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Test ID 10INE Factor X Inhibitor Evaluation, Plasma

Ordering Guidance

This test is for factor X inhibitors only. If the presence or type of inhibitor is unknown, order APROL / Prolonged Clot Time Profile, Plasma first, except for when screening studies are needed for patients with known hemophilia A or B.

Shipping Instructions

Send all vials in the same shipping container.

Necessary Information

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

Specimen Required

Patient Preparation:

1. Patient must not be receiving Coumadin (warfarin) or heparin therapy

2. Fasting preferred

Specimen Type: Platelet-poor plasma

Collection Container/Tube: Light-blue top (3.2% sodium citrate)

Submission Container/Tube: 3 Plastic vials

Specimen Volume: 3 mL in 3 plastic vials, each containing 1 mL

Collection Instructions:

1. Specimen must be collected prior to factor replacement therapy.

2. For complete instructions, see Coagulation Guidelines for Specimen Handling and Processing in Special Instructions.

3. Centrifuge, transfer all plasma into a plastic vial, and centrifuge plasma again.

4. Aliquot plasma (1-2 mL per aliquot) into 3 separate plastic vials, leaving 0.25 mL in the bottom of centrifuged vial.

5. Freeze plasma immediately (no longer than 4 hours after collection) at -20° C or, ideally, ≤-40° 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.

Useful For

Detection and quantitation of inhibitor to coagulation factor X


This test is not useful for the detection of a lupus-like circulating anticoagulant inhibitor, a nonspecific circulating anticoagulant, or other inhibitors that are not specific for coagulation factors.

Profile Information

Test ID Reporting Name Available Separately Always Performed
10INT FX Inhib Profile Tech Interp No Yes
F_10 Coag Factor X Assay, P Yes Yes

Reflex Tests

Test ID Reporting Name Available Separately Always Performed
10AIH FX Inhib Profile Prof Interp No No
10_IS Factor X Inhib Scrn No No
GBETH General Factor Bethesda Units, P No No

Testing Algorithm

Testing begins with coagulation factor X activity assay with dilutions to evaluate assay inhibition; if the factor X activity assay is normal or increased, then a technical interpretation will be provided.


If the factor X activity assay is decreased, then an inhibitor screen will be performed at an additional charge to look for specific factor X inhibition and a professional interpretation will be provided. If specific inhibition is apparent, the titer of the inhibitor will be determined.

Method Name

Optical Clot-Based

Reporting Name

Factor X Inhib Profile, P

Specimen Type

Plasma Na Cit

Specimen Minimum Volume

2 mL plasma in 2 plastic vials, 1 mL each

Specimen Stability Information

Specimen Type Temperature Time Special Container
Plasma Na Cit Frozen 14 days

Reject Due To

Gross hemolysis Reject
Gross lipemia Reject
Gross icterus Reject

Clinical Information

Coagulation factor inhibitors arise in patients who are congenitally deficient in a specific factor in response to factor replacement therapy, or they can either occur spontaneously without known cause or in response to a variety of medical conditions including the postpartum state, immunologic disorders, certain antibiotic therapies, some malignancies, and advanced age.


Inhibitors of factor VIII coagulant activity are the most commonly occurring of the specific factor inhibitors.

Reference Values


Adults: 70-150%

Normal, full-term newborn infants or healthy premature infants may have decreased levels (≥15-20%) which may not reach adult levels for ≥180 days postnatal.*

*See Pediatric Hemostasis References section in Coagulation Guidelines for Specimen Handling and Processing in Special Instructions.


Normally, there is no inhibitor, ie, negative result.


If the screening assays indicate the presence of an inhibitor, it will be quantitated and reported in Bethesda (or equivalent) units.


Occasionally, a potent lupus-like anticoagulant may cause false-positive testing for a specific factor inhibitor (eg, factor VIII or IX); see Ordering Guidance.

Clinical Reference

1. Hematology: Basic Principles and Practice. Seventh edition. Edited by R Hoffman, EJ Benz Jr, LE Silberstein, et al. Elsevier, 2018

2. Kasper CK: Treatment of factor VIII inhibitors. Prog Hemost Thromb 1989;9:57-86

Method Description

Screening for inhibitors of specific coagulation factors is represented by the inhibitor assay for factor X. This assay consists of measuring the difference in factor X activity (prothrombin time based assay) before and after incubation of a mixture of normal plasma and patient's plasma for 1 hour at 37° C. For optimal sensitivity, the factor X value of the normal plasma is adjusted to approximately 20%, because the factor X assay is more sensitive in this area of the curve. In addition, an excess of patient's plasma will make the test more sensitive to small amounts of inhibitors.(Owen CA Jr, Bowie EJW, Thompson JH Jr: The Diagnosis of Bleeding Disorders. Second edition. Little, Brown, and Company, 1975, pp 143-145; Meijer P, Verbruggen HW, Spannagi M: Chapter 33: Clotting factors and inhibitors: Assays and Interpretation. In Laboratory Hematology Practice. Edited by K Kottke-Marchant. Wiley Blackwell Publishing, 2012, pp 435-446)


If the inhibitor screen is positive for an inhibitor of factor X, the inhibitor will be quantitated by the "Bethesda assay." In the Bethesda procedure, inhibitors are quantified by mixing equal volumes of serially diluted plasma with normal plasma. This mixture is incubated 2 hours at 37° C, and its factor X activity is measured and compared to a control run at the same time. The difference between the factor X activity of the patient's incubation mixture and that of the control is used to calculate titer. The residual factor X activity is converted to "Bethesda units": 50% residual factor X is equal to 1 Bethesda unit.(Kasper CK, Aldedort LM, Counts RB, et al: A more uniform measurement of factor VIII inhibitors. Thromb Diath Haemorrh 1975;34:869-872; Meijer P, Verbruggen HW, Spannagi M: Chapter 33: Clotting factors and inhibitors: Assays and Interpretation. In Laboratory Hematology Practice. Edited by K Kottke-Marchant. Wiley Blackwell Publishing, 2012, pp 435-446)

Day(s) Performed

Monday through Friday

Report Available

1 to 3 days

Test Classification

This test has been modified from the manufacturer's instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information

85390-Factor X Tech Interp

85260-Factor X Assay

85335-Factor X inhibitor Scrn (if appropriate)

85335-Bethesda units (if appropriate)

85390-Factor X Professional Interp (if appropriate)

LOINC Code Information

Test ID Test Order Name Order LOINC Value
10INE Factor X Inhib Profile, P 96457-7


Result ID Test Result Name Result LOINC Value
10INT FX Inhib Profile Tech Interp 69049-5
F_10 Coag Factor X Assay, P 3218-5


If not ordering electronically, complete, print, and send a Coagulation Test Request (T753) with the specimen.