[Clinical significance and problems of technique in the immunochemical determination of the third complement component C 3 (beta 1 C/beta 1 A protein) (author's transl)].

Immunochemical determination of the third complement (C) component C 3 as beta 1 C/beta 1 A protein has found extensive use in the clinical evaluation of immunopathological conditions. C 3 changes are associated with inborn C defects and acquired diseases. In the latter case, diminished C3 levels are of the greatest practical relevance. Reduced C 3 values are not necessarily due to "classical" antibody-mediated C activation, as in immune-complex diseases, but may be caused also by "alternative" pathway activation and decreased C 3 synthesis. In addition, important technique factors have to be considered in the evaluation of immunochemically-determined C 3 values. Since the antigenic pattern of C 3 changes during in vitro ageing, not only the state of conversion of C 3 in the sample and in the reference serum (standard), but above all, the specificity of the anti-C 3-serum plays an important role in critically influencing the total error of the method. In order to avoid this error C 3 can be determined as beta 1 C protein using a beta 1 C standard which is, however, not easily available. On the other hand, beta 1 A quantitation causes less problems since standard sera usually contain C 3 as beta 1 A portein. However, an incubation period of 7 days at + 37 degrees C is necessary for complete C 3 conversion. This undesirable time delay has not yet been satisfactorily overcome by in vitro acceleration of C 3 conversion using different substances.