Hematocrit

What is Hematocrit?

Hematocrit (abbreviated as HCT or Hct) measures the percentage of your blood that consists of red blood cells. If your hematocrit is 45%, that means red blood cells make up 45% of your blood volume, with the remaining 55% being plasma (the liquid portion).

Hematocrit is closely related to hemoglobin — both are used to diagnose anemia (too few red blood cells) and polycythemia (too many red blood cells).

Normal Ranges

Group	Hematocrit Level
Adult men	38.3–48.6%
Adult women	35.5–44.9%
Pregnant women	31–41%

Normal ranges vary by altitude, hydration status, and individual factors.

Why It Matters

Anemia Detection

Low hematocrit indicates anemia — your blood has fewer red blood cells than normal, reducing oxygen delivery to tissues. This causes fatigue, weakness, and shortness of breath.

Polycythemia Detection

High hematocrit means your blood is thicker than normal, with too many red blood cells. This increases risk of blood clots, stroke, and heart attack.

Hydration Status

Hematocrit changes with hydration:

• Dehydration concentrates your blood, raising hematocrit
• Overhydration dilutes your blood, lowering hematocrit

This is why doctors look at trends over time, not just single values.

Low Hematocrit (Anemia)

Common Causes

• Iron deficiency — The most common cause
• Blood loss — Heavy periods, gastrointestinal bleeding, surgery
• Vitamin deficiencies — B12, folate
• Chronic disease — Kidney disease, cancer, autoimmune conditions
• Bone marrow problems — Reduced red blood cell production

Symptoms

• Fatigue and weakness
• Pale skin
• Shortness of breath
• Dizziness
• Cold extremities

High Hematocrit (Polycythemia)

Common Causes

• Dehydration — Most common temporary cause
• Smoking — Chronic hypoxia triggers production
• Lung disease — COPD, sleep apnea
• Living at altitude — Body compensates for lower oxygen
• Polycythemia vera — Rare bone marrow disorder

Risks

• Blood clots
• Stroke
• Heart attack
• Headaches and dizziness

Hematocrit vs. Hemoglobin

Both measure red blood cells, but differently:

• Hematocrit — Percentage of blood volume that's red blood cells
• Hemoglobin — Amount of oxygen-carrying protein

They typically rise and fall together. Both are part of a standard CBC (complete blood count).

What Affects Hematocrit

• Hydration — Major factor in short-term changes
• Altitude — Higher elevation increases hematocrit
• Pregnancy — Blood volume increases, diluting hematocrit
• Time of day — Slight variations throughout the day
• Exercise — Intense training can affect levels

How Often to Test

• Routine checkups: Included in standard CBC, typically every 1–2 years
• Pregnant women: Multiple times during pregnancy
• Chronic conditions: As directed by your doctor

Related Biomarkers

• Hemoglobin
• Red Blood Cells
• Ferritin