For the past several years, on a consistent basis, an asymptomatic 70-year-old Asian American man has had the following hematologic findings (with lab ranges) indicative of hemoconcentration: WBCs 6.7 ´ 106/L (4.4-10.1), RBCs 5.62 ´ 109/L (4.31-5.48), hemoglobin 18.7 g/dL (13.5-17.01), hematocrit 51.1% (40.0-50.1), mean corpuscular volume 90.9 fL (80-100), mean corpuscular hemoglobin 33.3 pg (28-33), mean corpuscular hemoglobin concentration 36.6% (32-36), red-cell distribution width-standard deviation 42% (37-47), red-cell distribution width coefficient of variation 13% (12-15), and platelets 137 ´ 106/L (150-400). His creatinine is 1.0 mg/dL. An erythropoietin assay is within normal limits. His collecting system appears normal. Brain and kidney scans are unremarkable. IV pyelogram shows multiple cysts, greater in number on the right; there are no masses or stones. The patient, who drinks lots of fluids, denies polyuria and polydipsia. There is no history of diabetes mellitus or diabetes insipidus. The man stands 5 ft 6 in tall, weighs 148 lb, and is physically quite active. What could be causing this clinical picture?
—Mahmud Syed, MD, Troy, Mich.

While this patient’s borderline elevations of hemoglobin and hematocrit with a normal WBC, slightly decreased platelet count, and erythropoietin level “within normal limits” suggest polycythemia, this diagnosis is unlikely. In primary polycythemia, or polycythemia vera, the hematocrit is usually elevated to higher levels and erythropoietin production is suppressed. Moreover, other clinical signs, such as splenomegaly, thrombosis, pruritus, and erythromelalgia, and lab findings, such as leukocytosis or thrombocytosis, tend to rule out polycythyemia vera. In secondary polycythemia, the erythropoietin level is elevated above the “normal range”; the most common cause is hypoxemia due to lung disease. An alternative explanation may be a contracted plasma volume rather than an elevated RBC volume. This can be proven by performing a radiolabeled RBC (chromium) and albumin study for these parameters. More information can be found in the article by Tefferi A, Hanson CA, and Inwards DJ, “How to interpret and pursue an abnormal complete blood cell count in adults” (Mayo Clin Proc. 2005;80:923-936).
—Michael Flamm, MD, assistant professor of clinical medicine, College of Physicians & Surgeons,
Columbia University Medical Center, New York City