What is the role of blood count studies in the diagnosis of polycythemia vera (PV)?

Updated: Aug 20, 2019
  • Author: Srikanth Nagalla, MBBS, MS, FACP; Chief Editor: Koyamangalath Krishnan, MD, FRCP, FACP  more...
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Answer

Automated red blood cell counts and hematocrit values (including hemoglobin levels) may be deceptive with regard to the total red blood cell mass in patients with polycythemia vera (PV). Direct measurement of the red blood cell mass should show an increase with a normal or slightly decreased plasma volume. This is a nuclear medicine test that uses radiochromium-labeled red blood cells to measure actual red blood cell and plasma volume. However, patients with hemoglobin concentrations of at least 20 g/dL or hematocrit values of at least 60% in males and 56% in females always have an elevated red blood cell mass.

The red blood cells in patients with PV are usually normochromic and normocytic, unless the patient has been bleeding from underlying peptic ulcer disease or phlebotomy treatment (in which case the cells may be hypochromic and microcytic, reflecting low iron stores). See the image below.

This blood film at 10,000X magnification shows a g This blood film at 10,000X magnification shows a giant platelet and an eosinophil. Erythrocytes show signs of hypochromia as a result of repeated phlebotomies. Courtesy of U. Woermann, MD, Division of Instructional Media, Institute for Medical Education, University of Bern, Switzerland.

Findings that are often present in patients with PV, but are not required for diagnosis, include the following [1] :

  • Thrombocytosis (>400,000 platelets/µL)
  • Leukocytosis (>12,000/µL)
  • Leukocyte alkaline phosphatase score >100 units/L in the absence of fever or infection

The platelet count is elevated to 400,000-800,000/µL in approximately 50% of patients. The release of potassium into the serum caused by the increased number of platelets during in vitro coagulation may cause a pseudohyperkalemia in the serum, whereas the true plasma potassium level in vivo is actually within the reference range, as shown by measuring plasma levels and by the lack of electrocardiography (ECG) changes. Morphologic abnormalities in platelets include macrothrombocytes and granule-deficient platelets.

An elevated white blood cell count (>12,000/µL) occurs in approximately 60% of patients. It is mainly composed of neutrophils with a left shift and a few immature cells. Mild basophilia occurs in 60% of patients.

The leukocyte alkaline phosphatase (LAP) score is elevated (>100 U/L) in 70% of patients. This technique is only semiquantitative and is susceptible to interobserver and laboratory errors unless it can be performed by flow cytometry, which is not routinely available.


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