BACKGROUND Marked decreases in ionized magnesium (iMg) levels occur during large-volume leukapheresis (LVL); however, the effect of intravenous (IV) magnesium supplementation in this setting has not been carefully studied. STUDY DESIGN AND METHODS Thirty healthy allogeneic peripheral blood progenitor cell donors receiving citrate anticoagulant with IV calcium prophylaxis were randomized to receive either IV magnesium (0. 2 mg Mg per
mL acid citrate dextrose-A) or placebo during LVL, with a double-blind design. RESULTS Thirty subjects underwent 75 LVL pro- cedures, 37 with magnesium and 38 with placebo. Group characteristics were similar for sex, weight, citrate infusion rate (1. 36 mg/kg/min vs. 1. 37 mg/kg/min), and volume processed (16 L vs. 17 L). Serum iMg levels remained within the reference range with magnesium supplementation, but decreased 39+/-11 percent below baseline (p<10(-10)) after placebo, with greater decreases after consecutive procedures. Subjects receiving magnesium had more vigorous parathyroid hormone responses and higher glucose levels and also tended to have higher serum potassium and ionized calcium levels. Mild paresthesias, coldness, and nausea occurred in 28, 20, and 7 percent of donors, respectively, with no significant differences between groups. Severe symptoms (chest tightness) occurred in only one subject receiving placebo. CONCLUSION IV magnesium supplementation exerts a significant impact on serum magnesium levels, but does not reduce the frequency or severity of the relatively mild citrate-related effects observed in LVL performed with continuous IV calcium prophylaxis.