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Rotational thromboelastometry (ROTEM®)–based FIBTEM is used perioperatively to assess the extent of fibrin polymerization in whole blood. In FIBTEM, cytochalasin D eliminates the contribution of platelets to whole blood clotting, but changing levels in fibrin(ogen) and erythrocytes may differently affect clot formation. Because dynamic changes of hematocrit are not reflected in plasma fibrinogen measurements, we hypothesized that the lack of erythrocytes in isolated plasma measurements would affect the relationship between the Clauss method and whole blood–based FIBTEM during cardiac surgery. Therefore, in the current study we investigated the influence of perioperative hematocrit changes on FIBTEM and fibrinogen measurements.Blood samples were collected from 6 consenting healthy volunteers. FIBTEM tests were run before and after serial in vitro dilutions of whole blood with saline or autologous plasma (5:1, 2:1, and 1:1 v/v). We then evaluated the relationship between FIBTEM–maximal clot firmness (MCF) and the Clauss fibrinogen method in relation to hematocrit values before and after cardiac surgery. Pearson correlation coefficients were determined between laboratory test results and ROTEM variables.Upon in vitro hematocrit reduction, FIBTEM-MCF was progressively decreased depending on the extent of saline dilution, but it was increased by 31% after 1:1 volume replacement with autologous plasma (P < 0.05). In samples from cardiac patients (150 measurements in 50 patients), the overall correlation coefficient between FIBTEM-MCF and plasma fibrinogen was 0.80 (P < 0.001). In hemodiluted blood samples (during surgery or at intensive care unit), FIBTEM-MCF 10 mm corresponded to plasma fibrinogen levels of 200 mg/dL. In the subgroup analysis (n = 50 each), according to hematocrit levels (<25%, ≥25% to 30%, ≥30%), plasma fibrinogen levels of 200 mg/dL corresponded to 11 mm, 10 mm, and 8 mm of FIBTEM-MCF, respectively. The correlation between FIBTEM-MCF and plasma fibrinogen was higher at lower hematocrit (<25%) than at higher hematocrit (>30%) (r = 0.88 and 0.67, respectively).Perioperative changes in hematocrit affect the correlation between plasma fibrinogen levels and FIBTEM-MCF values. The higher correlation between FIBTEM-MCF and plasma fibrinogen with lower hematocrit (<25%) indicates that FIBTEM is a practical method to determine the need for fibrinogen replacement in bleeding patients who typically develop perioperative anemia.