Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 68-year-old man is evaluated for a 3-year history of dyspnea on exertion. He experiences no headaches or blurred vision. Medical history is notable for a stroke 2 years ago. He is a smoker with an 80-pack-year smoking history. Medications are hydrochlorothiazide, lisinopril, aspirin, and simvastatin.

On physical examination temperature is 36.7 °C (98.0 °F), blood pressure is 145/84 mm Hg, pulse rate is 88/min, and respiration rate is 16/min. Oxygen saturation breathing ambient air is 88%. He has facial plethora. He has no carotid bruits. Cardiac sounds are distant. Pulmonary examination reveals distant breath sounds with scattered wheezing. No hepatosplenomegaly is palpated. No digital clubbing is observed.

Laboratory studies show a hemoglobin level of 18.2 g/dL (182 g/L), leukocyte count of 8000/µL (8 × 10⁹/L) with a normal differential, and platelet count of 225,000/µL (225 × 10⁹/L). Erythropoietin level is 30 mU/mL (30 U/L).

The patient is advised to quit smoking.

Which of the following is the most appropriate next step in management?

A: Bone marrow biopsy
B: JAK2 V617F testing
C: Phlebotomy
D: Supplemental oxygen

MKSAP Answer and Critique

The correct answer is D: Supplemental oxygen.

The patient should receive supplemental oxygen. He probably has secondary erythrocytosis due to hypoxic lung disease and may benefit from oxygen therapy based on his documented hypoxemia (arterial PO₂ ≤55 mm Hg [7.3 kPa] or arterial oxygen saturation ≤88%). The erythropoietin level is elevated owing to hypoxemia, causing an erythrocyte mass. No splenomegaly is present on physical examination, and the lung examination suggests COPD.

Because of the patient’s elevated erythropoietin level and hypoxemia, a bone marrow biopsy is not required to exclude polycythemia vera (PV). No leukocytosis, basophilia, or thrombocytosis is seen on laboratory studies, all of which are common in PV.

The JAK2 V617F activating mutation defines PV, and 95% of PV harbors this mutation, with the other 5% presenting with variations of the mutation. NoJAK2-negative PV exists. However, JAK2 testing is expensive and is not required when a clear cause of secondary erythrocytosis is apparent.

The appropriate management of secondary erythrocytosis is control of the underlying cause. In most circumstances, therapeutic phlebotomy is not recommended for secondary erythrocytosis, because the increased erythrocyte mass is compensating for an unmet tissue oxygenation need.

Key Point

• Secondary erythrocytosis can be caused by hypoxemia, so patients may benefit from oxygen supplementation.

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