ADAMTS13 Activity Assay

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.