Dt. Le et al., THE INTERNATIONAL NORMALIZED RATIO (INR) FOR MONITORING WARFARIN THERAPY - RELIABILITY AND RELATION TO OTHER MONITORING METHODS, Annals of internal medicine, 120(7), 1994, pp. 552-558
Objective: To enhance understanding of the reliability of the internat
ional normalized ratio (INR) for monitoring warfarin therapy and its r
elation to other monitoring techniques. Design: Prospective cohort stu
dy. Setting: A university hospital. Patients: 79 patients attending an
anticoagulation clinic. Measurements: International normalized ratios
obtained with a portable capillary monitor (Coumatrak) and the follow
ing from a simultaneous plasma sample: INRs from prothrombin times don
e with six thromboplastins, prothrombin-proconvertin (P&P) test activi
ty, specific prothrombin activity, and native prothrombin antigen. Res
ults: Converting to INRs failed to standardize prothrombin time result
s obtained with high- and low-sensitivity thromboplastins. Coumatrak I
NRs correlated best with INRs obtained with high-sensitivity thrombopl
astins. 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 mu g/mL
, and a specific prothrombin activity range of 43% to 21%. Conclusions
: Low-sensitivity thromboplastins may give erroneously high INRs in th
e upper therapeutic range. Plasma prothrombin times should be done wit
h a high-sensitivity thromboplastin, particularly in patients maintain
ed at the upper limit of the therapeutic range. An INR so obtained cor
related well with an INR obtained with a portable capillary blood moni
tor.