H emostasis is a complex interaction among 1) the fluid phase of procoagulants that generate thrombin, 2) the natural anticoagulants that regulate thrombin, 3) the fibrinolytic system that controls the size of thrombus and maintains vascular patency, and 4) the cellular phase that includes platelets and endothelium. To date, no single global test exists to assess these 4 components of hemostasis. The in vitro coagulation tests, the PT and PTT, were developed to identify the cause of bleeding in a symptomatic bleeding patient; they have never been shown to assess the risk of bleeding in a nonbleeding patient. 16,20,35 Because different sensitivities of tissue thromboplastin reagents account for the variability of PT, the INR was introduced to monitor warfarin therapy. The INR is a calculated value derived by the following formula: (patient PT/mean normal PT) ISI , where ISI is a value assigned to the PT reagent when compared with a WHO reference standard with an ISI of 1.0. 31,43,44 The INR was standardized on plasmas from patients on chronic warfarin therapy that affects only VKD factors II, VII, IX, and X. Use of the INR is therefore inappropriate in other medical conditions in which clotting factors other than VKD factors are affected.10 However, the INR is often (mis)used in clinical practice to assess hemostasis in patients not receiving warfarin therapy; thus, patients with mildly elevated INRs are reflexively transfused with FFP, usually perioperatively.In the past, less-sensitive thromboplastin reagents (ISI > 2.0) caused a prolonged PT only when factor VII Coagulation factor levels in neurosurgical patients with mild prolongation of prothrombin time: effect on plasma transfusion therapy Object. Neurosurgical patients often have mildly prolonged prothrombin time (PT) or international normalized ratio (INR). In the absence of liver disease this mild prolongation appears to be due to the use of very sensitive PT reagents. Therefore, the authors performed relevant coagulation factor assays to assess coagulopathy in such patients. They also compared plasma transfusion practices in their hospital before and after the study.Methods. The authors tested 30 plasma specimens from 25 patients with an INR of 1.3-1.7 for coagulation factors II, VII, and VIII. They also evaluated plasma orders during the 5-month study period and compared them with similar poststudy periods following changes in plasma transfusion guidelines based on the study results.Results. At the time of plasma orders the median INR was 1.35 (range 1.3-1.7, normal reference range 0.9-1.2) with a corresponding median PT of 13.6 seconds (range 12.8-17.6 seconds). All partial thromboplastin times were normal (median 29.0 seconds, range 19.3-33.7 seconds). The median factor VII level was 57% (range 25%-124%), whereas the hemostatic levels recommended for major surgery are 15%-25%. Factors II and VIII levels were also within the hemostatic range (median 72% and 118%, respectively). Based on these scientific data, plasma transfusion guidelines were mod...