What is Hematocrit?
Hematocrit (Hct), also called packed cell volume (PCV), is the percentage of total blood volume occupied by red blood cells. If your hematocrit is 45%, this means red blood cells comprise 45% of your blood volume, with plasma making up the remaining 55%.
How is Hematocrit Measured?
Modern analyzers calculate hematocrit from RBC count and mean corpuscular volume (MCV):
- Formula: Hematocrit (%) = RBC count (million/μL) × MCV (fL) / 10
- Traditional method: Centrifuge blood in capillary tube and measure packed RBC column height
- Automated analyzers: Calculate hematocrit from measured RBC parameters
Relationship to Hemoglobin (Rule of Three)
In healthy individuals, hematocrit and hemoglobin maintain a predictable relationship:
- Rule of Three: Hematocrit (%) ≈ Hemoglobin (g/dL) × 3
- Example: Hemoglobin 15 g/dL × 3 = Hematocrit ~45%
- Deviation from rule: Suggests abnormal RBC size (microcytosis or macrocytosis) or lab error
What Does Hematocrit Tell Us?
Hematocrit provides valuable clinical information:
- Oxygen-carrying capacity: Higher hematocrit means more oxygen delivery (up to a point)
- Blood viscosity: Elevated hematocrit increases blood thickness, raising thrombosis risk
- Hydration status: Hematocrit is sensitive to fluid balance changes
- Transfusion guide: One unit of packed RBCs typically raises hematocrit by ~3%
Hematocrit vs Hemoglobin: Both measure oxygen-carrying capacity but from different perspectives. Hemoglobin measures the actual oxygen-binding protein (more clinically important), while hematocrit measures the volume occupied by RBCs (more affected by hydration). Both typically rise and fall together, but discrepancies can reveal important conditions.
Hematocrit normal ranges vary by age, sex, pregnancy status, and altitude. Men have higher hematocrit than women due to testosterone's stimulatory effect on erythropoiesis and women's menstrual blood loss.
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| Population | Normal Range (%) | SI Units (Fraction) |
|---|---|---|
| Adult Men | 41.5-50.4% | 0.415-0.504 |
| Adult Women (non-pregnant) | 35.9-44.6% | 0.359-0.446 |
| Pregnant Women | 30.0-39.0% | 0.300-0.390 |
| Newborns (0-2 weeks) | 44.0-64.0% | 0.440-0.640 |
| Infants (2-6 months) | 28.0-42.0% | 0.280-0.420 |
| Children (6 months-12 years) | 33.0-45.0% | 0.330-0.450 |
| Adolescents (13-18 years) | 36.0-49.0% | 0.360-0.490 |
Important Considerations:
- Hydration sensitivity: Hematocrit is extremely sensitive to volume status; dehydration falsely elevates, overhydration falsely lowers
- Pregnancy changes: Physiologic hemodilution causes hematocrit nadir at 28-32 weeks gestation
- Altitude adjustment: Hematocrit increases at high altitude (physiologic adaptation to hypoxia)
- Critical values: Hct <20% or >60% should be reported immediately to provider
- Smokers: Chronic smokers have elevated baseline hematocrit (compensation for CO binding)
Low Hematocrit (Anemia)
Low hematocrit indicates anemia (reduced oxygen-carrying capacity) or hemodilution (fluid overload). Causes parallel those of low hemoglobin.
True Anemia (Low RBC Mass)
- Blood loss: Acute hemorrhage (trauma, GI bleeding, surgery) or chronic blood loss (heavy menstruation, occult GI bleeding)
- Decreased RBC production: Iron deficiency, vitamin B12/folate deficiency, bone marrow failure, chronic kidney disease, hypothyroidism
- Increased RBC destruction: Hemolytic anemia (autoimmune, hereditary spherocytosis, G6PD deficiency, sickle cell disease)
- Bone marrow disorders: Aplastic anemia, myelodysplastic syndrome, leukemia, marrow infiltration
Dilutional (Pseudoanemia)
- IV fluid administration: Aggressive fluid resuscitation dilutes RBC concentration
- Pregnancy: Physiologic plasma volume expansion exceeds RBC mass increase
- SIADH: Syndrome of inappropriate ADH causing water retention
- Congestive heart failure: Fluid retention dilutes blood
Elevated Hematocrit (Polycythemia)
Elevated hematocrit increases blood viscosity, raising risks of thrombosis (stroke, MI, DVT) and hypertension. Classified as true polycythemia or hemoconcentration.
True Polycythemia (Increased RBC Mass)
- Polycythemia vera: Myeloproliferative neoplasm with JAK2 mutation causing autonomous RBC overproduction
- Chronic hypoxia: COPD, sleep apnea, high altitude living, cyanotic heart disease (appropriate EPO response)
- Renal disease: Renal cell carcinoma, polycystic kidney disease (inappropriate EPO secretion)
- EPO-secreting tumors: Hepatocellular carcinoma, cerebellar hemangioblastoma
- Exogenous EPO: Erythropoietin therapy, athletic doping
- Testosterone therapy: Stimulates erythropoiesis
Relative Polycythemia (Hemoconcentration)
- Dehydration: Most common cause; volume depletion from vomiting, diarrhea, diuretics, burns, heat exposure
- Gaisböck syndrome: Apparent polycythemia in obese, hypertensive men due to reduced plasma volume
Hematocrit in Acute Blood Loss
Critical Concept: In acute hemorrhage, initial hematocrit may be normal because both plasma and RBCs are lost proportionally. Only after hemodilution (fluid shifts into vascular space or IV resuscitation) does hematocrit fall. Serial measurements over 12-24 hours reveal true degree of blood loss. Don't rely on initial hematocrit to exclude significant bleeding.
