A New One-Step Method for the Functional Assay of the Fourth Component (C4) of Human and Guinea Pig Complement

Abstract A simple one-step method for the molecular titration of functional C4 has been developed. This method utilizes serum from the C4D strain of guinea pigs as a reagent supplying all components except C4 in relative excess. Guinea pig C4 titers obtained with this method are slightly higher than titers obtained with classical functional method. In assay of human C4, titers obtained are more than 4-fold higher than those obtained with the classical functional method. The correlation between the classical and C4D methods of C4 titration is excellent and the C4D method has less inherent variability as shown by an analysis of variance. Some of the factors which determine the end point in C4 titration by this method have been examined in detail as has been the usefulness of the method for sera of various species. The method has been used in a longitudinal study of C4 levels in nine normal subjects. In this group the range of normal titers was wide. Most, but not all, individuals showed remarkable stability in their C4 titers over long periods of time.