ADAMTS13 Activity Assay

Description: 
The ADAMTS-13 activity assay is a chromogenic ELISA employing a recombinant fragment of a region of the A2 domain of VWF, which contains the cleavage site of ADAMTS-13, the VWF73 region. The process of manufacturing the recombinant VWF73 region involves tagging the N-terminal with glutathione S-transferase (GST), a property that is used in the design of the assay. The ELISA plate is coated with anti-GST antibody and the GST-VWF73 added to act as a substrate. The resultant antibody:antigen complex is then incubated with test samples whereby the ADAMTS-13 cleaves the immobilised recombinant VWF73, exposing the cleavage site. The plate is washed to remove unbound protein and then incubated with a horse radish peroxidise (HRP) conjugated monoclonal antibody to N10, the C-terminal edge residue of the VWF-A2 domain generated after ADAMTS-13 cleavage. The HRP is provided with a substrate, the product of which is coloured, and the degree of colour formation is directly proportional to the ADAMTS-13 activity that cleaved VWF73 and exposed N10.
Clinical details: 
Vascular endothelial cells and platelets contain ultra-large VWF multimers that are highly adhesive. However, they are only transiently detectable in plasma as they are cleaved by the circulating protease ADAMTS-13 (A zinc and calcium dependent Disintegrin and Metalloprotease with ThromboSpondin type 1 motifs, member 13), also known as VWF-cleaving protease. ADAMTS-13 degrades the ultra-large multimers into smaller forms ranging in size from 500 to ~20 000kD.

Deficiency of ADAMTS13 leads to a condition called thrombotic thrombocytopenic purpura (TTP), which can be either congenital or acquired. In congenital TTP, also known as Upshaw-Schulman syndrome, mutations cause deficiency of ADAMTS13 which generally affects synthesis or secretion of the enzyme rather than causing production of dysfunctional molecules. In acquired TTP, an IgG inhibitory antibody prevents the normal action of ADAMTS13. Each mechanism results in an excess of ultra-large VWF multimers being produced from the Weibel-Palade bodies which become anchored to the endothelium, where they can bind platelets via GpIbα on the platelet membrane. This can cause aggregation of platelets which results in microvascular thrombosis and haemolytic anaemia. Coagulation screening tests are usually normal or only slightly disturbed, which helps to distinguish TTP from disseminated intravascular coagulation, though thrombocytopenia and a microangiopathic haemolytic anaemia will be evident from a full blood count and blood film. Haemoglobin is usually below 10.5 g/dL and MCV normal unless there is marked red cell fragmentation, which will be evident from the blood film. The red cell fragments arise from being sheared as they travel past and through the micro-thrombi, which is the cause of the intravascular haemolysis. Reticulocytes are increased, which can increase the MCV, and nucleated red cells can be seen in the peripheral blood, both indicators of a bone marrow response to the haemolysis. Serum lactate dehydrogenase is also elevated. Neurological signs, such as coma, stroke, seizures and even personality change, are a presenting feature due to formation of thrombi in the cerebral circulation.
Reference range: 

66.4 – 107.9 

Synonyms or keywords: 
Thrombotic thrombocytopenic purpura (TTP), ADAMTS13 activity, adamts13, adam
Units: 
%
Sample type and Volume required: 
External requests: Citrated platelet poor plasma
500µl x 1 aliquot
Internal requests: please refer to EPR label
Turnaround time: 
5 - 7 working days.
Special sample instructions: 

Sample should be analysed or manipulated & stored in the laboratory within 4 hours of venepuncture. Please ensure 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.

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Last updated: 08/03/2017