Clinics and Departments

Laboratory Services

Lab Dept:


Test Name:


General Information

Lab Order Codes:



Factor 8 Inhibitor Normal Plasma

CPT Codes:

85335 – Factor inhibitor test
85325 – Heparin neutralization (if applicable)

Test Includes:

Bethesda Assay, PTT inhibitor/inactivator assay, possibly heparin neutralizaton by hepzyme if necessary


Test Indications:

The assay is designed specifically to measure the concentration of anti-Factor 8 antibodies in patients with hemophilia A. One Bethesda Unit is the amount of antibody that will inactivate 0.5 U/mL (50%) of Factor 8. Patients with spontaneous (acquired) antibodies will give variable results because of the unique conditions of the test and the nature of these types of antibodies.

Lab Testing Section:

Coagulation - Sendouts

Referred to:

Fairview University Medical Center (Fairview/Atlas test: BETH2)

Phone Numbers:

MIN Lab: 612-813-6280

STP Lab: 651-220-6550

Test Availability:

Daily, 24 hours. Performed as needed.

Turnaround Time:

Results are reported within 2 - 4 days.

Special Instructions:

Indicate on request form when the last factor VIII concentrate or cryoprecipitate was administered. No Heparin can be present in the specimen. If the thrombin time is long, hepzyme is added to neutralize the heparin at an additional charge.


Specimen Type:

Whole blood


Light Blue top tube (Buffered Na Citrate 3.2%)

Draw Volume:

Three 3 mL tubes, each containing 2.7 mL blood (8.1 mL). (Minimum volume: Two-3 mL tubes, each containing 2.7 mL blood (5.4 mL))

Processed Volume:

Specimens will be processed at reference lab facility if received within 4 hours of collection. Otherwise, specimens need to be processed at referring facility and provide minimum of two 0.5 mL specimens frozen at -70. Follow directions below under Special Processing.


Routine venipuncture. Do Not use the first 2 mL’s of blood collected. For correct anticoagulant to blood ratio fill to fill line on tube.

If the patient’s hematocrit is >55%, contact laboratory to obtain a special tube. If the patient’s hematocrit is greater than 55%, contact laboratory to obtain a special tube. Mix thoroughly by gentle inversion.

If the patient has a coagulation abnormality, apply direct pressure tot the puncture site for 10 minutes; apply a pressure dressing. Instruct the patient to leave the bandage on for 12 hours.

Special Processing:

Lab Staff: Do Not centrifuge. Do Not freeze. Send to reference lab in original Vacutainer at room temperature if within 4 hours of collection. Forward promptly.

If specimen is >4 hours old prior to shipment, process specimen as follows:
Spin sample collected in blue top tube(s) for 5 minutes on the
Stat Spin centrifuge, remove plasma and transfer to a 4 mL BCS sample cup(s), spin remaining plasma again for 5 minutes in the Stat Spin Centrifuge. Transfer plasma into two labeled 10x75 mL plastic tubes with a minimum of 0.5 mL in each. Freeze at -70 and send specimens on dry ice to Fairview University.

Patient Preparation:


Sample Rejection:

Patient on heparin or collection in a heparin containing tube; unprocessed specimens greater than 24 hours old or received frozen; clotted specimens; insufficient volume; mislabeled or unlabeled specimens


Reference Range:

0 Bethesda Units/mL (Beth U/mL)

Critical Values:



Recent Factor 8 infusion may invalidate results.


Measurement of residual factor VIII on mixtures of patient plasma mixed with normal pooled plasma and incubated for 2 hours at 37C.


Fairview University Laboratory Web Page February 2014


6/2/2004: Draw volume changed from three 5 mL tubes to three 3 mL tubes.
5/24/2010: Tubing of patient specimens is no longer prohibited.
2/24/2014: Specimen processing update.

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