Relationship between chromogenic factor X and international normalized ratio differs during early warfarin initiation compared with chronic warfarin administration.
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.
Rosborough
Terry K
TK
Medical Education Department and Coagulation Laboratory, Abbott Northwestern Hospital, Minneapolis, Minnesota 55407, USA.
Jacobsen
Jennifer M
JM
Shepherd
Michele F
MF
D000328:Adult; D000465:Algorithms; D000925:Anticoagulants / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use; D002863:Chromogenic Compounds / Q000032:analysis; D016903:Drug Monitoring / Q000379:methods / Q000706:statistics & numerical data*; D015951:Factor Xa / Q000032:analysis*; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D007297:Inpatients; D019934:International Normalized Ratio; D008297:Male; D009842:Oligopeptides / Q000032:analysis*; D010045:Outpatients; D011517:Prothrombin Time; D013997:Time Factors; D014859:Warfarin / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use
D000925:Anticoagulants; D002863:Chromogenic Compounds; D009842:Oligopeptides; D014859:Warfarin; C015608:benzoyl-Ile-Glu-Gly-Arg-p-nitroanilide; D015951:Factor Xa