Selective IgM deficiency in an adult presenting with Streptococcus pneumoniae septic arthritis

Septic arthritis caused by Streptococcus pneumoniae is uncommon. Most of the patients who have invasive pneumococcal infection have underlying diseases associated with impaired immune function. We report a case of polyarticular pneumococcal septic arthritis in a previously healthy adult as the first manifestation of selective immunoglobulin (Ig)M deficiency. The patient had no evidence of autoimmune disease or malignancy. Serum IgG, IgA, and complement levels were normal. Numbers of lymphocyte subsets were in normal range except that of CD4+ cells, which was slightly low. Invasive pneumococcal disease in a healthy adult should lead to further investigation for underlying diseases including primary immunodeficiencies.