What is hematocrit, what are the normal reference ranges for men, women, and children, and what do low or high hematocrit values indicate?

Hematocrit (Hct): Definition, Reference Ranges, and Clinical Significance

What is Hematocrit?

Hematocrit represents the percentage of blood volume occupied by red blood cells, calculated as red blood cell volume divided by total blood volume 1. Hematocrit is conventionally defined as approximately three times the hemoglobin value (g/dL), though this relationship varies with age and is not constant across all populations 2. Hemoglobin measurement is more accurate and stable than hematocrit for clinical decision-making, as hematocrit can falsely increase by 2-4% with prolonged sample storage (>8 hours) due to mean corpuscular volume changes, while hemoglobin remains stable 3.

Normal Reference Ranges

Adult Males

• Hematocrit: 42-54% (mean 47% ± 6%) 3
• Hemoglobin: 13.0-16.5 g/dL 4
• Post-pubertal males have higher values due to testosterone effects 3

Adult Females

• Premenopausal: Hematocrit 38-46% (mean 41% ± 5%) 3
• Postmenopausal: Hematocrit 38-46% (mean 47% ± 6%) 3
• Hemoglobin: 12.0-15.5 g/dL 5
• Lower limit of normal is 11.0 g/dL for premenopausal women (5th percentile) 5

Children

• Hematocrit and hemoglobin values show significant age-dependent variations throughout childhood 2
• The relationship between hematocrit and hemoglobin is non-linear in children, with an asymptotic trend toward the 3:1 ratio only achieved in older ages 2

Important Adjustments

Altitude corrections are mandatory for individuals living above 3,000 feet (914 meters) to avoid misdiagnosis 4, 5:

• 1,000 meters: +0.2 g/dL hemoglobin
• 1,500 meters: +0.5 g/dL hemoglobin
• 2,000 meters: +0.8 g/dL hemoglobin
• 2,500 meters: +1.3 g/dL hemoglobin
• 3,000 meters: +1.9 g/dL hemoglobin 3

Race-based variations exist: African American females typically have hemoglobin levels 0.5-1.0 g/dL lower than Caucasian females, with lower limits as low as 9.6 g/dL in some populations 5. Black males and females show lower baseline values even after income adjustment, representing normal variation rather than iron deficiency 4.

Smoking increases hemoglobin by 0.3-1.0 g/dL and requires separate adjustment beyond altitude 5.

Low Hematocrit Values (Anemia)

Diagnostic Thresholds

Anemia is defined as:

• Men: Hemoglobin <13.0 g/dL (Hct <39%) 4
• Women: Hemoglobin <12.0 g/dL (Hct <36%) 5
• These thresholds must be adjusted upward for altitude and smoking 4, 5

Clinical Significance

Low hematocrit indicates reduced oxygen-carrying capacity 6. Anemia is not a normal consequence of aging; any low hemoglobin in elderly patients signals underlying pathology requiring investigation 3. In critically ill surgical patients, peripheral blood hematocrit may not accurately reflect true red blood cell volume due to plasma volume shifts, with mean differences of 3.4 ± 7.8 percentage points between measured and normalized hematocrit 1.

Common Causes

• Iron deficiency (most common)
• Chronic kidney disease
• Chronic inflammation
• Blood loss (acute or chronic)
• Bone marrow disorders
• Nutritional deficiencies (B12, folate)

Transfusion Thresholds

Current guidelines recommend restrictive transfusion strategies with a threshold of 7-8 g/dL hemoglobin (Hct 20-24%) in stable hospitalized patients, as large clinical studies demonstrate better outcomes compared to liberal transfusion approaches 6. Each unit (approximately 300 mL) of packed red blood cells increases hematocrit by approximately 1.9% ± 1.2%, though variability is substantial 7.

High Hematocrit Values (Erythrocytosis)

Diagnostic Thresholds

True erythrocytosis is defined as:

• Men: Hemoglobin >18.5 g/dL OR Hematocrit >52-55% 3
• Women: Hemoglobin >16.5 g/dL OR Hematocrit >48-49.5% 3

These thresholds trigger mandatory workup for primary versus secondary causes 3.

Primary Polycythemia Vera

Polycythemia vera (PV) diagnosis requires WHO 2016 criteria 3:

Major criteria:

1. Hemoglobin >16.5 g/dL (women) or >18.5 g/dL (men), OR Hematocrit >48% (women) or >49% (men)
2. JAK2 mutation present (detects 95-97% of PV cases)
3. Bone marrow biopsy showing hypercellularity with trilineage myeloproliferation

Minor criterion:

• Subnormal serum erythropoietin level

Diagnosis requires all three major criteria OR first two major criteria plus one minor criterion 3.

