Hematocrit
Share of red blood cells in total blood volume, expressed as a percentage
Also known as
Definition
Hematocrit states what percentage of blood volume consists of red blood cells (erythrocytes); the rest is plasma, white blood cells and platelets. It depends on the number and size of red blood cells and is almost always measured as part of a complete blood count, usually together with hemoglobin. A value of 42 means that 42% of the blood consists of red blood cells.
| Parameter | Value |
|---|---|
| Unit | % (or l/l) |
| Reference Range | Men: 37–48% | Women: 34–43%. The value depends on sex, age, smoking and the altitude where you live. Labs differ; the reference range on your own report is what counts. Discuss deviations with your doctor. |
Agree on your personal target range with your doctor.
Some labs report hematocrit as a fraction in l/l: 0.42 l/l equals 42%.
↓ What a low value means
Low hematocrit can be linked to anemia, for example from acute or chronic blood loss, a deficiency of iron, folate, vitamin B12, vitamin B6 or copper, accelerated breakdown of red blood cells (hemolysis), chronic kidney disease or bone marrow disorders. It falls during pregnancy because the blood contains more fluid; excess body water lowers it too. Discuss deviations with your doctor.
↑ What a high value means
High hematocrit is most often linked to dehydration, because plasma volume falls. The body can also make more red blood cells, for example at high altitude, with smoking, with lung disease or congenital heart disease, or with the bone marrow disorder polycythemia vera. During testosterone therapy, the Endocrine Society guideline foresees hematocrit monitoring. Your doctor interprets the result.
When to test
Hematocrit is almost always measured with the complete blood count, for example at a routine checkup, with fatigue, dizziness or shortness of breath, before and after surgery, during pregnancy and to monitor chronic conditions or treatments. No special preparation is needed. Because hydration and altitude affect the value, measurements taken under similar conditions are easier to compare.
Frequently asked questions
Why does hematocrit rise at altitude? +
With less oxygen in the air, the body makes more red blood cells. After a stay at higher altitude, hematocrit can therefore be elevated for several weeks without an underlying disease.
How does it differ from hemoglobin? +
Hemoglobin measures the amount of the oxygen-carrying protein in the blood; hematocrit measures the volume share of red blood cells. The two are closely related and usually measured together.
Why is hematocrit monitored during testosterone therapy? +
The Endocrine Society guideline foresees measuring hematocrit alongside testosterone during testosterone therapy. It lists elevated hematocrit among the conditions in which it advises against starting therapy. How often it is measured is decided by the treating doctor.
Can dehydration affect the value? +
Yes. According to MedlinePlus, dehydration is the most common cause of a high hematocrit, because there is less plasma in the blood. Conversely, excess body water lowers the value.
Hematocrit
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.