Lab Test

Bethesda Inhibitor

Bethesda assay, Factor Inhibitor, Factor VIII Inhibitor, Factor IX Inhibitor, Bethesda Inhibitor (Quantitative Inhibitor Assay) , Antrim #26525, EPIC: LAB5169, SOFT: BETHI


Specimens with hematocrit values greater than 55% must have anticoagulant adjusted. Specimens must be re-collected using a specified whole blood to sodium citrate ratio. Contact the performing Laboratory for specific instructions.

Specimen Collection Criteria

Collect: Two Light Blue-top Becton-Dickinson 3.2% Sodium Citrate tubes.


Physician Office/Drawsite Specimen Preparation

For optimal results, maintain whole blood specimen at room temperature (20-26°C or 68-78.8°F) prior to transport. Transport whole blood to the Laboratory within 4 hours of collection. If transport within 4 hours is not possible, the following procedure must be followed for accurate results:

  1. Centrifuge the capped tubes at 1500 x g for 15 minutes.
  2. Transfer plasma with plastic pipette into a plastic polypropylene centrifuge tube, cap and centrifuge an additional 15 minutes at 1500 x g to obtain platelet poor plasma (PPP) which has a platelet count less than 10 bil/L. Plasma with a platelet count of less than 10 bil/L is critical for accurate results.
  3. Immediately remove only the top two-thirds of the PPP and transfer into a plastic tube.
  4. Freeze (-20°C/-4°F or below) the PPP immediately.
  5. Transport frozen on DRY ICE. Specimen must remain frozen during transport.   

Preparation for Courier Transport

Transport: Whole blood at room temperature (20-25°C or 68-77°F), or plasma obtained through the process listed above, frozen (-20°C/-4°F or below).

Rejection Criteria

  • Glass tubes and 3.8% Sodium Citrate tubes.
  • Specimens that are clotted or grossly hemolyzed.
  • Inappropriate specimen volume.
  • Thawed or partially thawed specimens.  


Monday - Friday, 7:00 a.m. - 3:30 p.m.
Results available within 2 business days.

Reference Range

Less than 1.0 Bethesda Units.

Test Methodology

aPTT. Change in optical density (Siemens BCS Series).


This assay is elevated when circulating inhibitors are present.

Clinical Utility

This assay is used to quantitate specific factor inhibitors, the most common being Factor VIII inhibitors. Factor VIII inhibitors have been associated with hemophilia A, a variety of immunological disorders, and may be an acquired inhibitor in elderly or postpartum patients without underlying disease. The Bethesda assay is also used to quantitate Factor IX inhibitors in Hemophilia B.

CPT Code


Test Codes

Antrim #26525, EPIC: LAB5169, SOFT: BETHI

Last Updated


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This directory currently reflects information only for specimens collected and/or processed at the
Farmington Hills, Grosse Pointe, Royal Oak, and Troy campuses.