What are the significant Hemoglobin (Hb) and Hematocrit (Hct) values for polycythemia?

From the Guidelines

Significant hemoglobin (Hb) and hematocrit (Hct) values for diagnosing polycythemia are generally Hb >18.5 g/dL in men and >16.5 g/dL in women, with Hct >49% in men and >48% in women, as per the 2001 WHO criteria 1. These values are based on the World Health Organization criteria for polycythemia vera (PV), which include hemoglobin/hematocrit thresholds as major diagnostic criteria, along with bone marrow findings and presence of JAK2 mutations. The increased red cell mass in polycythemia leads to blood hyperviscosity, which can cause symptoms like headaches, dizziness, visual disturbances, and increases the risk of thrombotic events. Some key points to consider when evaluating elevated hemoglobin and hematocrit include:

• Distinguishing between absolute polycythemia (true increase in red cell mass) and relative polycythemia (decreased plasma volume)
• Dehydration, stress, or living at high altitude can cause relative polycythemia, which typically resolves when the underlying cause is addressed
• Persistent elevations warrant further investigation for polycythemia vera or secondary causes like chronic hypoxia or certain tumors
• Targeting hematocrit levels to <45% can lower the risk of thrombosis, as shown in the CYTO-PV trial 1. It's also important to note that the management of polycythemia vera involves phlebotomy to maintain the hematocrit at less than 45% and low-dose aspirin, as well as cytoreduction in high-risk patients 1. Overall, the diagnosis and management of polycythemia require careful consideration of these factors to minimize the risk of morbidity and mortality.

From the Research

Significant Hemoglobin (Hb) and Hematocrit (Hct) Values for Polycythemia

• Hemoglobin values:

  • 16.5 mg/dL in men 2

  • 16.0 mg/dL in women 2

• Hematocrit values:
  • <45% is the goal for therapeutic phlebotomy 2, 3

  • 52% in men and >48% in women for diagnosis according to BCSH guidelines 4, 5

Diagnostic Thresholds

• The best cut-off for Hb to discriminate JAK2-mutated ET from PV was 16.5 g/dL for males and 16.0 g/dL for females 5
• For HCT, this was 49% in males and 48% in females 5

Clinical Implications

• Hematocrit control below 45% is associated with a lower rate of thrombosis in polycythemia vera 3
• Patients requiring 3 or more phlebotomies per year showed a worse hematocrit control and a higher rate of thrombosis 3