Thank you for your inquiry.
As you know, 5 half-lives are required before a drug is eliminated (from a clinical standpoint) from the body. For immunoglobulin administered intravenously, we have very good data as to the half-life of the agents employed. The half-life of intravenous immunoglobulin is usually between 30 and 40 days (1), but may vary with the preparation involved. Unfortunately, there are far less data, at least according to that which I could find, regarding the half-life of subcutaneously administered immunoglobulin. In fact, looking at the package inserts of various preparations, there appears to be no definitive information in this regard.
However, I was able to find one study which looked at this issue and found that the median half-life of the total serum IgG was 40.6 days (2).
From this information, as you can see, perhaps the maximum time for clearance in your patient, at least based upon the single source I could find, would be as long as approximately 200 days. Thus about 6 to 7 months after the last administration would be appropriate.
Thank you again for your inquiry and we hope this response is helpful to you.
1.Immune globulin subcutaneous (human) Vivaglobin. Pharmacy and Therapeutics 2009 (June); 34(6):2-21.
2.Gustafson R, et al. Rapid subcutaneous immunoglobulin administered every second week results in high and stable immunoglobulin G levels in patients with primary antibody deficiencies. Clinical and Experimental Immunology 2008 (May); 152(2):274-279.
Phil Lieberman, M.D.