THE DETECTION OF D-DIMER IN PLASMA BY ENZYME-IMMUNOASSAY - IMPROVED DISCRIMINATION IS OBTAINED WITH A MORE SPECIFIC SIGNAL ANTIBODY

Most commercial D-dimer enzyme immunoassays employ two antibodies, a f ibrin specific capture monoclonal and a less specific antibody cross-r eacting with epitopes present on fibrinogen. Two different ELISA syste ms were compared to test whether this cross-reaction can lead to overe stimation of the true levels of cross-linked fibrin derivatives. Both assays used the same D-dimer specific antibody for capture, DD-3B6/22, but utilized signalling antibodies which differed in fibrinogen react ivity. The assays gave low results for 210 normal samples (conventiona l ELISA, 39 +/- 45 ng/ml; non-fibrinogen reactive ELISA, 23 +/- 20 ng/ ml) with a high degree of elevation for 53 patients with active thromb osis (conventional ELISA, 901 +/- 649 ng/ml; non-fibrinogen reactive E LISA, 1906 +/- 1725 ng/ml). However a dramatic difference between resu lts was seen when fibrinogenolysis was induced by treatment of 20 norm al plasmas with high levels of t-PA and plasminogen. The D-dimer level s estimated with the conventional fibrinogen-reactive signal antibody rose 25-fold (normal, 36 +/- 27 ng/ml; treated, 833 +/- 272 ng/ml), wh ereas no increase was obtained with the non-fibrinogen reactive ELISA (normal, 24 +/- 21 ng/ml; treated, 27 +/- 22 ng/ml). These results sug gest that a more accurate estimation of D-dimer levels in samples cont aining high levels of fibrinogen derivatives is achieved in assays inc orporating non-fibrinogen reactive antibodies.