The clinical significance of differences between point-of-care and laboratory INR methods in over-anticoagulated patients

Patients receiving warfarin are at increased risk of bleeding when their International Normalised Ratio (INR)4.5. Although not standardised above 4.5 the INR is measured in over-anticoagulated patients, consequently we have examined the reliability of INR results ≥4.5. We assessed: the relationship between different prothrombin time systems for INRs4.5; the relationships between the INR and levels of vitamin K-dependent coagulation factors (VKD-CF) and thrombin generation test (TGT) parameters; and the impact that variation in results would have on warfarin dosing.INRs were performed using a CoaguChek XS Plus point-of-care (POC) device (measuring range 0.6-8.0). For POC INRs ≥4.5, laboratory INRs were also measured using a recombinant tissue factor (rTF) and a rabbit brain (RBT) thromboplastin.There was good correlation between POC (INR ≥4.5,8.0) and Lab INRs (rTF n=154, rs=0.87, p0.0001; RBT n=102, rs=0.76, p0.0001); and significant correlations between each of the VKD-CF and the INR, the strongest being with FVII (POC INR rs=-0.53 p0.0001; Lab rTF-INR rs=-0.70 p0.0001). TGT peak thrombin and ETP also showed good correlations with INR values (R(2)0.71). Using POC and Lab rTF-INR, 109/154 (71%), or POC and Lab RBT-INR 75/102 (74%) results exhibited dosage concordance and/or were within 0.5 INR units. In the remaining patients variation in warfarin dosing was generally slight.Our data suggest that CoaguChek XS Plus INRs4.5 and8.0 are comparable to laboratory INRs (both methods) and it is probably unnecessary to perform laboratory INRs for clinical management of patients with INRs4.5 including those8.0.