Secondary erythrocytosis due to hemoglobin San Diego.

High-oxygen-affinity hemoglobin variants are a rare clinical entity that can present with secondary erythrocytosis. Herein, the author reports a case of a 41-year-old man with a 4-year history of secondary erythrocytosis of unknown etiology. Physical examination was unremarkable except for plethora of the palms. Myeloproliferative neoplasms and common causes of secondary erythrocytosis were ruled out. The P50 oxygen-hemoglobin dissociation curve was left shifted. Hemoglobin electrophoresis was silent; however, globin mass spectrometry revealed a β-globin variant. Globin sequencing confirmed hemoglobin San Diego. This case highlights the fact that rare high-oxygen-affinity variants should be considered in the differential diagnoses of secondary erythrocytosis.