EPO and Sleep apnea: Increased erythropoietin (EPO) level can lead to secondary erythrocytosis from different disorders, i.e., chronic lung and heart disease, chronic increase in carbon monoxide due to smoking, sleep apnea, renal cysts and tumors, liver diseases, high-altitude living, and EPO-secreting tumors (cerebellar hemangioblastoma, renal cell carcinoma, hepatocellular carcinoma, or uterine leiomyoma).