Fibrin D-dimer testing for venous and arterial thrombotic disease.

The present issue of Seminars in Vascular Medicine assembles a series of articles that nicely elucidate the current knowledge on the use of fibrin D-dimer testing for venous and arterial thrombotic disease in vascular medicine. In the first article Dempfle provides relevant background information on D-dimer assays used for thrombosis exclusion. D-dimer antigen assays are heterogeneous concerning epitope specificity and calibration. Dimerized D-domains (D-dimers), D-trimers, and D-tetramers are needed for the branching of fibrin fibrils to cross-linked fibrin during clot formation. Fragment D-dimer is a specific indicator for plasmin proteolysis of cross-linked fibrin as the essential part of a blood clot. Proteolysis of cross-linked fibrin generates various fragments, which contain dimerized D-domains. Proteolytic fragment of cross-linked fibrin also include fibrin fragments formed by elastase or other proteolytic enzymes. The monoclonal antibodies used in D-dimer antigen assays react with epitopes of fibrin compounds containing dimerized D-domains. Three main types of immune assays are available for the measurement of D-dimers. Qualitative latex agglutination assays are insufficiently sensitive for use in the exclusion of venous thrombosis. The whole blood red cell agglutination uses bivalent antibodies against D-dimer antigen and red blood cells. In the presence of sufficient D-dimer antigen the red cells agglutinate. Automated latex-enhanced light-scattering assays use latex particles coated with monoclonal antibodies against D-dimer antigen and are performed on various automatic laboratory analyzers with photometric detection systems. An automated enzyme-linked immunoassay (ELISA) has been developed for the VIDAS analyzer. Fibrin derivatives detected by D-dimer antigen assays are highly heterogeneous. Manufacturers of D-dimer assays use either purified D-dimer, cross-linked fibrin degradation products, or pooled plasma for calibration. The concentration label is based on either the amount of D-dimer present in the solution or the amount of fibrinogen used for the preparation of fibrin degradation products and expressed as D-dimer units (concentration) or fibrinogen equivalent nuts (FEU). Standardization and harmonization of D-dimer assays using standardized preparation for D-dimer concentration are warranted. In the second article Meijer and Kluft discuss the current status of standardization and harmonization of the available quantitative D-dimer methods. The different quantitative D-dimer tests can differ significantly, which is mainly caused by the variety of fibrin degradation products in plasma, the specificity of antibodies against D-dimer antigen, and the calibrator used in the D-dimer assay. Depending on the stage of degradation of cross linked fibrin in various diseases like diffuse intravascular coagulation or venous thrombosis, the patient samples contain various amounts of large and smaller fibrin degradation and D-dimer products. Depending on the specificity of antibodies used in different D-dimer assays, the mixtures of smaller and larger fibrin D-dimer degradation products are recognized by the different methods with different reactivity. These variations hamper the exchange and comparability of text results of the various quantitative D-dimer methods. Within the context of the scientific standardization committee (SSC) on fibrinogen of the International Society on Thrombosis and Haemostasis (ISTH), standardization of quantitative D-dimer assays including latex immune assays (LIAs) and ELISA was not possible, but harmonization of D-dimer assays using a pooled patient sample seems to be feasible. The principle of harmonization of D-dimer assays is based on the use of a method-specific conversion factor, which is the ratio between the consensus value of all methods included and the value of a particular method for pooled plasma. The authors developed a harmonization model for conversion of the D-dimer results over a wide range by transformation of the measured method-specific D-dimer result to the harmonized D-dimer concentration using the slope and intercept of the method-specific regression line and the reference line regression line. One limitation of the harmonization model is the fact that the reference regression line is based on the results