Hematocrit measures what proportion of your blood is made up of red blood cells, the cells that carry oxygen. Blood is part cells and part fluid, and hematocrit tells you how much of the total is cells. It appears automatically on a complete blood count, often shortened to CBC, and is sometimes written as HCT or called the packed cell volume.
This article explains what hematocrit measures and what a high or low result can mean. It also explains why the result changes with hydration, not only with how many red blood cells you have.
The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.
If you spin a blood sample quickly, the red blood cells settle to the bottom and the pale fluid, called plasma, sits on top. Hematocrit is the fraction of the sample that the packed red blood cells occupy. Modern analyzers calculate it rather than spinning every sample, but the idea is the same.
Because hematocrit is a proportion, it depends on two things: the number of red blood cells, and the amount of fluid they are suspended in. That second factor is the part patients often do not expect. If you are dehydrated, there is less plasma, so the same number of cells occupies a larger share, and the hematocrit rises. If you have received a lot of intravenous fluid, the reverse happens.
Your report may show hematocrit as a percentage, or as a decimal fraction such as 0.42, printed next to a reference range. A typical adult range is about 40 to 52 percent for men, and 36 to 48 percent for women, and exact ranges vary by laboratory.
Hemoglobin and hematocrit measure closely related things and almost always move together. Hemoglobin measures the amount of the oxygen-carrying protein in your blood. Hematocrit measures the space the cells containing it take up.
As a rough guide, hematocrit is usually around three times the hemoglobin value when hemoglobin is reported in grams per decilitre. Doctors generally treat hemoglobin as the more reliable of the two, because it is measured directly while hematocrit is often calculated. When the two do not agree, that mismatch can clue you in that something affected the sample.
In practice, clinicians usually define and monitor anemia by hemoglobin rather than hematocrit. Hematocrit is useful supporting information, not the main number.
Hematocrit is measured on a small sample of blood, usually taken from a vein in your arm. No fasting is needed. It is reported automatically as part of a complete blood count.
Because hydration affects the result, your doctor may consider whether you were unwell, had been vomiting, or had received fluids around the time of the test.
Hematocrit is read alongside your hemoglobin, your red blood cell count, and the other red cell measurements.
Small changes are common and often reflect hydration rather than anything about blood production. Your own previous results are the most useful comparison.
Common reasons hematocrit can be low
Common reasons hematocrit can be high
A hematocrit slightly outside the range in someone who feels well is frequently repeated rather than investigated, particularly if hydration could explain it.
If your hematocrit is low along with your hemoglobin, your doctor will use the MCV and RDW to decide which tests to order next, usually iron studies or vitamin levels. If it is persistently high with no obvious cause, the usual steps are to review smoking, sleep, and any testosterone treatment. Further testing for a marrow cause is sometimes arranged. Treatment always addresses the underlying reason rather than the hematocrit itself.