Fibrinogen determination according to Clauss: commutability assessment of International and commercial standards and quality control samples

Background: Many clinical laboratories use a clotting rate assay according to Clauss for the determination of fibrinogen in citrated plasma. The aim of the present study was to assess the commutability of the current International Standard for fibrinogen (coded 09/264), three commercial fibrinogen standards, and 10 freeze-dried plasma quality control samples from various sources. Methods: Clotting rate assays according to Clauss were performed on three automated instruments (Sysmex CA1500, STA-Rack Evolution and ACL-Top 700), using three commercial thrombin reagents (Siemens, Stago, and Instrumentation Laboratory). Relationships between the results obtained with the three instruments were determined with 25 fresh-frozen plasma samples obtained from patients. The deviations of the assay results obtained with the freeze-dried samples were compared with the deviations obtained with the freshfrozen samples, according to approved CLSI guideline C53A. Results: Freezing and thawing had no influence on the assay results. There were significant differences in the mean assay results (fibrinogen, g/L) for the fresh-frozen plasma samples between the three automated instruments: 2.51 (STA-Rack Evolution), 2.25 (ACL-Top 700) and 2.20 (Sysmex CA1500). Similar differences were observed for several freeze-dried plasma samples. Some freeze-dried plasma samples, including the International Standard, were out of the 95% confidence interval for the relationship between STA-Rack Evolution and Sysmex CA1500. Conclusions: Some freeze-dried plasmas including the international standard for fibrinogen are not commutable among automated instruments for fibrinogen clotting rate assays according to Clauss. Our results have consequences for all interested parties in the traceability chain (WHO, industry, external quality assessment schemes, clinical laboratories).