Alterations in the half-life and clearance of IgG during therapy with intravenous γ-globulin in 16 patients with severe primary humoral immunodeficiency

Published studies of the metabolism of human IgG using trace amounts of radiolabeled IgG demonstrated that the elimination of native IgG followed first-order kinetics but that the half-life of IgG was shortest in patients with the highest serum concentrations of IgG. To evaluate the effect of increasing the serum concentration of IgG on the metabolism of IgG, we determined the half-life and clearance of IgG and tetanus antibody in 16 patients with severe primary humoral immunodeficiency diseases while they received several doses of intravenous -globulin (IVIG). Each patient received 100 mg/kg of IVIG each month and the half-life and clearance of IVIG were determined by following the decline in the serum IgG concentration. The dose of IVIG was adjusted to give a minimum IgG level of 200 mg/dl and the half-life was reevaluated. The dose was again adjusted to give minimum concentrations of 450 mg/dl and two additional studies were performed. Mean doses of IVIG infused increased from 100 to 346 mg/kg. The mean trough serum IgG concentration was 191 mg/dl on the standard dose and increased to 427 mg/dl at the highest dose. The serum half-lives of IgG were highly variable, ranging from 22 to 96 days. The mean decreased from 43 days in the first to 33 days in the third and fourth studies, and the clearances of IgG increased from 1.8339 to 2.4302 mg/kg/day, but the differences were not statistically significant. Patients with the highest serum IgG concentrations tended to have the longest half-lives, suggesting that intrinsic IgG production might falsely prolong the calculated half-life of IgG. However, it was possible to determine the half-life in these patients by measuring the decline in tetanus antibody. The half-life measured by this technique in the third and fourth studies was 27 and 36 days and the clearances were 2.988 and 3.648 ml/kg/day. Knowledge of the metabolism of IgG in these patients may lead to more appropriate guidelines for determination of dosage.