Factor VIII inhibitor profile (Nijmegen-Bethesda & ELISA)

Description: 
FVIII inhibitors are progressive so dilutions of test plasma (i.e. reducing concentrations of inhibitor) are incubated for 2 hours at 37°C in the presence of a fixed amount of exogenous FVIII in the Bethesda assay. One unit of inhibitor is defined as the amount of inhibitor that will neutralise 50% of a normal plasma containing 100% FVIII after the 2 hour incubation. The amount of residual FVIII in each tube is used to calculate the inhibitor level.

Our laboratories employ a modified Nijmegen-Bethesda assay which uses buffered normal plasma and dilutions in 4% BSA to reduce pH drift and maintain constant protein concentration, that can otherwise falsely suggest the presence of a weak inhibitor.

FVIII antibodies can also be detected using indirect enzyme-linked immunosorbent assay (ELISA). FVIII antibodies in patient sera, whether inhibitory or 'immune', are captured in microtitre plates coated with recombinant FVIII. Unbound material is then washed off and a solution of antibody to human immunoglobulin that is conjugated to an enzyme is added to ‘tag’ onto any captured FVIII antibodies. Unbound conjugate is washed off and a substrate for the enzyme is added, the product of the enzyme-substrate reaction being coloured. Colour intensity is in direct proportion to the degree of conjugate-binding, itself proportional to the amount of FVIII antibody capture. The assay is not quantitative and is reported as either positive or negative based on whether the colour intensity exceeds a kit-specific cut-off value.
Clinical details: 
Acquired inhibitors to FVIII are antibodies that arise in 10-15% of patients with hereditary haemophilia A as a result of recognising therapeutic FVIII products as 'foreign' protein, or in patients with previously normal haemostasis as part of an autoimmune process. Inhibitors in congenital haemophiliacs complicate treatment as the infused FVIII has a shorter survival time, and patients with acquired haemophilia can present with severe bleeding. Some patients generate antibodies that are not inhibitory but may cause increased clearance upon removal of immune complexes from the circulation.
Reference range: 

Not detected

Units: 
Nijmegen-Bethesda NBU/ml
Sample type and Volume required: 
External requests: Citrated platelet poor plasma
150µL and 1mL x 2 aliquots separately
Internal requests: please refer to EPR label

Turnaround time: 
7 - 10 days
Special sample instructions: 

The samples should be analysed or manipulated & stored in the laboratory within 4 hours of venepuncture. Please ensure citrate sample tubes are filled exactly to the fill-line as underfilling creates a dilution error and leads to inaccurate results.

Contacts:
Diagnostic Haemostasis and Thrombosis Department
020 7188 2797
St Thomas' Hospital
North Wing - 4th and 5th Floors
Westminster Bridge Road
London SE1 7EH

Laboratory opening times
24/7
For clinical advice or interpretation of results, please contact the laboratory in the first instance.

Print as a PDF

Last updated: 09/03/2017