Hematocrit (HCT)
Complete guide to hematocrit testing — normal ranges, causes of high and low hematocrit, and when to get tested.
What is Hematocrit?
Hematocrit (HCT) measures the percentage of your blood volume occupied by red blood cells. It reflects the blood's oxygen-carrying capacity and viscosity (thickness).
Why is it Important?
Hematocrit is used to:
- Diagnose anemia (low HCT) or polycythemia (high HCT)
- Assess dehydration or overhydration
- Monitor blood disorders and treatments
- Guide blood transfusion decisions
Normal Ranges
| Group | Range | Unit |
|---|---|---|
| Men | 40 – 54 | % |
| Women | 36 – 48 | % |
| Children (6-12 years) | 35 – 45 | % |
| Newborns | 45 – 61 | % |
What Do Abnormal Results Mean?
Low Hematocrit
Common causes include:
- Anemia (iron, B12, folate deficiency)
- Acute or chronic blood loss
- Kidney disease (reduced erythropoietin)
- Bone marrow failure
- Hemolysis (red blood cell destruction)
- Overhydration (dilutional effect)
- Pregnancy
High Hematocrit
Common causes include:
- Dehydration (concentration effect)
- Polycythemia vera (bone marrow disorder)
- Chronic hypoxia (lung disease, high altitude)
- Smoking
- Testosterone therapy
- Kidney tumors
- Congenital heart disease
Clinical Implications
- HCT > 55%: Increased blood viscosity, risk of blood clots
- HCT < 30%: Reduced oxygen delivery, fatigue, weakness
- Rapid changes: Can indicate acute bleeding or dehydration
Relationship with Hemoglobin
Hematocrit is roughly 3 times the hemoglobin value:
- Hemoglobin 15 g/dL ≈ Hematocrit 45%
When Should You Get Tested?
Hematocrit is part of routine complete blood count (CBC). Your doctor may order it if you have symptoms of anemia, dehydration, or blood disorders.
This information is for educational purposes only and is not a substitute for professional medical advice.
Last reviewed: 2026-02-13