During normovolemic hemodilution, e.g. during the replacement of blood loss with crystalloids and/or colloids in a bleeding patient, the first procoagulant factor that decreases below a level requiring substitution is fibrinogen (i.e. < 100-150 mg/dL). Fibrinogen and fibrin are elementary for clot formation at the site of vessel damage. The lack of fibrinogen results in the formation of unstable clots unable to resist to blood flow and stop bleeding. Thus, the early substitution of fibrinogen represents a new relevant aspect in the treatment of a massively bleeding patient. The prerequisite for a reasonable use of fibrinogen concentrate is the correct measurement of its plasma concentration. In their laboratory investigation, Fenger-Eriksen and coworkers describe the effects of a 30 and 50% in vitro hemodilution of human plasma with 6% HES 130.000/0.4, 5% human albumin and 0.9% physiologic saline on different laboratory methods measuring the plasma fibrinogen concentration. Importantly, the gold standard (i.e. the measurement of antigen levels of fibrinogen) decreased exactly by 30 and 50%, respectively, after the 30 and 50% dilution of the plasma irrespective of the diluents used indicating that the diluents themselves had no effect on the fibrinogen concentration. After dilution with HES, the functional photometric Clauss methods overestimated the real fibrinogen concentration, whereas the mechanical Clauss method was more or less uninfluenced. Only thromboelastography revealed a potential fibrin polymerization defect induced by HES. Dilution with human albumin and saline did not influence the measurements in a similar way. The authors conclude that: 1) the measurement of plasma fibrinogen concentration with the photometric Clauss methods is problematic; 2) mechanical Clauss methods should be preferred; and 3) currently, the best method to detect HES-induced fibrin polymerization disturbances is thromboelastography. – Oliver Habler