ORIGINAL ARTICLESRelationship between chromogenic factor X and international normalized ratio differs during early warfarin initiation compared with chronic warfarin administrationRosborough, Terry K; Jacobsen, Jennifer M; Shepherd, Michele FAuthor Information Medical Education Department and Coagulation Laboratory, Abbott Northwestern Hospital, Minneapolis, Minnesota, USA Received 21 January, 2009 Revised 7 March, 2009 Accepted 6 April, 2009 Correspondence to Michele F. Shepherd, PharmD, Medical Education Office 11313, Abbott Northwestern Hospital, 800 East 28th Street, Minneapolis, MN 55407, USA Tel: +1 612 863 5149; fax: +1 612 863 3672; e-mail: email@example.com Blood Coagulation & Fibrinolysis: September 2009 - Volume 20 - Issue 6 - p 433-435 doi: 10.1097/MBC.0b013e32832ca31f Buy Metrics Abstract Chromogenic factor X (CFX) monitoring is necessary in patients with potential international normalized ratio (INR) artifacts during warfarin therapy. The relationship of CFX with the INR needs to be quantitated to have warfarin protocols that are equivalent with either test as a monitoring parameter. This study investigated whether the CFX/INR relationship is different during warfarin initiation compared with that during chronic warfarin therapy. Outpatients (N = 164) taking chronic doses of warfarin and inpatients (N = 137) initiating warfarin therapy had plasma samples tested for CFX and INR. The best fit mathematical relationship of CFX and INR was determined for both groups. A six hundred and twenty-five bed, adult-only, private, tertiary care teaching hospital was the setting of the study. The best fit equation for chronic warfarin patients was quadratic using a reciprocal transformation of the INR. The best fit equation for the warfarin initiation patients was linear using logarithmic transformation of CFX and INR. The predicted CFX from INRs over the range of 1.4–2.2 was 7–18% higher in the warfarin initiation patients than in the chronic warfarin patients. Translation of CFX values into equivalent INRs for use in warfarin initiation and maintenance protocols is improved when using equations specific to the patient situation. © 2009 Lippincott Williams & Wilkins, Inc.