Management of Polycythemia Vera

Maintain hematocrit strictly <45% through therapeutic phlebotomy to reduce thrombotic risk 3. The landmark CYTO-PV trial demonstrated that hematocrit <45% reduced cardiovascular death or major thrombosis to 2.7% versus 9.8% with hematocrit 45-50% (HR 3.91, P=0.007) 3. A slightly lower target of approximately 42% is reasonable for women and African Americans due to physiological baseline differences 3.

Low-dose aspirin (81-100 mg daily) is the second cornerstone of therapy for thrombosis prevention 3.

When phlebotomy is performed, remove 300-450 mL per session and replace with equal volume of saline or dextrose to prevent hemoconcentration and stroke risk 3.

Secondary Erythrocytosis

Secondary causes include:

Hypoxic causes:

• Chronic obstructive pulmonary disease
• Obstructive sleep apnea (requires sleep study if suspected) 3
• Cyanotic congenital heart disease with right-to-left shunting 3
• Smoking ("smoker's polycythemia" from carbon monoxide exposure) 3
• High altitude residence (physiologic adaptation) 4

Non-hypoxic causes:

• Testosterone therapy (prescribed or unprescribed) 3
• Erythropoietin-producing tumors (renal cell carcinoma, hepatocellular carcinoma, pheochromocytoma) 3
• Exogenous erythropoietin administration 3

Relative polycythemia (plasma volume depletion):

• Dehydration
• Diuretic use
• Burns
• Stress polycythemia (Gaisböck syndrome) 3

Management of Secondary Erythrocytosis

Treatment of the underlying condition is the primary intervention 3:

• Smoking cessation for smoker's polycythemia (typically resolves erythrocytosis) 3
• CPAP therapy for obstructive sleep apnea 3
• Dose adjustment or discontinuation of testosterone if causative 3
• Management of chronic lung disease 3

Therapeutic phlebotomy in secondary erythrocytosis is indicated ONLY when ALL of the following criteria are met 3:

1. Hemoglobin >20 g/dL AND Hematocrit >65%
2. Documented symptoms of hyperviscosity (headache, blurred vision, confusion, bleeding)
3. Patient is adequately hydrated
4. Iron deficiency has been excluded
5. Hematocrit remains elevated above baseline despite hydration

First-line therapy for suspected hyperviscosity is aggressive rehydration with oral fluids or intravenous normal saline, NOT phlebotomy 3.

Critical Pitfalls to Avoid

Never perform routine or repeated phlebotomies in secondary erythrocytosis 3. This causes iron depletion, decreased oxygen-carrying capacity, and paradoxically increases stroke risk 3. In cyanotic congenital heart disease, the elevated hematocrit is a compensatory mechanism to optimize oxygen transport and should not be interrupted 3.

Iron deficiency frequently coexists with erythrocytosis and requires opposite management (iron supplementation rather than phlebotomy) 3. Iron-deficient red blood cells have reduced oxygen-carrying capacity and deformability, increasing stroke risk 3. Mean corpuscular volume is unreliable for screening iron deficiency in erythrocytosis; serum ferritin, transferrin saturation, and iron levels are required 3.

If transferrin saturation <20%, initiate cautious oral iron supplementation with close hemoglobin monitoring, as rapid increases in red cell mass can occur 3.

Never perform phlebotomy without equal-volume fluid replacement, as this increases hemoconcentration and stroke risk 3.

Initial Workup for Elevated Hematocrit

Order the following tests immediately 3:

• Complete blood count with red cell indices (MCV, MCH, MCHC, RDW)
• Reticulocyte count
• Peripheral blood smear review
• Serum ferritin and transferrin saturation
• C-reactive protein
• JAK2 mutation testing (exon 14 V617F and exon 12)

If JAK2 mutation is positive, perform bone marrow aspirate and biopsy to confirm PV diagnosis 3.

If JAK2 mutation is negative, systematically evaluate secondary causes including sleep study, pulmonary function tests, chest imaging, renal imaging, medication review (especially testosterone), and erythropoietin level 3.

Refer immediately to hematology if 3:

• JAK2 mutation is positive
• Hemoglobin >20 g/dL with symptoms of hyperviscosity
• Unexplained splenomegaly
• Diagnosis remains unclear after initial workup
• Cytopenias coexist with erythrocytosis