Myeloperoxidase (MPO) deficiency is a rare inherited (genetic) primary immunodeficiency disorder characterized by a reduction or lack of the immune enzyme, myeloperoxidase, in white blood cells (neutrophils and monocytes). Myeloperoxidase deficiency causes an impaired ability for the immune system to kill microbes.

Most individuals with myeloperoxidase deficiency do not have any symptoms. However, myeloperoxidase deficiency may lead to severe infections, in particular fungal infections, such as Candida albicans, especially in individuals who also have diabetes mellitus.

Most individuals with myeloperoxidase deficiency do not need any treatment. Patients who also have diabetes mellitus and myeloperoxidase deficiency may need rapid treatment with antibiotics for localized and systemic infections, especially fungal infections, such as Candida albicans.