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When to Use the Chromogenic X Assay

A question about the chromogenic factor X assay from Eryn Cramer:

I am working as a student in an anticoagulation clinic and we are looking for information on the frequency of chromogenic factor X (CFX) monitoring before returning to INR monitoring for patients with antiphospholipid syndrome (APS).  Are there any recomendations for CFX test frequencies?  Also, it appears that the CFX varies depending on the reagents used.  How does that allow for a correlation to the INR, since INR is standardized?

I sent Eryn’s question to Dave McGlasson, who has published a great deal of research on the CFX:

Eryn: Interesting question.  In my opinion no one who has a lupus anticoagulant (LA) that prolongs the PT and does not correct in mixing studies should be monitored using the PT (INR) as it cannot be validated. Likewise, laboratories should not use the clottable factor X method when LA is present, as the thromboplastin for the factor X assay may also be partially neutralized by LA.

The CFX is a valid alternative as it is insensitive to many of the variables that affect the PT and clottable factor X including LA. The therapeutic range 22–40% compares well to an INR of 2.0–3.0.

Although test frequency is the same as you would use for the PT/INR, the question of how long a person should be monitored with the CFX before returning to the INR management is a little involved.  As long as the inhibitor remains demonstrable the PT/INR reagent is likely to give an inaccurate result and should be avoided, and in many cases this is indefinite. Please see our publication; McGlasson DL, Romick BG, Rubal BL. Comparison of a chromogenic factor X assay with international normalized ratio for monitoring oral anticoagulation therapy. Blood Coagulation and Fibrinolysis 2008. 19:513-7.

Two companies distribute CFX kits, Diapharma, whose kit is FDA cleared, and Aniara, whose kit is produced by Biophen.  I have compared these kits using a variety of specimen types and conclude there is no difference statistically or clinically.  In our study, both methods are robust, stable and reproducible.

I’d like to add that it seems the INR may be unreliable above 4.0 even though its formula is standardized. Several studies have shown a remarkable variation in results from identical specimens.  We have completed studies in both humans and swine in which the INRs varied from 3.5 to 7.9 on identical specimens using several test systems. Please see:

McGlasson DL. A comparison of INRs after local calibration of thromboplastin international sensitivity indexes. Clin Lab Sci 2002; 15: 91-5.
McGlasson DL, Brickey DA, Doe RH.  Oral anticoagulant therapy and international normalized ratios in swine.  Lab Animal Sci 1998; 48: 371-3.
McGlasson DL, Hickman JR, More LE, et al.  Discrepancies in international normalized ratios (INR) in swine and humans.  Clin Lab Sc 1998; 11: 156-60.
I hope this helps, and thank you for your question.

David L. McGlasson, MS, CLS/NCA
59th Clinical Research Division
Wilford Hall Medical Center
Lackland AFB, TX 78236-9908

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