Minimising changes in plasma calcium and magnesium concentrations during plasmapheresis.

Hypocalcaemic tetany is a known complication of plasmapheresis. It has two causes. Intravenously administered 4.5% human albumin solution (HAS) has no calcium or magnesium, so the replacement of plasma with this fluid depletes these ions. The citrate in fresh frozen plasma (FFP) chelates divalent cations, so the exchange with this at the end reduces the proportion of calcium and magnesium that is ionised. We studied the effect of supplementing HAS with 2 mmol/l calcium chloride and 0.8 mmol/l magnesium sulphate on the changes in ionised and total calcium and magnesium concentrations throughout plasmapheresis. The supplements prevented the falls in these concentrations that is otherwise seen during the HAS infusion, and, thus, the transient fall in ionised calcium concentration induced by the citrate in the FFP was not so profound, reaching 0.92 instead of 0.78 mmol/l (P = 0.002). Supplementation with calcium and magnesium during HAS maintains their balance and prevents tetany during the FFP infusion.