SULM – Schweizerische Union für Labormedizin | Union Suisse de Médecine de Laboratoire | Swiss Union of Laboratory Medicine

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V Koustosov1 , T Bombeli1 , J Fehr1

1University Hospital of Zürich, Division of Hematology, Dept. Internal Medicine. USZ Gerinnungslabor, Sternwartstrasse 14, 8091 Zürich

Introduction: The classical clinical entity of idiopathic thrombotic thrombocytopenic purpura (TTP) has been closely associated with a severe deficiency of the von Willebrand factor (vWF)-cleaving protease (ADAMTS-13). Levels < 5 % are highly specific for TTP basically excluding hemolytic uremic syndrome (HUS). Deficiency of ADAMTS-13 can either be inherited or, more frequently, caused by inhibitory autoantibodies to ADAMTS-13. The classical picture of TTP with the triad of neurological signs, renal impairment and fever, however, is rather rare. Instead, TTP often manifests oligosymptomatic exhibiting only laboratory signs with thrombocytopenia and hemolysis or also may involve other organs. In such cases the rapid knowledge of the ADAMTS-13 activity is very useful helping to establish a correct diagnosis and to initiate the appropriate therapy of the patient.
Methods&Results: We have therefore designed a rapid, sensitive and simple assay for the determination of ADAMTS-13 activity and ADAMTS-13 inhibitors. The test is based on the measurement of the ristocetin-cofactor activity of urea-denaturated vWF concentrate after incubation with diluted BaCl2-preactivated patient plasma. Modifications of that assay led to the development of a kinetic test that is performed in only 50-60 min. Mixing studies with patient plasma and normal pool plasma revealed that 30 minutes incubation at room temperature is sufficient for most samples to detect an inhibitor. With a single mixing study in ratio of 1:1 it is possible to discriminate inhibitor levels in the range of 0,5-4 BU/ml. The simultaneous determination of the ADAMTS-13 activity and ADAMTS-13 inhibitors has a turn-around time < 2 hours, thereby offering valuable information to the clinician very rapidly.
Conclusion: Using this assay, determination of ADAMTS-13 activity and its inhibitors can be performed easily, rapidly, and accurately, providing simultaneous information about the severity of ADAMTS-13 deficiency and its cause.

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