A patient presents with a hematocrit of 55%, elevated red cell mass, normal arterial oxygen saturation, elevated LAP score, and thrombocytosis. What is the most probable cause of these laboratory findings?

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The combination of elevated hematocrit, elevated red cell mass, normal arterial oxygen saturation, elevated LAP score, and thrombocytosis strongly suggests the diagnosis of polycythemia vera. Secondary erythrocytosis and relative erythrocytosis can also cause an elevated hematocrit, but they would not be associated with thrombocytosis and elevated LAP score. Idiopathic myelofibrosis would present with a normal or decreased red cell mass, and essential thrombocythemia would have a normal red cell mass with thrombocytosis.

A Patient Presents with a Hematocrit of 55%, Elevated Red