Solid-phase radioimmunoassay of fragment D of human fibrinogen by use of a low-affinity monoclonal antibody.

Increased concentrations of plasmin-catalyzed degradation products of fibrin and fibrinogen in body fluids have been correlated with many disease states. We have developed monoclonal antibodies that react with one of these products, fragment D. Antibodies binding fragment D but not fibrinogen were found to have low affinity constants (Ka = 10(7) L/mol). A double monoclonal-antibody assay was developed in which a specific antibody attached to Sepharose beads is used to concentrate fragment D selectively from a test sample. Fragment D bound to the washed beads was then quantified with a high affinity (Ka = 10(10) L/mol) monoclonal antibody that reacts equally well with fragment D and fibrinogen. This solid-phase assay is shown to be specific and about 10-fold more sensitive than a standard liquid-phase competition radioimmunoassay in which the same antibody was used.