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

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Abstract
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 [Canadian] 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 yr) or sex of the subjects and remained unchanged over 9 mo. 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% 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 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.