AbstractBackground and ObjectivesImmunoadsorptions (IA) are used to remove autoantibodies from the plasma in autoimmune disorders. In this study, we evaluated the effects of a single‐use, recombinant staphylococcal protein A‐based immunoadsorber on blood composition of the patient.Materials and MethodsIn a cohort of patients with myasthenia gravis or stiff‐person syndrome, essential parameters of blood cell count, coagulation, clinical chemistry or plasma proteins and immunoglobulins (Ig) were measured before and after IA (n = 11).ResultsIn average, IA reduced the levels of total IgG, IgG1, IgG2 and IgG4 by approximately 60%, the acetylcholine receptor autoantibody levels by more than 70%. IgG3, IgA or IgM were diminished to a lower extent. In contrast to fibrinogen or other coagulation factors, the column markedly removed vitamin K‐dependent coagulation factors II, VII, IX and X by approximately 40%–70%. Accordingly, international normalized ratio and activated partial thromboplastin time were increased after IA by 59.1% and 32.7%, respectively. Coagulation tests almost returned to baseline values within 24 h. Blood cell count, electrolytes, total protein or albumin were not essentially affected. No clinical events occurred.ConclusionThe single‐use, multiple‐pass protein A adsorber column is highly efficient to remove IgG1, IgG2 and IgG4 or specific acetylcholine receptor autoantibodies from the plasma. Coagulation parameters should be monitored, since the column has the capacity to largely reduce vitamin K‐dependent factors.