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Test Code F9INH Factor IX Inhibitor Evaluation

Reporting Name

Factor IX Inhib Profile

Useful For

Detection and titering of coagulation inhibitor to the specific factor requested, primarily factor IX in patients with hemophilia B

Profile Information

Test ID Reporting Name Available Separately Always Performed
F_9 Coag Factor IX Assay, P Yes Yes

Reflex Tests

Test ID Reporting Name Available Separately Always Performed
IBETH Bethesda Units No No
F9_IS Factor IX Inhib Scrn No No
CCCR Special Coagulation Interpretation No No

Testing Algorithm

Testing begins with coagulation factor IX activity assay with dilutions to evaluate assay inhibition; if the factor IX activity assay is decreased, an inhibitor will be performed to look for specific factor IX inhibition. If specific inhibition is apparent, it will be titered.

Method Name

F_9: Activated Partial Thromboplastin Time-Based Clotting Assay

F9_IS, IBETH: Clot-Based Assay

Performing Laboratory

Mayo Medical Laboratories in Rochester

Specimen Type

Plasma Na Cit

Specimen Required

See Coagulation Studies in Special Instructions.


Specimen Type: Platelet-poor plasma

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

Submission Container/Tube: Plastic vials

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

Collection Instructions:

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

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

3. Freeze specimens immediately at ≤-40 degrees C, if possible.

4. Send specimens in the same shipping container.

Additional Information:

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

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

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

Specimen Minimum Volume

2 mL

Specimen Stability Information

Specimen Type Temperature Time
Plasma Na Cit Frozen 14 days

Reject Due To


Mild OK; Gross reject


Mild OK; Gross reject


Mild OK; Gross reject



Special Instructions

Reference Values


Adults: 65-140%

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

*See Pediatric Hemostasis References in Coagulation Studies in Special Instructions.






0 Units

Day(s) and Time(s) Performed

Monday through Friday; Varies

CPT Code Information

85250-Factor IX activity assay

85335-Bethesda titer (if appropriate)

85335-Factor IX inhibitor screen (if appropriate)

85390-26-Special coagulation interpretation (if appropriate)

LOINC Code Information

Test ID Test Order Name Order LOINC Value
F9INH Factor IX Inhib Profile 3187-2


Result ID Test Result Name Result LOINC Value
F_9 Coag Factor IX Assay, P 3187-2

Specimen Retention Time

See individual unit codes

Method Description

This assay consists of measuring the difference in factor IX activity (partial thromboplastin time 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 IX value of the normal plasma is adjusted to approximately 20%, because the factor IX 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. 2nd edition. Boston, MA, Little, Brown, and Company, 1975, pp 143-145)


If the inhibitor screen is positive for an inhibitor of factor IX, 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 IX activity is measured and compared to a control run at the same time. The difference between the factor IX activity of the patient's incubation mixture and that of the control is used to calculate the titer. The residual factor IX activity is converted to Bethesda units: 50% residual factor IX is equal to 1 Bethesda unit. Assays using the same basic principle as the Bethesda assay are used to quantitate the inhibitors of other coagulation factors.(Kasper CK, Aldedort LM, Counts RB, et al: A more uniform measurement of factor VIII inhibitors. Thromb Diath Haemorrh 1975;34:869-872)

Analytic Time

1 day


This test is not useful for detecting presence of inhibitors directed against other clotting factors and is not useful for the detection of a nonspecific circulating anticoagulant.


This assay will not detect presence of lupus anticoagulants.


If presence or type of inhibitor is unknown, PROCT / Prolonged Clotting Time or LUPPR / Lupus Anticoagulant Profile should be ordered.


Occasionally, a potent lupus-like anticoagulant may cause false-positive testing for a specific factor inhibitor (eg, factor VIII or IX). See preceding caution statement.

Clinical Information

Factor IX inhibitors arise in patients with severe hemophilia B after factor IX transfusion. Patients with factor IX inhibitors may also develop anaphylactic reactions in response to factor IX infusions. Acquired factor IX inhibitors, occurring in previously healthy people, are exceedingly rare.


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.

Test Classification

See Individual Test IDs