Factor X Chromogenic Activity Assay, Plasma (FXCH)

Test Code: FXCHSO

Overview
Ordering
Specimen
Performing
Clinical/Interpretive
Contacts
Coding

Useful For

Monitoring warfarin anticoagulant therapy, especially in patients whose plasma contains lupus anticoagulants that interfere with baseline prothrombin time/international normalized ratio and in patients receiving the drug Argatroban who are being transitioned to warfarin

This assay should not be used for monitoring heparin, or oral direct factor Xa inhibitors such as rivaroxaban (Xarelto), apixaban (Eliquis), or edoxaban (Savaysa).

Specimen Requirements

Fasting Required	Specimen Type	Preferred Container/Tube	Acceptable Container/Tube	Specimen Volume	Minimum Volume
No	Platelet-poor Plasma	Citrate Blue Top Tube (BTT)		1 mL	0.5 mL

Collection Processing Instructions

1. Spin down, remove plasma, and spin plasma again.

2. Freeze plasma immediately (no longer than 4 hours after collection) at -20 degrees C or, ideally, < or =-40 degrees C.

Additional Information:

1. Double-centrifuged specimen is critical for accurate results as platelet contamination may cause spurious results.

2. Each coagulation assay requested should have its own vial.

3. If priority specimen, mark request form, give reason, and request a call-back.

Specimen Stability Information

Specimen Type	Temperature	Time
Plasma	Frozen	14 days

Rejection Criteria

Gross hemolysis

Gross lipemia

Gross icterus

Interference

Liver disease and vitamin K deficiency may lower factor X levels. If factor X deficiency is suspected, order F_10/Coagulation Factor X Activity Assay, Plasma.

Performing Laboratory Information

Performing Location	Day(s) Test Performed	Report Available	Methodology/Instrumentation
Mayo Clinic Laboratories	Monday through Friday	1 to 3 days	Chromogenic

Reference Lab

Mayo Clinic Laboratories

Reference Range Information

> or =18 years of age: 60%-140%

Chromogenic Factor X activity generally correlates with the one-stage factor X activity. In full term or premature neonates, infants, and children, the one-stage factor X activity* is lower than adult reference range and progressively rises to the adult reference range by adolescence. However, no similar data for the chromogenic factor X activity have been published.

Interpretation

A chromogenic factor X activity of approximately 20% to 40% corresponds to the usual warfarin international normalized ratio range (ie, 2.0-3.0).

Outreach CPTs

CPT	Modifier (if needed)	Quantity	Description	Comments
85260

Ordering Applications

Ordering Application	Description

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