Erythrocytosis: relative and absolute

5 ASCP MLS practice questions on Erythrocytosis: relative and absolute, part of Hematology. The first 5 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   A 26-year-old non-smoker has had a raised hemoglobin since childhood, as has his father. Results: Hgb 19.4 g/dL (194 g/L), oxygen saturation 98%, carboxyhemoglobin 1%, serum erythropoietin normal, JAK2 V617F not detected, P50 17 mm Hg [2.3 kPa] (reference 24 to 28 mm Hg [3.2 to 3.7 kPa]). What is the most likely cause?

   1. ARelative erythrocytosis from low plasma volume
   2. BHigh-oxygen-affinity hemoglobin variant
   3. CPolycythemia vera with a JAK2 exon 12 mutation
   4. DErythropoietin-secreting renal tumor

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   Answer: B · High-oxygen-affinity hemoglobin variant

   A low P50 means hemoglobin holds oxygen too tightly and releases less to the tissues, so the kidneys sense hypoxia and keep erythropoietin normal or raised even though arterial saturation is normal. A lifelong, familial erythrocytosis with a low P50 and a normal carboxyhemoglobin points to an inherited high-affinity hemoglobin. Many of these variants are not separated by routine electrophoresis, so P50 measurement or globin gene sequencing is used. Polycythemia vera, including the JAK2 exon 12 form, has a low erythropoietin and a normal P50.

   • A: Relative erythrocytosis has a normal red cell mass and a normal P50.
   • C: Polycythemia vera, including exon 12 types, has a low erythropoietin and a normal P50.
   • D: A renal tumor would not lower the P50 or run in families.

2. Question 2 · Medium

   Which result best separates polycythemia vera from a hypoxic secondary erythrocytosis?

   1. AA low serum erythropoietin
   2. BA low arterial oxygen saturation
   3. CA raised red cell mass
   4. DA raised carboxyhemoglobin

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   Answer: A · A low serum erythropoietin

   In polycythemia vera, a JAK2-driven clone makes red cells without needing erythropoietin, and feedback lowers the serum erythropoietin. Hypoxia from lung disease, sleep apnea or altitude raises erythropoietin, and carbon monoxide from smoking does the same. A low oxygen saturation or raised carboxyhemoglobin therefore points to a secondary cause. Red cell mass is raised in both kinds of absolute erythrocytosis.

   • B: A low oxygen saturation points to hypoxic secondary erythrocytosis.
   • C: Red cell mass is raised in both primary and secondary erythrocytosis.
   • D: A raised carboxyhemoglobin points to smoking-related secondary erythrocytosis.

3. Question 3 · Easy

   In relative erythrocytosis, the hematocrit is raised because:

   1. AErythropoietin is increased
   2. BPlasma volume is reduced
   3. CRed cell mass is increased
   4. DRed cells survive longer

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   Answer: B · Plasma volume is reduced

   Relative erythrocytosis is a fall in plasma volume with a normal red cell mass, as in dehydration, diuretic use, burns or severe vomiting. The hemoglobin and hematocrit rise only because the blood is more concentrated. Absolute erythrocytosis is a true increase in red cell mass, either primary (polycythemia vera) or secondary to erythropoietin. Correcting the fluid loss brings the hematocrit back to normal.

   • A: Raised erythropoietin causes secondary absolute erythrocytosis.
   • C: A raised red cell mass defines absolute, not relative, erythrocytosis.
   • D: Red cell survival does not explain a raised hematocrit in dehydration.

4. Question 4 · Easy

   Which condition causes a secondary erythrocytosis driven by tissue hypoxia?

   1. AChronic obstructive lung disease
   2. BLoop diuretic therapy
   3. CJAK2-mutated polycythemia vera
   4. DDehydration from severe diarrhea

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   Answer: A · Chronic obstructive lung disease

   Chronic lung disease lowers arterial oxygen, the kidneys sense tissue hypoxia and release more erythropoietin, and the marrow makes more red cells. High altitude, cyanotic heart disease, sleep apnea and smoking (through carbon monoxide) act the same way. Dehydration and diuretics cause relative erythrocytosis by lowering plasma volume. Polycythemia vera is a primary disorder with a low erythropoietin.

   • B: Diuretics cause relative erythrocytosis by reducing plasma volume.
   • C: Polycythemia vera is a primary clonal disorder, not driven by hypoxia.
   • D: Diarrhea lowers plasma volume and causes relative erythrocytosis.

5. Question 5 · Medium

   A 58-year-old man has a hematocrit of 56% (0.56 L/L), a normal oxygen saturation, a raised serum erythropoietin and a mass in one kidney. His erythrocytosis is best classified as:

   1. APrimary, polycythemia vera
   2. BRelative, from low plasma volume
   3. CSecondary, from tissue hypoxia
   4. DSecondary, from inappropriate EPO release

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   Answer: D · Secondary, from inappropriate EPO release

   Renal cell carcinoma, and less often hepatocellular carcinoma, cerebellar hemangioblastoma and uterine leiomyoma, can secrete erythropoietin without any hypoxia. Because the oxygen saturation is normal, the raised erythropoietin is inappropriate. Polycythemia vera would have a low erythropoietin. Imaging of the kidneys is part of the workup of an unexplained secondary erythrocytosis.

   • A: Polycythemia vera has a low, not raised, erythropoietin.
   • B: Relative erythrocytosis has a normal red cell mass and no raised erythropoietin.
   • C: Tissue hypoxia is unlikely with a normal oxygen saturation.