What is Considered a High Hematocrit Level?
A hematocrit level is considered elevated when it exceeds the 95th percentile of the normal distribution adjusted for sex and race: specifically, hematocrit >52% in adult males/post-menopausal females and >45% in menstruating females should prompt clinical evaluation. 1
Sex-Specific Normal Reference Ranges
The normal hematocrit ranges vary significantly by sex due to hormonal influences:
- Adult males and post-menopausal females: 47 ± 6% (normal range approximately 41-53%) 1
- Menstruating females: 41 ± 5% (normal range approximately 36-46%) 1
These gender differences emerge at puberty due to testosterone and estrogen effects. 1
Clinical Thresholds for Action
When to Suspect Polycythemia Vera or Erythrocytosis
The diagnostic evaluation should be initiated when: 2
- Hemoglobin/hematocrit exceeds the 95th percentile for sex and race
- Documented increase above individual baseline, regardless of absolute value
- Borderline-high hematocrit accompanied by PV-related features (thrombocytosis, leukocytosis, splenomegaly, aquagenic pruritus, unusual thrombosis, erythromelalgia)
Formal Diagnostic Criteria
For suspected polycythemia vera, erythrocytosis is defined as: 3
- Hemoglobin >18.5 g/dL in men or >16.5 g/dL in women
- Hematocrit >55% in men or >49.5% in women
Critical Thresholds for Intervention
Therapeutic Phlebotomy Indications
Phlebotomy is indicated only when hematocrit exceeds 65% with associated symptoms of hyperviscosity (after excluding dehydration). 3, 1 This is a critical threshold because:
- Repeated routine phlebotomies below this level are contraindicated due to risks of iron depletion, decreased oxygen-carrying capacity, and stroke 3
- The measurement of red cell mass at hematocrit >60% without obvious hemoconcentration is redundant since it is almost always elevated 2
Target Hematocrit in Polycythemia Vera
For confirmed polycythemia vera, hematocrit must be maintained strictly below 45% through therapeutic phlebotomy to reduce thrombotic risk. 2, 3 This recommendation is based on retrospective studies showing progressive increase in vascular occlusive episodes at hematocrit levels >44%, with suboptimal cerebral blood flow between 46-52%. 2
A lower target of 42% is reasonable for women and African Americans due to physiological differences in baseline hematocrit values. 2
Special Considerations and Pitfalls
Altitude Adjustments
Normal hematocrit increases with altitude due to compensatory erythropoietin production: 1
- At 2,000m: +0.8 g/dL hemoglobin increase
- At 3,000m: +1.9 g/dL hemoglobin increase
- At 4,000m: +3.5 g/dL hemoglobin increase
At 4,000 meters elevation, normal ranges extend to hematocrit 45-61% in men and 41-56% in women. 4
Thrombotic Risk Considerations
Hematocrit ≥46% in men is associated with significantly increased thrombotic risk in the general population. Men with hematocrit in the upper 20th percentile (≥46%) have a 1.5-fold increased risk of total venous thromboembolism and 2.4-fold increased risk of unprovoked venous thromboembolism compared to those in the lower 40th percentile. 5
High hematocrit (top 5 percentiles: women >45%, men >48%) is associated with 1.5-fold increased risk of arterial thrombosis in the heart. 6
Common Diagnostic Pitfalls
Do not rely on a single measurement: Borderline elevated hematocrit requires confirmation with repeated measurements, as single values are unreliable. 3
Consider relative vs. absolute polycythemia: Dehydration, diuretic use, burns, and stress polycythemia can cause elevated hematocrit through plasma volume depletion rather than true red cell mass increase. 3
Hemoglobin is more reliable than hematocrit: Hematocrit can falsely increase by 2-4% with prolonged sample storage and is affected by hyperglycemia, while hemoglobin remains stable. 1, 3
Iron deficiency can mask polycythemia: Coexisting iron deficiency may lower hematocrit into the normal range despite underlying polycythemia vera, or cause microcytic polycythemia with elevated RBC count but reduced hemoglobin. 2, 3