The international normalized ratio (INR) for monitoring warfarin therapy: reliability and relation to other monitoring methods.
OBJECTIVE
To enhance understanding of the reliability of the international normalized ratio (INR) for monitoring warfarin therapy and its relation to other monitoring techniques.


METHODS
Prospective cohort study.


METHODS
A university hospital.


METHODS
79 patients attending an anticoagulation clinic.


METHODS
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.


RESULTS
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%.


CONCLUSIONS
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.
Le
D T
DT
University of California, San Diego, La Jolla.
Weibert
R T
RT
Sevilla
B K
BK
Donnelly
K J
KJ
Rapaport
S I
SI
D001324:Autoantigens / Q000097:blood; D002196:Capillaries; D016903:Drug Monitoring / Q000379:methods / Q000592:standards*; D005167:Factor VII / Q000378:metabolism; D006801:Humans; D011446:Prospective Studies; D011516:Prothrombin / Q000276:immunology; D011517:Prothrombin Time; D012044:Regression Analysis; D013925:Thromboplastin / Q000378:metabolism; D014859:Warfarin / Q000627:therapeutic use*
D001324:Autoantigens; D014859:Warfarin; D005167:Factor VII; D011516:Prothrombin; D013925:Thromboplastin