Polycythemia and erythrocytosis

5 DOH Abu Dhabi (HAAD) lab licensing practice questions on Polycythemia and erythrocytosis, 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 · Medium

   Neonatal polycythemia is usually defined as a venous hematocrit above 0.65 L/L (65%). Which maternal condition is a common cause?

   1. ADiabetes mellitus
   2. BIron deficiency anemia
   3. CAnti-D alloimmunization
   4. DFolate deficiency

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   Answer: A · Diabetes mellitus

   Maternal hyperglycemia causes fetal hyperinsulinemia, which raises fetal oxygen use and leads to relative fetal hypoxia. The fetus responds with more erythropoietin and a higher red cell mass. Other causes include placental insufficiency, delayed cord clamping and being the recipient twin in twin-to-twin transfusion. Capillary samples read higher than venous ones, so the diagnosis is confirmed on a venous sample.

   • B: Maternal iron deficiency does not raise the newborn's hematocrit.
   • C: Anti-D causes fetal hemolysis and anemia, not polycythemia.
   • D: Maternal folate deficiency does not drive fetal red cell overproduction.

2. Question 2 · Hard

   A 50-year-old obese man who has never smoked reports loud snoring and daytime sleepiness. Results: hemoglobin 186 g/L (18.6 g/dL), hematocrit 0.56 L/L, normal WBC and platelets, red cell mass 132% of predicted, daytime SpO2 96%, carboxyhemoglobin 1%, normal P50, serum erythropoietin at the upper reference limit, JAK2 V617F and exon 12 negative, and a normal renal ultrasound. The most likely cause is:

   1. AObstructive sleep apnea
   2. BStress (relative) erythrocytosis
   3. CA high-oxygen-affinity hemoglobin
   4. DPolycythemia vera

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   Answer: A · Obstructive sleep apnea

   The red cell mass is raised, so this is an absolute erythrocytosis, and an erythropoietin that is not low, with negative JAK2 testing, points to a secondary cause. Daytime saturation can be normal in sleep apnea because the hypoxemia happens at night, during apneas. The normal carboxyhemoglobin, P50 and renal imaging exclude smoking, a high-affinity hemoglobin and an erythropoietin-secreting kidney lesion. Overnight oximetry or a sleep study confirms the diagnosis.

   • B: A red cell mass of 132% of predicted is absolute, not relative, erythrocytosis.
   • C: A high-affinity hemoglobin would give a low P50, usually with a family history.
   • D: Polycythemia vera usually has a low erythropoietin and a JAK2 mutation.

3. Question 3 · Easy

   Which of these medicines commonly causes a secondary erythrocytosis?

   1. AHydroxyurea
   2. BRuxolitinib
   3. CTestosterone
   4. DAspirin

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   Answer: C · Testosterone

   Testosterone and other androgens stimulate erythropoietin release and suppress hepcidin, which makes more iron available to the marrow, so the hematocrit rises. Erythrocytosis is a common effect of testosterone replacement and of anabolic steroid misuse, so the hematocrit is checked before and during treatment. Erythropoiesis-stimulating agents are another drug cause. Hydroxyurea, ruxolitinib and aspirin are treatments for polycythemia vera, not causes of erythrocytosis.

   • A: Hydroxyurea suppresses the marrow and is used to treat polycythemia vera.
   • B: Ruxolitinib, a JAK inhibitor, lowers blood counts in polycythemia vera.
   • D: Low-dose aspirin lowers thrombosis risk in polycythemia vera but does not raise red cells.

4. Question 4 · Easy

   Stress (Gaisböck) erythrocytosis in obese, hypertensive smokers is a form of:

   1. ASecondary erythrocytosis from tissue hypoxia
   2. BPrimary erythrocytosis from a JAK2 mutation
   3. CRelative erythrocytosis from a low plasma volume
   4. DSecondary erythrocytosis from a renal tumor

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   Answer: C · Relative erythrocytosis from a low plasma volume

   In stress (Gaisböck) erythrocytosis the red cell mass is normal and the plasma volume is reduced, so the hematocrit is raised only relatively. It is typically seen in middle-aged men with obesity, hypertension, smoking, alcohol use or diuretic therapy. A measured red cell mass within the reference interval separates it from absolute erythrocytosis. Correcting the risk factors, such as stopping diuretics or smoking, often lowers the hematocrit.

   • A: Hypoxic secondary erythrocytosis has a truly raised red cell mass.
   • B: JAK2-driven polycythemia vera raises the red cell mass itself.
   • D: Erythropoietin-secreting renal tumors cause an absolute, not relative, erythrocytosis.

5. Question 5 · Medium

   A heavy smoker has a hematocrit of 0.55 L/L (55%) and a pulse oximeter reading of 98%. Why does the normal oximeter reading not exclude a hypoxic cause?

   1. ASmoking shifts the oxygen dissociation curve right
   2. BNicotine expands the plasma volume and dilutes the result
   3. COximeters measure dissolved oxygen, not hemoglobin
   4. DStandard oximeters read carboxyhemoglobin as oxyhemoglobin

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   Answer: D · Standard oximeters read carboxyhemoglobin as oxyhemoglobin

   Carbon monoxide binds hemoglobin more than 200 times as strongly as oxygen and shifts the dissociation curve to the left, so tissues receive less oxygen and erythropoietin rises. Two-wavelength pulse oximeters cannot tell carboxyhemoglobin from oxyhemoglobin, so the reading looks normal. CO-oximetry on a blood gas analyzer measures carboxyhemoglobin directly, and heavy smokers often have levels of 5% or more. Smoking also lowers plasma volume, adding a relative component.

   • A: Carbon monoxide shifts the curve to the left, which worsens oxygen release.
   • B: Plasma volume tends to fall in smokers, and plasma volume does not affect oximetry.
   • C: Pulse oximeters estimate hemoglobin saturation from light absorbance, not dissolved oxygen.