Factor X or anti-Xa Assay. Which Do I Use?

Factor X or anti-Xa Assay. Which Do I Use?

David L. McGlasson, MS, MLS(ASCP)cm, GA Fritsma, MS, MLS(ASCP)cm
59th Clinical Research Division
JBSA Lackland, TX

What is the therapeutic range for Factor X?

Each lab must determine its own reference range. Studies have shown that normal FX levels are about 70 – 130%, and therapeutic range is about 20 – 40%. A list of reference ranges found in the literature and at various reference laboratories can be obtained from DiaPharma.

What CPT code should I use for Factor X?

You can bill to CPT Code 85260

What can I use for calibrators & controls for Factor X?

Currently there are no calibrators or controls on the market that are assayed for chromogenic FX, just clotting FX. Most labs use normal plasma calibrators and controls. If you calibrate your assay using normal plasma, you can determine the % of normal FX in your patient sample. Calibration and control plasma is available from DiaPharma that are traceable to the International Standards supplied by NIBSC in accordance with WHO recommendations.

Do you have applications for the DiaPharma Factor X kit on automated coagulation analyzers?

Yes, we have applications for the ACL Advance, AMAX, BCS, STA, STA Compact, STA-R, and SysmexCA 1500, as well as for microplates. Our applications library is growing, so please check with DiaPharma for the latest updates.

Why can’t I just use a FX clotting assay?

Presence of a lupus inhibitor or thrombin inhibitor can interfere with the clotting assay. LA patients can produce antibodies that interfere with the phospholipid-dependent clotting reactions that are part of most PT assays. In contrast, the chromogenic factor X reaction does not require a phospholipid membrane surface, and the presence of an LA or DTI does not affect the chromogenic factor X assay.

What is the utility of the DiaPharma Factor X kit? Am I expected to use this on ALL my Coumadin patients?

No, only for those where the INR is unreliable and unstable. For example, it is useful in patients with lupus anticoagulant where the LA falsely prolongs the PT, therefore giving unreliable INRs. Also, Argatroban has been shown to prolong the PT, so patients being bridged from Argatroban to warfarin can be monitored using the DiaPharma Factor X kit.

What is the DiaPharma Factor X kit?

The DiaPharma Factor X Kit is an in vitro diagnostic test kit for the quantitative determination of Factor X activity in human citrated plasma. Factor X activity is useful for monitoring patients on oral anticoagulant therapy (warfarin) where baseline INR values are prolonged and INR results are not reliable, such as in OAC patients with lupus inhibitors. The DiaPharma Factor X Kit is also useful for screening for factor X deficiencies.

If I want to determine FX in plasma, but do not want to buy a kit, what other options are there?

S-2337™ was previously included in the now discontinued Chromogenix Coatest® FX kit, so another substrate must be employed. S-2765™, which is also a substrate for FXa, can therefore be used, and a method based on the activation of FX in the presence of calcium using Russell’s Viper Venom (RVV) as the activator is described in the Chromogenix catalog. DiaPharma has composed a new DiaPharma Factor X kit based on S-2765™.

Protein concentrations in plasma


Weight kDa



Fibrinogen 330 3000 9
Prothrombin 72 150 2
Factor V 330 20 0.05
Factor VII 50 0.5 0.01
Factor VIII 330 0.1 0.0003
Factor IX 56 5 0.09
Factor X 59 8 0.13
Factor XI 160 5 0.03
Factor XII 80 30 0.4
Factor XIII 320 10 0.03
Protein C 62 4 0.06
Protein S 70 10 (free) 0.14
Protein Z 62 2 0.03
Prekallikrein 86 50 0.6
HMW kininogen 120 70 0.6
Fibronectin 450 300 0.7
Plasminogen 92 200 2
t-PA 60 0.005 0.0001
Urokinase 53 0.004 0.0001
Antithrombin 58 145 2.5
Heparin Cofactor II 66 80 1.2
Plasmin Inhibitor 63 60 1
Protein C Inhibitor 57 4 0.07
α2-Macroglobulin 725 2000 3