False-Positive reaction for Fibrin degradation products due to a Monoclonal (IgM Lambda) Cryoglobulin with warm-reactive antibody activity for Rabbit IgG

The serum of a patient with prolymphocytic leukemia and a hyperviscosity syndrome contained high levels of IgM, with a monoclonal peak on protein electrophoresis, and had a high titer for fibrin(ogen) degradation products, using a rabbit anti-fibrinogen reagent, without other evidence for disseminated intravascular coagulation. A monoclonal IgM lambda cryoglobulin was identified. It retained reactivity with the fibrin(ogen) degradation products reagent, but failed to react with human IgG by latex fixation. Binding activity for rabbit IgG (and absence of binding activity for human, goat, and bovine IgG) was also demonstrable by double diffusion in agarose, and by enzyme-linked immunoassay. By enzyme-linked immunoassay, binding of both F(ab) 2 and Fab and, to a lesser extent, Fc fragments of rabbit IgG was found. Increased binding of heat-denatured rabbit IgG was observed, without inhibition by similarly denatured IgG of other species. The false-positive fibrin(ogen) degradation products reaction in the serum of this patient was due to a unique monoclonal IgM lambda cryoglobulin with warm-reactive antibody activity for an epitope displayed on native and denatured rabbit IgG, but absent on native or denatured human, goat, or bovine IgG, and present in both F(ab) 2 and Fc regions of the IgG molecule.