two prospective cohorts were stored at −80°C to −70°C for 1–3 years. The CEA level in the stored specimen was measured with an immunoassay, and its level in the fresh specimen was extracted from medical records. The Bland–Altman method, Passing‐Bablok regression, and Deming regression were used to analyze the agreement of CEA between the fresh and frozen pleural fluid. In addition, we used receiver operating characteristic (ROC) curves to evaluate the diagnostic accuracy of CEA in the fresh and frozen specimens for MPE. RESULTS: A total of 210 participants were enrolled. The median CEA levels in frozen and fresh pleural fluid specimens were similar (frozen, 2.32 ng/mL; fresh, 2.59 ng/mL; p < 0.01). The slopes and intercepts in the Passing‐Bablok regression (intercept 0.01, slope 1.04) and Deming regression (intercept 0.65; slope 1.00) were not statistically significant (p > 0.05 for all). No significant difference was observed between the area under the ROC curves of CEA in the fresh and frozen specimens (p > 0.05 for all). CONCLUSION: Pleural fluid CEA is seemingly stable when stored at –80°C to −70°C for 1–3 years. Frozen storage does not significantly affect the diagnostic accuracy of CEA for MPE.