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HelpOrder Code 8INHE Factor VIII Inhibitor Evaluation, Plasma
Ordering Guidance
This test is for factor VIII inhibitors only. If the patient is known to have hemophilia A, this is the correct test to order. If the presence or type of inhibitor is unknown, first order APROL / Prolonged Clot Time Profile, Plasma. When screening studies are needed for patients with known hemophilia B, order 9INHE / Factor IX Inhibitor Evaluation, Plasma.
Shipping Instructions
Send all vials in the same shipping container.
Necessary Information
Specimen Required
Specimen Type: Platelet-poor plasma
Patient Preparation:
1. Patient must not be receiving Coumadin (warfarin) or heparin therapy
2. Fasting preferred
Collection Container/Tube: Light-blue top (3.2% sodium citrate)
Submission Container/Tube: 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.
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, at -40° C or below.
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
Detecting the presence and titer of a specific factor inhibitor directed against coagulation factor VIII
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 |
|---|---|---|---|
| 8INHT | FVIII Inhib Profile Tech Interp | No | Yes |
| F8A | Coag Factor VIII Activity Assay, P | Yes | Yes |
Reflex Tests
| Test ID | Reporting Name | Available Separately | Always Performed |
|---|---|---|---|
| 8AINH | FVIII Inhib Profile Prof Interp | No | No |
| 8BETH | FVIII Bethesda Units, P | No | No |
| F8IS | Coag Factor VIII Assay Inhib Scrn,P | No | No |
Testing Algorithm
Testing begins with coagulation factor VIII activity assay with dilutions to evaluate assay inhibition; if the factor VIII activity assay is normal or increased, a technical interpretation will be provided.
If the factor VIII activity assay is decreased, an inhibitor screen will be performed at an additional charge to look for specific factor VIII inhibition and a professional interpretation will be provided. If specific inhibition is apparent, the titer of the inhibitor will be determined.
Special Instructions
Method Name
F8A, F8IS, 8BETH: Optical Clot-Based
8INHT: Technical Interpretation
8AINH: Medical Interpretation
Reporting Name
Factor VIII Inhib Profile, PSpecimen Type
Plasma Na CitSpecimen Minimum Volume
2 Plastic vials, each containing 1 mL
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 |
Reference Values
FACTOR VIII ACTIVITY ASSAY
Adults: 55-200%
Normal, full-term newborn infants or healthy premature infants typically have levels greater or equal to 40%.*
*See Pediatric Hemostasis References in Coagulation Guidelines for Specimen Handling and Processing.
FACTOR VIII INHIBITOR SCREEN:
Negative
GENERAL FACTOR BETHESDA UNITS:
≤0.5 Bethesda Units
Method Description
The factor VIII assay is performed on the Instrumentation Laboratory ACL TOP using the activated partial thromboplastin time (aPTT) method and a factor-deficient substrate. Patient plasma is combined and incubated with a factor VIII-deficient substrate (normal plasma depleted of factor VIII by immunoadsorption) and an aPTT reagent. After a specified incubation time, calcium is added to trigger the coagulation process in the mixture. Then the time to clot formation is measured optically using a wavelength of 671 nm.(Owen CA Jr, Bowie EJW, Thompson JH Jr. Diagnosis of Bleeding Disorders. 2nd ed. Little, Brown and Company; 1975; Cielsa B. Defects of plasma clotting factors. In: Hematology in Practice. 3rd ed. FA Davis; 2019:chap 17)
The factor VIII inhibitor screen consists of measuring the difference in factor VIII activity (partial thromboplastin 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 VIII value of the normal plasma is adjusted to approximately 20%, because the factor VIII 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 ed. Little, Brown, and Company; 1975:143-145; Cielsa B. Defects of plasma clotting factors. In: Hematology in Practice. 3rd ed. FA Davis; 2019:chap 17)
If the inhibitor screen is positive for an inhibitor of factor VIII, 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 VIII activity is measured and compared to a control run at the same time. The difference between the factor VIII activity of the patient's incubation mixture and that of the control is used to calculate the titer. The residual factor VIII activity is converted to Bethesda units: 50% residual factor VIII is equal to 1 Bethesda unit. Assays using the same basic principle as the Bethesda assay are used to quantitate the inhibitors of the 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; Cielsa B. Defects of plasma clotting factors. In: Hematology in Practice. 3rd ed. FA Davis; 2019:chap 17)
Day(s) Performed
Monday through Friday
Performing Laboratory
Mayo Clinic Laboratories in Rochester
CPT Code Information
85390-Factor VIII Tech Interp
85240-Factor VIII activity assay
85335-Bethesda titer (if appropriate)
85335-Factor VIII inhibitor screen (if appropriate)
85390-Factor VIII Professional Interp (if appropriate)
Forms
If not ordering electronically, complete, print, and send a Coagulation Test Request (T753) with the specimen.