Assessing Hematocrit with Hemoglobin (Rule of Three)
Rule of Three: Hematocrit (%) ≈ Hemoglobin (g/dL) × 3
Examples:
- Hemoglobin 15 g/dL → Expected Hematocrit ~45%
- Hemoglobin 10 g/dL → Expected Hematocrit ~30%
Deviation from Rule of Three suggests:
- Microcytic anemia (low MCV): Hct lower than expected (many small RBCs)
- Macrocytic anemia (high MCV): Hct higher than expected (fewer large RBCs)
- Lab error or sample problem
Transfusion Guidelines Using Hematocrit
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| Clinical Scenario | Hematocrit Threshold | Target Post-Transfusion |
|---|---|---|
| Stable patients (restrictive) | <21% (Hgb <7 g/dL) | 21-27% (Hgb 7-9 g/dL) |
| Active bleeding | <24% (Hgb <8 g/dL) | 24-30% (Hgb 8-10 g/dL) |
| Acute coronary syndrome | <24% (Hgb <8 g/dL) | 24-30% (Hgb 8-10 g/dL) |
| Symptomatic anemia | Individualized based on symptoms | Relief of symptoms |
Expected Response to Transfusion:
- One unit packed RBCs: Raises hematocrit by ~3% (or hemoglobin by ~1 g/dL)
- Post-transfusion check: Measure hematocrit 15-30 minutes after transfusion to assess response
- Poor response: Ongoing bleeding, hemolysis, or antibody-mediated RBC destruction
Hematocrit in Polycythemia Management
Hyperviscosity Risk: Hematocrit >54-60% significantly increases blood viscosity
- Complications: Thrombosis (stroke, MI, DVT, PE), hypertension, headache, dizziness
- Polycythemia vera target: Maintain Hct <45% with phlebotomy (remove 500mL blood every 2-3 days until target reached)
- Monitoring: Serial hematocrit measurements to guide phlebotomy frequency
Hydration Status Assessment
Hematocrit helps assess volume status, especially when comparing to baseline:
- Dehydration: Hematocrit elevated above baseline (hemoconcentration)
- Overhydration: Hematocrit decreased below baseline (hemodilution)
- Limitations: Acute changes may not be apparent for several hours; consider clinical context
Factors That Increase Hematocrit
- Dehydration: Most common cause of elevated hematocrit; volume depletion from any cause
- Diuretics: Decrease plasma volume, concentrating RBCs
- Altitude: Chronic high altitude causes true polycythemia (physiologic adaptation)
- Smoking: Carbon monoxide binding stimulates EPO production
- Medications: Erythropoietin (EPO), testosterone, anabolic steroids
- Living at altitude: Even short-term altitude exposure increases hematocrit
Factors That Decrease Hematocrit
- IV fluid administration: Hemodilution from aggressive fluid resuscitation
- Pregnancy: Physiologic plasma volume expansion (40-50% increase)
- SIADH: Water retention dilutes blood
- Overhydration: Excessive oral or IV fluid intake
- CHF: Fluid retention causing dilutional effect
- Blood draw timing: Supine position lowers hematocrit ~3% versus standing
Pre-analytical Errors
- Prolonged tourniquet time: Hemoconcentration causes falsely high hematocrit
- Clotted sample: Falsely low hematocrit; redraw required
- Inadequate mixing: EDTA tube must be inverted 8-10 times to prevent clotting
- Sample storage: RBCs swell over time; analyze within 6 hours for accurate results
Technical Considerations
- Microclots: Can falsely lower hematocrit
- Extreme leukocytosis: WBC >100,000 can falsely elevate hematocrit
- Lipemia: Severe hypertriglyceridemia can interfere with optical measurement
Clinical Pearl
"Rule of Three is your friend": Hematocrit should equal hemoglobin × 3. If Hgb is 12 g/dL but Hct is 48%, something's wrong—check for lab error or abnormal RBC morphology. This simple rule catches many lab errors.
Acute bleeding and hematocrit lag: In acute hemorrhage, the initial hematocrit may be deceptively normal because both plasma and RBCs are lost together. It takes 12-24 hours for hemodilution to occur and hematocrit to drop. Never rule out significant bleeding based on normal initial hematocrit in the ED.
Clinical Pearl
Hydration status indicator: Serial hematocrit measurements help assess fluid balance. If hematocrit drops after IV fluids without blood loss, the patient was dehydrated. If hematocrit stays high despite fluids, consider ongoing losses or polycythemia.
Transfusion math: One unit of packed RBCs raises hematocrit by ~3% (or hemoglobin by ~1 g/dL) in an average adult. Use this to plan transfusion needs. If Hct is 21% and target is 30%, need ~3 units.
Clinical Pearl
Pregnancy anemia is expected: Pregnant women normally have hematocrit 30-39% due to plasma volume expansion. This is physiologic, not pathologic. However, Hct <30% in pregnancy warrants iron studies as iron deficiency is common.
Clinical Pearl
Polycythemia vera threshold: In polycythemia vera, maintain Hct <45% with phlebotomy to minimize thrombotic risk. Higher hematocrit exponentially increases viscosity and cardiovascular events. Target is strict: <45%, not "around 45%."
Dehydration vs. true polycythemia: If hematocrit is elevated, check hydration status and repeat when euvolemic. If Hct remains high after adequate hydration, then pursue polycythemia workup (EPO level, JAK2 mutation). Most "high hematocrits" in the ED are just dehydration.
Hyperviscosity syndrome: Hematocrit >60% causes hyperviscosity with neurologic symptoms (headache, confusion, vision changes), increased thrombosis risk, and paradoxical bleeding. This is a medical emergency requiring urgent phlebotomy.
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