Comparison of a chromogenic factor X assay with international normalized ratio for monitoring oral anticoagulation therapy

The purpose of the present study was to compare the international normalized ratio with a chromogenic factor X (CFX) assay for monitoring patients on oral anticoagulant therapy using the DiaPharma CFX method on a STA-R Evolution coagulation analyzer. International normalized ratio values were correlated with the CFX for determining normal, subtherapeutic, therapeutic and supratherapeutic ranges for these patients. Specimens were analyzed and grouped as normal or patients on oral anticoagulant therapy with international normalized ratios of less than 2.0, 2.0–3.0, and more than 3.0. Three hundred and nine randomly selected oral anticoagulant therapy patients were tested. The range of international normalized ratio and CFX in oral anticoagulant therapy patients was 0.92–12.76 and 9–132%, respectively. CFX was inversely related to international normalized ratio; R = 0.964 (P < 0.0001) (CFX = 13.2 + (5.3/international normalized ratio) + (81.3/international normalized ratio). Results by group were as followsnormal (n = 30), CFX range 72–131%, mean CFX 96%; international normalized ratio less than 2.0 (n = 70), CFX range 32–132%, mean CFX 53%; international normalized ratio 2.0–3.0 (n = 135), CFX range 18–48%, mean CFX 28%; international normalized ratio more than 3.0 (n = 104), CFX range 9–46%, mean CFX 21%. Sensitivity and specificity crossed at a CFX of 35.5%, which yielded a sensitivity of 91.7% and a specificity of 91.9% for discriminating international normalized ratio of at least 2.0. Area under the curve on receiver–operator curve using international normalized ratio was 0.984 (P < 0.001). In this randomly selected group of oral anticoagulant therapy patients and normal individuals at varying levels of anticoagulation, CFX correlated well with international normalized ratio as determined by R = 0.964. The data suggests that the CFX can be a useful tool for monitoring oral anticoagulation in patient populations in which confounders to international normalized ratio may be present. Further investigation with the use of CFX for monitoring is warranted in large patient populations on oral anticoagulant therapy, including follow-up for clinical outcomes.