A 35-year-old man presents to an emergency department (ED) complaining of fatigue of several months’ duration and the development of chest pain. He has a past medical history of metastatic testicular cancer that was successfully treated in 2004; he currently has no evidence of disease. An EKG shows no evidence of myocardial infarction or concern for pulmonary embolism, and a chest radiograph is unremarkable. Laboratory studies revealed WBC 18,000/µL, hematocrit (Hct) 67%, hemoglobin 23 g/dL, and platelets 600,000/µL.
The patient is clinically stable so he is discharged from the ED and told to follow-up with a hematologist.
What is the next step in working up a patient who presents with erythrocytosis?