Red blood cell-bound C3d in normal subjects and in random hospital patients

Red blood cells (RBC) drawn into EDTA from 103 normal adults, 26 newborn infants, and 203 randomly selected hospital patients, and into citrate-phosphate-dextrose (CPD) from 87 Red Cross donors, were evaluated for RBC-bound C3d by agglutination with anti-C3d and by a radiolabeled anti-antiglobulin technique. Positive agglutination reactions were observed only with RBC from 26 hospital patients; however, by the radiolabeled antiglobulin method, C3d was demonstrable on all RBC, but its amount varied widely among different subjects. A normal range for RBC-bound C3d was established. RBC-bound C3d levels were not related to the age (18-88 years) or sex of the subjects and remained unchanged over 9 months. Red blood cells taken into CPD had more RBC-bound C3d than cells taken into EDTA; cord RBC had less than adult RBC. In 39 percent of the hospital patients (excluding those with autoimmune hemolytic anemia) RBC-bound C3d was above the normal range and a good correlation was found between values obtained by the radiolabeled anti-antiglobulin method and by agglutination titration scores. There was a threshold level of RBC-bound C3d below which agglutination reactions were negative. Evaluation of RBC-bound C3d in health and disease is important for determining optimal characteristics of anticomplement antiglobulin reagents.