The effect of serum immunoglobulin concentration on immune complex detection by polyethylene glycol

The addition of 4% polyethylene glycol (PEG) to serum and quantitation of immunoglobulins in the dissolved precipitate has been advocated as a simple, reliable method for detecting circulating immune complexes. Because pathological sera, which often yield positive results in this assay, may contain increased concentrations of immunoglobulins compared to normal control sera, we have determined the relationship between total serum immunoglobulin concentration and the quantity of immunoglobulins precipitated by 4% PEG. When IgG was added to normal serum, the quantity and percentage of IgG in the precipitate was directly proportional to total serum IgG concentration. This concentration-dependent precipitation appeared to be unrelated to the presence of aggregates in the IgG preparation, the serum concentration of albumin, or interactions with serum complement. With normal serum, concentrated to yield a wide range of endogenous immunoglobulin concentrations, the percentage of IgG, IgM and IgA in the PEG precipitates was likewise directly proportional to the total serum concentration of these immunoglobulins. In view of these findings, this method is likely to give false-positive results in pathological sera containing increased immunoglobulin concentrations and is probably invalid as a means for detecting circulating immune complexes. However, with a final concentration of 2% PEG, successful discrimination between aggregated immunoglobulin and monomeric IgG may be achieved.