What Hematocrit Level Indicates in Adult Patients
Hematocrit reflects the percentage of blood volume occupied by red blood cells and serves as a primary screening tool for anemia, polycythemia, and hydration status in adults without significant medical history.
Normal Reference Ranges
The interpretation of hematocrit must account for sex-specific differences:
- Adult males: Normal range 42-54%, with anemia defined as hematocrit corresponding to hemoglobin <13.5 g/dL 1
- Adult females: Normal range 38-46%, with anemia defined as hematocrit corresponding to hemoglobin <12.0 g/dL 1
- Hematocrit-to-hemoglobin ratio: Typically follows a 3:1 relationship (e.g., hematocrit of 45% corresponds to hemoglobin of 15 g/dL) 2
Clinical Significance of Abnormal Values
Elevated Hematocrit
Hematocrit >60% in males or >55% in females always indicates absolute polycythemia and requires evaluation 3. The differential diagnosis includes:
- Primary polycythemia (Polycythemia Vera): Consider when hematocrit is elevated with thrombocytosis, leukocytosis, splenomegaly, or aquagenic pruritus 1, 3
- Secondary polycythemia: Due to chronic hypoxia, smoking, or inappropriate erythropoietin production 1
- Relative polycythemia: Reduced plasma volume with normal red cell mass, often seen with dehydration 3
For hematocrit values between the upper normal limit and 60% (males) or 55% (females), blood volume studies may be needed to distinguish true polycythemia from hemoconcentration, unless clinical features strongly suggest a specific diagnosis 1, 3.
Low Hematocrit
Hematocrit values corresponding to hemoglobin <13.5 g/dL (males) or <12.0 g/dL (females) indicate anemia requiring further evaluation 1. The workup should include:
- Complete blood count with red cell indices to assess mean corpuscular volume 4
- Reticulocyte count to evaluate bone marrow response 4
- Iron studies (serum ferritin and transferrin saturation) 4
- Additional testing based on red cell morphology (vitamin B12/folate for macrocytosis, hemoglobinopathy evaluation for microcytosis) 4
Important Caveats and Pitfalls
Altitude Adjustments
Normal hematocrit values increase with altitude, and failure to adjust leads to misdiagnosis 2. At elevations ≥3,000 feet:
- Hemoglobin increases approximately 0.9 g/dL per 1000 meters of altitude 2
- At 5,700 feet (1,737 meters), expected hemoglobin increases by ~0.65 g/dL above sea level values 2
- Corresponding hematocrit adjustments follow the 3:1 ratio 2
Hemodilution vs. True Anemia
In fluid-resuscitated or hypervolemic patients, peripheral hematocrit may overestimate anemia severity 5. Research demonstrates that:
- In hypervolemic critically ill patients, peripheral hematocrit overdiagnosed anemia in 46.7% of cases compared to normalized hematocrit adjusted for blood volume 5
- Hematocrit alone cannot distinguish true red cell mass reduction from plasma volume expansion 5
Measurement Considerations
Hemoglobin measurement is preferred over hematocrit for clinical decision-making 4. The rationale includes:
- Hematocrit can increase 2-4% with prolonged sample storage due to mean corpuscular volume changes 2
- Hemoglobin remains stable at room temperature and has better reproducibility 4
- Hematocrit is affected by hyperglycemia, which is not clinically relevant to oxygen-carrying capacity 4
Smoking Effects
Smoking causes independent elevation of hematocrit beyond other factors 2, 3. Smokers with elevated hematocrit should discontinue smoking before pursuing extensive evaluation, as this is the most common cause of isolated hematocrit elevation 3.
Risk Stratification
Elevated hematocrit itself is an independent risk factor for thrombosis, regardless of underlying cause 3. For polycythemia vera specifically: