A COMPARATIVE-STUDY OF LATEX D-DIMER REAGENTS

D-dimers are terminal fragments resulting from plasmin digestion of cr oss-linked fibrin. They are assayed in citrated plasma by agglutinatio n of latex particles coated with monoclonal antibodies directed agains t D-dimer epitopes. In the following study we have compared six: latex D-dimer reagents in several experiments: reactivity with freshly draw n and frozen plasma samples, with so-called ''D-dimer standards'', wit h increasing fibrinogen concentration, with fibrinogen and fibrin spli t products prepared in fresh normal samples and with ''purified'' D an d E fragments. We have also tested the influence of several diluting f luids and the effect of freezing and thawing. We have found remarkable titre differences of D-dimers between the six reagents in fresh and f rozen samples. The reagents do not recognize, or in a titre different from the labelled value, ELISA standards from Ortho and Stago, They do not react with fibrinogen and fibrinogen split products. Self prepare d fibrin split products are recognized by all latex reagents hut with amazing titre differences, also depending on the plasmin digestion tim e. According to our experiments the titre is dependent on the diluting fluid used. None of the tested latex reagents react with pure E fragm ents hut all react, in a rather strong way, with pure D fragments exce pt one. We have tested six commonly available D-dimer latex reagents s howing quite different titre reading of fresh and frozen plasma sample s and of self prepared fibrin split products. Perhaps these results ca n be explained by differences in specificity and/or affinity of the co ating monoclonal antibodies for the D-dimer cross-link, Perhaps some o f the used monoclonal antibodies are also sensitive to the presence of early and intermediate fibrinolysis products and not only to terminal fragments (D-dimers). The reaction of all but one of the reagents wit h purified D fragments is in favour of a lack of specificity of the mo noclonal antibodies for the D-dimer cross-link, We conclude that D-dim er titres of plasma samples assayed hy latex reagents have no absolute value making interlaboratory comparison of D-dimer results impossible especially when different reagents are used. The only meaningful appl ication of such reagents is the follow up of the D-dimer titre in a sa me patient for example during therapy.