Chromogenix factor X assay

In 1997, Moll and Ortel published a study concluding “lupus anticoagulants (LAs) can influence prothrombin times (PTs) and lead to INRs that do not accurately reflect the true level of [oral anticoagulant therapy (OAT)]. Use of the INR to standardize PTs is invalid for some patients with lupus anticoagulants. To prevent supratherapeutic or subtherapeutic anticoagulation, these patients must be individually monitored with a test that is insensitive to lupus anticoagulants.” They concluded: “chromogenic factor X levels and prothrombin-proconvertin times correlated well with each other and with established therapeutic ranges.” This study established an oral anticoagulant therapeutic range of 22-40% for the chromogenic factor X assay. The reference is Moll S, Ortel TL. Monitoring warfarin therapy in patients with lupus anticoagulants. Ann Intern Med 1997;127:177-85.

We expanded on the work of Moll and Ortel with two studies to support substituting the chromogenic factor X (CFX) assay for the PT/INR in the presence of LA.In our initial study, we correlated DiaPharma® CFX and Diagnostic Stago® PT/INR results from 309 stable anticoagulation clinic patients using an STA-R Evolution® coagulation analyzer. Results were distributed to sub-therapeutic (n=70; INR <2.0), therapeutic (n=135; INR 2.0-3.0), and super-therapeutic (n=104; INR >3.0) ranges. We also assayed 30 non-OAT normal subjects.

The normal subject specimens generated a CFX range of 72-132%, mean 96%. The ranges for INR and CFX in OAT subjects were 0.92-12.76 and 9-131%, respectively. Regression revealed an inverse relationship with R = 0.964, p <0.0001. The sub-therapeutic arm generated a CFX range of 32-132%, mean 53%; therapeutic CFX range 18-48%, mean 28%; super-therapeutic CFX range 9-46%, mean 21%. Sensitivity and specificity crossed at 35.5% activity, which appears equivalent to an INR > 2.0. The CFX clinical sensitivity and specificity for INR > 2.0 were 91.7% and 91.9%, respectively and the area under the receiver-operating characteristic curve was 0.984. Our data suggest the CFX may be effectively employed to monitor OAT in an anticoagulation clinic population.

To address monitoring OAT in the presence of LA, we compared the CFX to a clottable factor X assay in McGlasson D, Shaklee P, Fanella T. A multi-laboratory evaluation of a chromogenic factor X assay for monitoring oral anticoagulation therapy. Clin Chem 2005; 51: D-34.

In this second study we compared a minimum of 6 repeat assays of 19 OAT patients (152 discrete assays) with LAs and unstable INRs using the DiaPharma CFX and Diagnostica Stago clot-based factor X assay (DFX). The DFX range was 2.7-101%, mean 22.8%, CFX 7.0-122.0, mean 33.1%, R = 0.841. The INR range was 0.97-4.5. Weak correlation and difference in means suggest the CFX was unaffected by LAs.

We propose to expand on these studies by including additional OAT patients with unstable INRs and LAs, and patients converting from direct thrombin inhibitors Argatroban, Lepirudin, and Bivalirudin to OAT.

It should be noted the name “chromogenic factor X assay” may easily be confused with “chromogenic anti-Xa assay,” which is used to monitor standard unfractionated heparin, low molecular weight heparin, and pentasaccharide formulations. This distinction should be highlighted to ordering clinicians.