[Interference in turbidimetric immunoassay for serum C-reactive protein due to serum protein abnormalities an immune...

Interference in immunoassays for CRP caused by serum protein abnormalities was studied with special reference to turbidimetric immunoassay(TIA) and latex photometric immunoassay(LPIA). One of the interfering factors in TIA is immune complex or agglutinating immunoglobulin which reacts with a chemical component like polyethylene glycol in reagent of the first reaction and causes remarkable turbidity in the initial phase. Since the turbidity decreases gradually and can not be eliminated within the first reaction, the second reaction is affected by the continuing reduction in absorbance, resulting in falsely low CRP values. As examples of this kind of interference, two cases, case 1 and 2 were presented. Case 1 involved malignant lymphoma with paraproteinemia of monoclonal IgA(kappa) and Case 2 was chronic viral hepatitis type C with type II cryoglobulinemia composed of monoclonal IgM(kappa) and polyclonal IgG. Rheumatoid factor(RF) is another factor that interferes in TIA which reacts with antibody in the reagent of the second reaction and causes falsely high CRP values. The effect of RF is especially remarkable in LPIA. As an example, a case of chronic rheumatoid arthritis(Case 3) with polyclonal IgM is presented. Predilution of serum(5-fold dilution with physiological saline solution) was proved to be effective in reducing the intensity of interference in TIA caused by immune complex or agglutinating immunoglobulin in Cases 1 and 2. Covalent binding of IgG(Fab')2 to latex particle instead of physical adsorption of IgG was shown to be efficient in eliminating interference caused by RF in LPIA in Case 3.