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

IntroductionPatients 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 INRs > 4.5; 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.MethodsINRs 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.ResultsThere was good correlation between POC (INR ≥ 4.5, < 8.0) and Lab INRs (rTF n = 154, rs = 0.87, p < 0.0001; RBT n = 102, rs = 0.76, p < 0.0001); and significant correlations between each of the VKD-CF and the INR, the strongest being with FVII (POC INR rs = -0.53 p < 0.0001; Lab rTF-INR rs = -0.70 p < 0.0001). TGT peak thrombin and ETP also showed good correlations with INR values (R2 > 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.ConclusionsOur data suggest that CoaguChek XS Plus INRs > 4.5 and < 8.0 are comparable to laboratory INRs (both methods) and it is probably unnecessary to perform laboratory INRs for clinical management of patients with INRs > 4.5 including those > 8.0.