warfarinized patients with a therapeutic international normalized ratio: the effect of low factor IX levels. J Thromb Haemost 2013; 11:1043-52.Summary. Objective: Bleeding is the main complication of warfarin therapy, even patients with an international normalized ratio (INR) in the target range can suffer bleeding, suggesting that INR does not perfectly reflect the therapeutic effect of warfarin. We hypothesized the INR might underestimate the level of anticoagulation in a subject with a lower factor (F) IX level than average. Methods and results: We modeled warfarin anticoagulation in our in vitro thrombin generation (TG) model by adjusting the levels of vitamin K-dependent factors to those of patients with an INR of 2-3. Variation in FIX had a marked effect on TG but had no effect on the prothrombin time (PT)-INR. A prospective observational, crosssectional clinical study including 341 consecutive patients admitted to the emergency department with an INR between 2 and 3, showed a statistically lower FIX activity in bleeders (P = 0.004) compared with others. No correlation was found between TG capacity and PT-INR results (P = 0.36). However, in patients, presenting with a warfarin-related hemorrhage, TG was significantly lower (P < 0.001) than others. A correlation on the boundary of significance was observed between TG capacity and FIX levels (P = 0.09). Conclusion: These data demonstrates that patients who bleed when their PT-INR is in the target range 2-3 might have defective TG related to a lower level of FIX than expected.

541 Bleeding occurs in from 10 – 16% of warfarin-treated patients. Having a PT-INR in the target range is associated with better outcomes. However, even patients with an INR in the target range of 2–3 can suffer bleeding, suggesting that INR does not perfectly reflect the therapeutic effect of warfarin. The goal of our studies was to determine whether the level of specific coagulation factors could predict the risk of bleeding while the INR was in the target range. We modeled warfarin anticoagulation in our previously published in vitro cell based-model by adjusting the levels of vitamin K-dependent factors to those of patients with an INR of 2–3. We then examined the effect of variations in the level of FIX. The cogulation reactions were initiated by monocyte-expressed tissue factor (assayed at 1pM). Variation in FIX had a marked effect on thrombin generation. However, in plasma with the same levels of factors, as expected, variations in FIX had no effect on the PT-INR. Thus, we hypothesized that a subject with a lower FIX level than average may have a lower level of thrombin generation than is indicated by the INR. The INR might, therefore, underestimate the level of anticoagulation in such a subject. If s/he is maintained in the “therapeutic range” as measured by the INR, s/he will actually be over-anticoagulated and prone to hemorrhage. A prospective, single centre clinical study has been carried out to test this hypothesis in warfarinized patients. Between October 2010 and June 2011, 312 consecutive patients admitted to the emergency department of Edouard Herriot Hospital in Lyon, with an INR between 1.8 and 3.2, were included in the study after obtaining informed consent. Twenty six patients were admitted for a bleeding episode, 18 for recurrent thrombosis and 268 for other medical reasons. Patients presenting with bleeding, 17 males and 9 females, were aged 74±14 years old compared to the rest of the patients aged 76±14. Among the 26 bleeders, 7 had a spontaneous intracranial haemorrhage, 2 had a trauma-induced intracranial haemorrhage, 12 presented a gastrointestinal bleeding and 5 exhibited muscle hematomas, severe epistaxis or urinary tract bleeding. PT-INR and vitamin K-dependent factor levels were determined in all patients. Thrombin generation capacity in platelet poor plasma was measured using Calibrated Automated Thrombin generation assay (Thrombinoscope bv, Maastricht, The Netherlands), with tissue factor 1pM and phospholipids PC:PS:PE 4μM. No statistically significant difference was observed in the PT-INR of bleeding patients (INR=2.4±0.4) and those having a thrombosis (INR=2.5±0.5) or patients admitted for other reasons (INR=2.6±0.2). Plasma prothrombin and factor × levels were also similar in all three groups. However, a statistically lower plasma factor IX activity was observed in bleeders (p=0.01, Mann Whitney test) compared to other groups, 47.6±20 IU/dL vs. 63±33 IU/dL. In all the warfarinized subjects with an INR between 1.8 and 3.2, no correlation was found between thrombin generation capacity and PT-INR results (p=0.85, Spearman correlation test). However, a statistically significant correlation was observed between thrombin generation capacity and factor IX levels (p=0.0002). In patients, presenting with warfarin-related haemorrhage, the endogenous thrombin potential (ETP) was significantly lower at 340±335 nM.min (p=0.05) then that of warfarinized subjects who did not suffer bleeding (ETP 406±215 nM.min). These data support our hypothesis based on our in vitro results and show that patients who bleed when their PT-INR is in the target range 2 – 3 might have defective thrombin generation related to a lower level of factor IX than expected. Thus, our results suggest that the appropriate target INR level might not be the same for all patients. Those with factor IX levels that differ significantly from the mean of the population might be managed best by selecting a target INR that is based on the level of thrombin generation. Of course, a “target range” for parameters of thrombin generation during warfarin therapy would need to be developed if the assay were to be used for this purpose. Disclosures: No relevant conflicts of interest to declare.