Influence of heparin on the fibrinogen level measured by the prothrombin time-derived method Junko Ichikawa, Keiko Okamura, Goro Kaneko, Keiko Nishiyama, Makiko Komori Department of Anesthesiology, Tokyo Women’s Medical University Medical Center East, Tokyo, Japan Background: With the prothrombin time (PT)-derived method fibrinogen levels are estimated by increase of absorbance during a clotting process triggered by tissue thromboplastin. If the PT is prolonged, however, the precision of this assay is inadequate. We therefore evaluated the effect of heparin on the plasma fibrinogen level using the PT-derived method during cardiac surgery. Methods: This study was a post-hoc analysis of a previous prospective observational study. A total of 78 patients scheduled for elective cardiac surgery were enrolled. Blood samples were obtained after anesthesia induction (baseline) and after initiation of the cardiopulmonary bypass (CPB). The fibrinogen concentration was determined by the PT-derived method using a human recombinant thromboplastin and photo-optical coagulation analyzer on the ACL TOP. Results and Discussion: After CPB initiation, nine (17.6%) patients had fibrinogen levels <40 mg dL-1 from 337.1±120.1 mg dL-1 at average heparin concentrations of 3.58 ± 0.52 U/mL. At heparin concentrations of 3 and 4 U/mL, the fibrinogen concentrations were significantly lower than those at heparin concentrations of 2 and 3 U/ml (Table1). The heparin concentration after initiation of CPB was correlated with the heparin-induced prolonged PT (r=0.49, P=0.0001). There was an inverse relationship between the PT values and the fibrinogen concentration (r=-0.36, P=0.001). This means that PT values influence the results of the PT-derived method, leading to a systematic underestimation of the absorbance changes. Higher thrombin concentrations produce a dense network of relatively thin fibrin strands, related to a non-turbid fibrin gel. Increasing heparin concentrations delayed the onset of thrombin formation, which might result in a late endpoint of clot formation. A plateau is not yet reached in the clotting curve when the reading is stopped at the time defined in assay protocol． Conclusions: This study showed that thrombin formation at the initiation of CPB and prolonged lag time of clotting depending on heparin concertation may lead to an underestimation of the fibrinogen level by the PT-derived method accompanied by increased PT.
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