Hematocrit is the percentage of your blood that’s made up of red blood cells. If your hematocrit is 42%, that means red blood cells account for 42% of your blood’s total volume, with the remaining 58% being plasma (the liquid part) and other components. It’s one of the most common measurements included in routine bloodwork, and it tells your doctor a lot about your body’s ability to carry oxygen to your tissues.
What the Number Actually Tells You
Red blood cells are your body’s oxygen delivery system. They pick up oxygen in the lungs and carry it to every organ and muscle. When your hematocrit is within the normal range, oxygen delivery works smoothly. When it’s too low, your tissues aren’t getting enough oxygen. When it’s too high, your blood becomes thicker and harder to pump, which creates its own set of problems.
Hematocrit is usually measured as part of a complete blood count (CBC), the standard panel most doctors order during checkups. The test itself requires a simple blood draw, and no fasting or special preparation is needed. Results come back as a percentage.
Normal Hematocrit Ranges
Normal ranges differ by sex, age, and whether you’re pregnant. Here are the general reference values:
- Adult men: 38% to 50%
- Adult women: 35.5% to 44.9%
- Pregnant women: above 33% (lower than usual because blood volume expands during pregnancy)
- Newborns: 44% to 64%
- Infants 2 to 6 months: 35% to 50%
- Children 1 to 6 years: 30% to 40%
You’ll notice slight variations in these ranges depending on the lab. That’s normal. Your results report will typically list the specific reference range used by the lab that processed your blood.
Newborns have a noticeably higher hematocrit because they’re transitioning from the low-oxygen environment of the womb. Those levels drop steadily over the first several months of life and gradually settle into the childhood range.
What Low Hematocrit Means
A hematocrit below the normal range usually points to anemia, meaning you don’t have enough functional red blood cells. The most common cause is iron deficiency, which can happen from heavy menstrual periods, slow bleeding from an ulcer, or simply not getting enough iron in your diet. Your body needs iron to build hemoglobin, the protein inside red blood cells that actually binds to oxygen.
Other nutrient deficiencies cause low hematocrit too. Not getting enough vitamin B-12 or folate impairs your bone marrow’s ability to produce healthy red blood cells. Some people can’t absorb B-12 properly even if they eat enough of it, a condition called pernicious anemia. Pregnancy raises the risk of low hematocrit because blood volume increases faster than the body can produce new red blood cells, especially without adequate folic acid and iron.
Chronic diseases are another major cause. Kidney disease, cancer, rheumatoid arthritis, Crohn’s disease, and HIV/AIDS can all suppress red blood cell production through ongoing inflammation. The body essentially diverts resources away from making blood cells as part of its inflammatory response. Aplastic anemia, a rare condition where the bone marrow fails to produce enough new blood cells of any type, can also drive hematocrit dangerously low.
When hematocrit drops, you may feel fatigued, weak, short of breath, dizzy, or cold in your hands and feet. These symptoms develop because your organs and muscles aren’t receiving the oxygen they need. Mild anemia sometimes causes no noticeable symptoms at all, which is why routine blood tests catch it before you feel anything.
What High Hematocrit Means
The single most common cause of a high hematocrit reading is dehydration. When you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough water, the liquid portion of your blood shrinks. The red blood cells themselves haven’t changed, but they now make up a larger share of the total volume. Rehydrating usually brings the number back to normal quickly.
Living at high altitude also raises hematocrit, and this is a normal adaptation rather than a sign of disease. With less oxygen available in thinner air, your body compensates by producing more red blood cells to capture what oxygen there is. People who move from sea level to mountainous areas often see their hematocrit climb over the first few weeks.
When high hematocrit isn’t explained by dehydration or altitude, the causes become more serious. Lung diseases like COPD reduce how much oxygen reaches your blood, triggering the same compensatory increase in red blood cell production. Heart failure and congenital heart defects can have a similar effect. Polycythemia vera, a blood cancer where the bone marrow overproduces red blood cells, is a less common but important cause.
Health Risks of Persistently High Levels
When hematocrit stays elevated over time, the blood becomes thicker and flows less easily through blood vessels. This increased viscosity raises the risk of blood clots, which can cause a stroke, heart attack, or blockage in the veins of the legs or lungs. The combination of thicker blood, slower flow, and abnormal platelet behavior makes clotting one of the most dangerous complications of chronically high hematocrit.
Polycythemia vera carries additional risks beyond clotting. The excess red blood cells force the spleen to work harder filtering old and damaged cells, causing it to enlarge. The condition can also lead to peptic ulcers (open sores in the stomach or upper intestine) and gout, a painful joint inflammation caused by elevated uric acid. Without treatment, polycythemia vera can be life-threatening.
Why Your Result Might Be Off Without a Disease
An abnormal hematocrit doesn’t automatically mean something is wrong. Beyond altitude and dehydration, several temporary factors can shift the number. Recent intense exercise, recent blood donation, or even the position of your arm during the blood draw can influence results. Pregnancy naturally lowers hematocrit because plasma volume expands by up to 50%, diluting the red blood cells even as the body produces more of them.
If your result falls outside the normal range, your doctor will typically look at the rest of your CBC for context. Red blood cell count, hemoglobin level, and the size and shape of your red blood cells all help narrow down whether the hematocrit reading reflects a real problem or a temporary fluctuation. A single abnormal result often leads to a repeat test before any further workup.
How Hematocrit Differs From Hemoglobin
These two values appear side by side on a CBC and measure related but different things. Hematocrit is the volume of red blood cells as a percentage of total blood. Hemoglobin is the amount of oxygen-carrying protein inside those red blood cells. They usually move in the same direction: if one is low, the other tends to be low too. But they can occasionally diverge, which gives your doctor useful diagnostic information. For example, certain conditions affect the size of red blood cells without changing the amount of hemoglobin inside them, causing the two numbers to tell slightly different stories.
In practice, both measurements serve as screening tools for anemia and blood disorders. Your doctor may focus on one or the other depending on the clinical situation, but for most people looking at their lab results, a normal hematocrit and a normal hemoglobin together are reassuring signs that red blood cell health is on track.