False interpretation of diagnostic serology tests for patients treated with pooled human immunoglobulin G infusions: a trap for the unwary.

Therapeutic immunoglobulin G (IgG) products are produced from numerous plasma donations, and are infused in many medical conditions. The serological testing of patients who have received IgG infusions may well produce falsely positive and misleading results from this infused IgG, rather than endogenously produced IgG. We present two example cases of clinical situations where this could cause concern. We tested multiple IgG products with a range of serological tests performed in infective or autoimmune conditions, including hepatitis B, syphilis, human immunodeficiency virus, human T-lymphotropic virus, anti-neutrophil cytoplasmic antibody (ANCA), anti-nuclear antibody (ANA), anti-cardiolipin antibodies and anti-double stranded DNA (dsDNA) antibody. We found positivity within these products for hepatitis B surface and core antibody, syphilis, ANCA, ANA, anti-cardiolipin IgG and dsDNA antibody, which may result from specific or non-specific reactivity. The serological testing of patients who have received IgG treatment detects the administered IgG in addition to IgG produced by the patient.