Prolonged diagnostic journey in delayed-onset adenosine deaminase deficiency.

Adenosine deaminase (ADA) deficiency typically presents as a severe combined immunodeficiency in early infancy, although its onset may be delayed in some cases. We encountered two patients diagnosed with ADA deficiency in adulthood. In addition to previously reported cases, we aimed to identify and characterize the clinical and immunological features associated with delayed- and late-onset ADA deficiency. Both patients presented with pneumonia and hypothyroidism during early childhood. The patients were subsequently treated with periodic immunoglobulin replacement and levothyroxine therapy. They experienced recurrent infections, including pneumonia and shingles, and were diagnosed with ADA deficiency in adulthood. A literature review revealed that patients diagnosed after the age of 10 years had a median interval of 18 years from disease onset to diagnosis. Patients with combined immunodeficiency and recurrent lower respiratory tract infections or autoimmune diseases require early measurement of ADA activity or genetic analysis.