What does it measure?
Hematocrit describes how much space red blood cells take up in a blood sample. It is usually shown as a percentage and comes with the CBC. It adds context to hemoglobin and the red-cell count because both cell number and cell size affect it. [1]
Before the test
- Sample
- Blood, usually collected with a complete blood count.
- Fasting
- The CBC alone usually needs no fasting. Other tests at the appointment may have different instructions.
What can the result mean?
A low hematocrit may go with anemia or extra fluid in the bloodstream. A high result may reflect dehydration or more red cells than expected. It does not directly measure how well your circulation works, your fitness, or the amount of oxygen reaching an organ. [1]
When is it useful, and how is it read?
When hematocrit is high or low, compare hemoglobin and RBC count and consider fluid balance. Persistently high results after the fluid context is addressed need a different assessment from a single concentrated sample. [1] [3]
What can affect it?
Fluid balance matters here. Pregnancy, recent intravenous fluids, or dehydration after vomiting can change the percentage without the same change in total red-cell mass. The laboratory range and your previous pattern help the clinician interpret a result fairly. [1]
The liquid part of blood matters too
Hematocrit is a proportion, so losing fluid can raise it without your body making more red cells. Extra plasma volume, including during pregnancy, can lower it by dilution. [1] [3]
Questions you might have
What does the percentage describe?
The share of the blood sample occupied by red cells. It is not the percentage of oxygen in your blood. [1]
Can a fluid change affect hematocrit?
Yes. Dehydration can raise the concentration; pregnancy can lower it as blood volume changes. Interpret it with hemoglobin and your situation. [1]
What comes next?
Read hematocrit together with hemoglobin and MCV, the average cell size. Tell your clinician about fluid loss, recent treatment, or bleeding. An unexpected result may need a repeat under steadier conditions, while a persistent change needs assessment of its cause. [1]
Could fluid balance be affecting this percentage, or do the other CBC results suggest anemia?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
