ACCURACY OF LABORATORY AND PORTABLE MONITOR INTERNATIONAL NORMALIZED RATIO DETERMINATIONS - COMPARISON WITH A CRITERION STANDARD

Background: Portable instruments that measure the prothrombin time and automatically calculate the international normalized ratio (INR) with the use of a drop of whole blood have simplified the treatment of pat ients who are receiving warfarin therapy. The accuracy of these portab le monitors has never been determined by comparing INR results with a criterion (gold) standard INR determination. Methods: Duplicate whole- blood INR determinations were made with two commercially available por table INR monitors. Duplicate frozen-plasma samples were measured with four different thromboplastin reagents, each with a different interna tional sensitivity index. The criterion standard INR was determined by using an international reference thromboplastin and the manual tilt-t ube technique. Agreement was evaluated by determining how accurately l aboratory and monitor INR determinations matched criterion standard va lues in designating a sample to be within or outside of currently reco mmended INR target ranges. Results: Two of the laboratory methods, whi ch used relatively sensitive thromboplastins, showed close agreement w ith the criterion standard, whereas two laboratory methods that used l ess sensitive thromboplastin reagents showed poor agreement. Both of t he portable monitors fell between these extremes. The two best laborat ory methods were significantly better (P<.003) than both monitors, whi ch in turn were better (P<.003) than the remaining two laboratories. C onclusions: There is large interlaboratory variation in the accuracy o f INR determinations. Laboratory methods that used insensitive (high i nternational sensitivity index) thromboplastins performed poorly. Accu racy of monitor measurements appears satisfactory.