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February 1992

Reliance on Prothrombin Time Ratios Causes Significant Errors in Anticoagulation Therapy

Author Affiliations

From the Department of Pharmacology (Drs Bussey and Force) and Division of Neurology (Ms Leonard), The University of Texas Health Science Center at San Antonio; and the College of Pharmacy, The University of Texas at Austin (Dr Bussey); and the College of Pharmacy, Oregon State University, Portland Campus at Oregon Health Science University (Dr Bianco). Dr Force is now with the College of Pharmacy, Ohio (Columbus) State University.

Arch Intern Med. 1992;152(2):278-282. doi:10.1001/archinte.1992.00400140032009

Background.—  The intensity of warfarin anticoagulation in the United States may be inappropriate if the international normalized ratio (INR) is not used, or if the international sensitivity index (ISI) of the thromboplastin is outside the range of 2.2 to 2.6.

Methods.—  Fifty-three hospital laboratories provided data on the sensitivity of their thromboplastin and whether they reported INR values. Additional data on thromboplastin sensitivity were obtained from 140 laboratories involved in the Stroke Prevention in Atrial Fibrillation study. The three major manufacturers of thromboplastin confirmed the range of thromboplastin sensitivity reported by the laboratories.

Results. —  Of 53 laboratories surveyed, 16 (30%) could not provide ISI data and only 11 (21%) reported INR results. Unlabeled thromboplastin was being used by 20% to 24% of laboratories, and only 8% to 20% were using thromboplastins with an ISI of 2.2 to 2.6. At the time the three manufacturers were contacted, they reported marketing thromboplastins with ISI values from 1.2 to 2.8, but none of the thromboplastins at that time had ISI values between 2.2 and 2.6.

Conclusion.—  Warfarin therapy in the United States is managed inappropriately because most laboratories do not report INRs and the variability in thromboplastin sensitivity produces misleading prothrombin time ratio results. Additionally, recent research may require reexamination if INR or ISI data were not provided.(Arch Intern Med. 1992;152:278-282)