Purpose of review: Subcutaneous IgG (SCIG) is becoming more popular, but there is still uncertainty about efficacy and optimal dosing. This review discusses recent pharmacokinetic studies and applications of SCIG therapy, and its efficacy in the context of emerging understanding of the relationship between dosing and efficacy of both intravenous IgG (IVIG) and SCIG replacement therapy for primary immunodeficiency diseases.
Recent findings: Three preparations of IgG have been licensed in the US in the past year. Their bioavailabilities are 65–70% of that of IVIG. Pooled analyses show that the efficacy of SCIG in preventing infections is proportional to the steady-state levels achieved, and similar to that of IVIG. Pharmacokinetic studies allow estimation of doses that will yield desired serum levels with both IVIG and SCIG, and when switching from one route to another.
Summary: Pooled analyses show that at equivalent total doses, weekly SCIG results in steady-state levels 10–20% higher than troughs on monthly IVIG. For most patients, the choice between routes should be based on individual preference, and the regimen should be individualized to achieve the desired outcomes.