The measurement of C-reactive protein in human sera; comparison of the clinical tests on the basis of a quantitative method.

C-reactive protein is a substance which appears in the blood in the course of a variety of disease processes. The existence of this protein in serum was first recognized as a result of its property of reacting to form a precipitate in the presence of the somatic C polysaccharide of Pneumococcus (1), and this reaction is responsible for the name which is at present used for the protein. Since the original description of the reaction, a number of papers have appeared concerning the purification and properties of this protein, and the range of diseases in which it is present in the blood (2-6). The determination of C-reactive protein in the blood as a measure of the activity of the disease process in acute rheumatic fever has been described by Anderson and McCarty (7). In this instance, a specific rabbit antiserum to human Creactive protein was employed to detect the presence of the protein in patients' sera by means of a precipitin test, and the results of this test were correlated with clinical and other laboratory data in a series of 45 patients with acute rheumatic fever. Like the erythrocyte sedimentation rate and the white blood cell count, the test for C-protein is not a diagnostic test for rheumatic fever, since it is positive in other infectious and non-infectious diseases. However, in this series of cases it appeared to be a more sensitive index of disease activity than any other that was available. The antigen-antibody test for estimating C-reactive protein is extremely sensitive in detecting small amounts of the substance in serum, but the procedure used is at best only semi-quantitative. The results are recorded on the basis of the volume of precipitate formed, ranging from zero in the absence of reaction to ++++++ in the case of a maximal reaction. It is obvious that readings of this sort may not be the same when made by different observers, and furthermore different lots of antiserum vary in the amount of precipitable antibody which they contain. In the present study an attempt has been made to obtain a more quantitative estimate of the amount of C-protein present in various sera, and to correlate the quantitative data with the results of the precipitin reaction. The general use of the C-protein precipitin test with rabbit antiserum as a clinical laboratory test in rheumatic fever is at present limited because of the difficulties involved in obtaining pure Cprotein preparations in quantities which allow large scale production of rabbit antiserum. An alternative to the use of rabbit antiserum for the detection of the C-reactive protein in serum is the original precipitation reaction which employs the relatively easily prepared C polysaccharide of the Pneumococcus (8). The latter reaction is best performed as a ring test in which a dilute solution of the carbohydrate is carefully layered on the serum to be tested. A visible precipitate forms at the interface, and the density of the ring is roughly proportional to the amount of C-protein present in the serum. The reaction is never obtainable between C polysaccharide and normal serum. Many earlier determinations done in this laboratory indicated that the carbohydrate does not give a visible reaction in the presence of low concentrations of C-protein which are readily detectable with the antiserum. In the present study a comparison of the sensitivity of the two indicator systems, C carbohydrate and antiserum, is presented. Because the capillary precipitin reaction between the C-protein of serum and the rabbit antiserum is only roughly quantitative, the desirability of determining the actual amount of protein responsible for the different titers of precipitin reaction given by patients' sera is evident. This determination has been undertaken in the present study.