Summary
The recommended therapeutic range of International Normalized Ratio (INR) for oral
anticoagulant treatment in patients with the antiphospholipid syndrome remains controversial.
As a part of this controversy, it has been suggested that lupus anticoagulants (LA)
could interfere with the determination of prothrombin time, thus questioning the validity
of monitoring the treatment of these patients using INR. To clarify this point, we
compared the values of INR obtained in the plasmas of two groups of patients, one
without LA (n = 47), and the other with LA (n = 43). INR were determined using 8 different
thromboplastin reagents on the same automated coagulation instrument. Chromogenic
factor X, which is supposed to be insensitive to the presence of LA, was also measured.
The results are the following: provided INR was calculated using calibrated reference
plasmas, there was no significant difference between INR values obtained with the
8 reagents, both in the non-LA and in the LA groups (CV: 5.9 and 6.7%, respectively).
Closer examination revealed that INR results obtained with one reagent (the recombinant
thromboplastin Innovin) diverged from those of the 7 others, leading to an overestimation
of INR, to a very large extent in some instances. However this effect was restricted
to a subset of the patient population with LA (6 out of 43). Finally, the relationship
between INR (average value obtained using the 8 reagents) and factor X was identical
in non-LA and in LA patient groups. We conclude that, provided the reagents which
display the LA interference are identified and excluded for this purpose, the INR
system is valid for monitoring oral anticoagulant treatment in patients with LA.
