In medical settings, “crit” is shorthand for hematocrit, a blood test that measures the percentage of your blood made up of red blood cells. If a nurse or doctor mentions your “crit is low” or checks your “crit,” they’re talking about this single number. Normal hematocrit runs 40% to 54% for men and 36% to 48% for women.
The word itself comes from Greek: “hemato” meaning blood and “krinein” meaning to separate. That’s a literal description of how the test works. A blood sample is spun in a centrifuge, which separates it into three visible layers: red blood cells packed at the bottom, a thin middle layer of white blood cells and platelets, and plasma (the liquid portion) on top. Your hematocrit is the volume of that bottom red blood cell layer compared to the total volume. It’s also called packed cell volume, or PCV.
What Hematocrit Actually Tells You
Red blood cells carry oxygen from your lungs to every tissue in your body. Your hematocrit reflects how much oxygen-carrying capacity your blood has. A crit of 45% means red blood cells make up 45% of your blood volume, with the remaining 55% being mostly plasma.
There’s a handy relationship between hematocrit and hemoglobin, the oxygen-carrying protein inside red blood cells. Your hemoglobin number, multiplied by three, should roughly equal your hematocrit. So a hemoglobin of 15 g/dL predicts a hematocrit around 45%. Lab technicians use this “Rule of Three” as a quick quality check. When the math doesn’t line up, it signals something unusual about the shape or content of the red blood cells, or a problem with the sample itself.
Normal Ranges by Age and Sex
Hematocrit varies depending on who you are. Adult men typically fall between 40% and 54%, while adult women range from 36% to 48%. The difference is largely driven by hormones, particularly testosterone, which stimulates red blood cell production.
Newborns have notably higher levels, ranging from 45% to 61%. This is a normal adaptation to the low-oxygen environment of the womb. By infancy, hematocrit drops to 32% to 42% before gradually rising toward adult levels through childhood and adolescence. Slight variations exist between labs, so your results should always be read against the specific reference range printed on your report.
What Low Hematocrit Means
A hematocrit below normal range generally points to anemia, meaning your blood isn’t carrying as much oxygen as it should. The most common symptoms are tiredness, weakness, and shortness of breath. As it drops further, you might notice dizziness, cold hands and feet, headaches, chest pain, or an irregular heartbeat.
Several things can drive hematocrit down:
- Iron deficiency: The most common cause worldwide, often from heavy menstrual periods, poor dietary intake, or slow blood loss from an ulcer or other internal source that depletes iron stores over time.
- Vitamin deficiencies: Low levels of B12 or folate impair the body’s ability to produce healthy red blood cells.
- Chronic disease: Conditions like kidney failure, cancer, and diabetes can suppress red blood cell production.
- Blood loss: Acute bleeding from surgery or injury, or chronic bleeding that outpaces the body’s ability to replace cells.
- Bone marrow problems: Conditions like aplastic anemia, where the marrow stops producing enough blood cells.
- Pregnancy: Blood volume expands significantly during pregnancy, which dilutes the red blood cell concentration and lowers hematocrit even when cell counts are adequate.
What High Hematocrit Means
An elevated hematocrit means your blood contains a higher-than-normal proportion of red blood cells. The main clinical concern is that thicker blood flows less easily, raising the risk of blood clots in both arteries and veins. Those clots can lead to strokes, heart attacks, or clots in deep veins.
Sometimes a high reading is misleading. Severe dehydration from vomiting or diarrhea shrinks the plasma volume without actually increasing red blood cells, so the percentage looks artificially high. Once fluids are restored, the number returns to normal.
True increases in red blood cell production fall into two categories. Primary polycythemia, most commonly polycythemia vera, is a bone marrow disorder where the body overproduces red blood cells on its own. Secondary polycythemia happens when something outside the marrow triggers extra production. Living at high altitude is a classic example: the body compensates for thinner air by making more red blood cells. Research on people living at around 3,000 meters (roughly 10,000 feet) shows consistently higher red blood cell counts, hemoglobin, and hematocrit compared to sea-level populations. Chronic lung conditions like COPD and severe asthma do the same thing, as do certain heart defects that reduce oxygen levels in the blood. Smoking compounds the effect by raising levels of carbon monoxide in the blood, further stimulating red blood cell production. Smokers living at high altitude show even greater elevations in hemoglobin and hematocrit than their non-smoking neighbors. Kidney tumors and some liver tumors can also secrete hormones that drive red blood cell production upward. Testosterone replacement therapy is another recognized cause.
How the Test Is Done
Hematocrit is part of a complete blood count (CBC), one of the most commonly ordered blood tests. A standard blood draw from your arm is all that’s needed. In urgent situations, point-of-care devices can measure hematocrit from a much smaller sample, often a fingerstick, and return results in minutes rather than waiting for a full lab run.
Modern labs rarely spin a tube and measure layers by hand anymore. Automated analyzers calculate hematocrit from other measurements in the CBC, such as the average size of your red blood cells multiplied by the total count. The traditional centrifuge method is still used as a reference standard and in settings without automated equipment. Blood gas analyzers in emergency and critical care settings can also estimate hematocrit using the electrical conductivity of the sample, which changes depending on how many red blood cells are present.
Why Your Doctor Watches This Number
Hematocrit is a fast, inexpensive snapshot of your blood’s oxygen-carrying capacity. It’s checked routinely during annual physicals, monitored closely during pregnancy, tracked after surgery or significant bleeding, and used to evaluate symptoms like unexplained fatigue or dizziness. In critical care, it helps guide decisions about fluid replacement and transfusion.
Because so many factors influence it, a single abnormal result rarely tells the whole story. Dehydration, altitude, recent exercise, and even the position of your arm during the blood draw can shift the number. A trend over time, combined with other values on the CBC, gives a much clearer picture than any one reading alone.