Le D T, Weibert R T, Sevilla B K, Donnelly K J, Rapaport S I
University of California, San Diego, La Jolla.
Ann Intern Med. 1994 Apr 1;120(7):552-8. doi: 10.7326/0003-4819-120-7-199404010-00004.
To enhance understanding of the reliability of the international normalized ratio (INR) for monitoring warfarin therapy and its relation to other monitoring techniques.
Prospective cohort study.
A university hospital.
79 patients attending an anticoagulation clinic.
International normalized ratios obtained with a portable capillary monitor (Coumatrak) and the following from a simultaneous plasma sample: INRs from prothrombin times done with six thromboplastins, prothrombin-proconvertin (P&P) test activity, specific prothrombin activity, and native prothrombin antigen.
Converting to INRs failed to standardize prothrombin time results obtained with high- and low-sensitivity thromboplastins. Coumatrak INRs correlated best with INRs obtained with high-sensitivity thromboplastins. The INR range of 2.0 to 3.0 corresponded to a P&P range of 30% to 13%, a native plasma prothrombin antigen range of 56 to 24 micrograms/mL, and a specific prothrombin activity range of 43% to 21%.
Low-sensitivity thromboplastins may give erroneously high INRs in the upper therapeutic range. Plasma prothrombin times should be done with a high-sensitivity thromboplastin, particularly in patients maintained at the upper limit of the therapeutic range. An INR so obtained correlated well with an INR obtained with a portable capillary blood monitor.
