Therapeutic immunoglobulin should be dosed by clinical outcome rather than by body weight in obese patients.

There are currently no data to support the suggestion that the dose of therapeutic immunoglobulin (Ig) should be capped in obese patients for pharmacokinetic (PK), safety and economic reasons. We compared IgG trough levels, increment and efficiency in matched pairs of obese and lean patients receiving either replacement or immunomodulatory immunoglobulin therapy. Thirty-one obese patients were matched with a clinically equivalent lean patient across a range of indications, including primary antibody deficiency or autoimmune peripheral neuropathy. Comprehensive matching was carried out using ongoing research databases at two centres in which the dose of Ig was based on clinical outcome, whether infection prevention or documented clinical neurological stability. The IgG trough or steady state levels, IgG increments and Ig efficiencies at times of clinical stability were compared between the obese and lean cohorts and within the matched pairs. This study shows that, at a population level, obese patients achieved a higher trough and increment (but not efficiency) for a given weight-adjusted dose compared with the lean patients. However at an individual patient level there were significant exceptions to this correlation, and upon sub-group analysis no significant difference was found between obese and lean patients receiving replacement therapy. Across all dose regimens a high body mass index (BMI) cannot be used to predict reliably the patients in whom dose restriction is clinically appropriate.