From the Guidelines
High hematocrit causes increased blood viscosity, leading to serious health complications, and therapeutic phlebotomy is recommended when hemoglobin is greater than 20 g/dL and hematocrit is greater than 65% with associated symptoms of hyperviscosity. When hematocrit rises above normal levels, blood becomes thicker and flows more slowly through blood vessels, putting strain on the cardiovascular system and raising the risk of blood clots, stroke, heart attack, and deep vein thrombosis 1. The increased red blood cell mass may result in an increase in blood viscosity, and symptoms may include headache, poor concentration, and fatigue.
Causes and Symptoms
Some common causes of elevated hematocrit include:
- Dehydration
- Living at high altitude
- Smoking
- Lung diseases
- Certain genetic disorders
- Polycythemia vera (a bone marrow disorder) Symptoms may also include dizziness, visual disturbances, and redness in the face, hands, and feet.
Treatment and Management
Treatment depends on the underlying cause but may include:
- Phlebotomy (blood removal) to relieve hyperviscosity symptoms
- Hydration
- Medication to suppress bone marrow production of red blood cells
- Addressing underlying conditions Regular monitoring of hematocrit levels is essential for those at risk, and repeated routine phlebotomies are not recommended due to the risk of iron depletion, decreased oxygen-carrying capacity, and stroke 1. According to the acc/aha 2008 guidelines, indications for therapeutic phlebotomy are hemoglobin greater than 20 g per dL and hematocrit greater than 65%, associated with headache, increasing fatigue, or other symptoms of hyperviscosity in the absence of dehydration or anemia 1.
Diagnosis and Evaluation
The diagnostic evaluation of polycythemia vera (PV) may be entertained if the hemoglobin/hematocrit level is higher than the 95th percentile of the normal distribution adjusted for sex and race, or if there is a documented increase in the hemoglobin/hematocrit level above the baseline for an individual patient, regardless of where the specific hematocrit level lies within the reference range 1. A diagnostic algorithm can be followed to make a working diagnosis of PV, including the determination of serum EPO and the examination of bone marrow histology 1.
From the Research
High Hematocrit Causes
- High hematocrit is associated with an increased risk of thrombosis, as seen in patients with polycythemia vera 2, 3, 4, 5
- Elevated hematocrit enhances platelet accumulation following vascular injury, which can lead to thrombus formation 6
- Hematocrit control below 45% is associated with a lower rate of thrombosis in polycythemia vera 3
- Phlebotomy is used to keep hematocrit below 45% in patients with polycythemia vera, and is associated with improved overall survival and decreased risk of thrombosis 2, 3, 5
- The risk of thrombosis is higher in patients with high hematocrit, and is also influenced by other factors such as age, previous cardiovascular complications, and the presence of JAK2 mutation 2, 3, 4
Clinical Implications
- Maintaining a normal hematocrit may reduce arterial thrombosis risk in humans 6
- Phlebotomy and hydroxyurea therapy are associated with improved overall survival and decreased risk of thrombosis in patients with polycythemia vera 5
- The clinical implications of phlebotomy requirement under hydroxyurea therapy are unknown, but patients requiring more frequent phlebotomies may have a higher risk of thrombosis 3
- Driver mutations, such as JAK2, CALR, or MPL, do not affect survival in essential thrombocythemia, but may influence the risk of thrombosis 2