Hematocrit is the percentage of your blood that is made up of red blood cells. If your result reads 42, that means 42% of your blood volume is red blood cells, with the remaining 58% being plasma, white blood cells, and platelets. It’s one of the standard measurements included in a complete blood count (CBC), the most commonly ordered blood test.
What Hematocrit Actually Measures
Your blood is a mixture of cells suspended in a liquid called plasma. Red blood cells make up the vast majority of those cells, and their job is carrying oxygen from your lungs to every tissue in your body. Hematocrit captures how much of your total blood volume those red blood cells occupy. A higher percentage means your blood is more densely packed with red blood cells; a lower percentage means it’s relatively thinner.
Hematocrit is closely related to hemoglobin, the oxygen-carrying protein inside red blood cells. In fact, there’s a simple relationship between the two: your hematocrit is roughly three times your hemoglobin number. If your hemoglobin is 14 g/dL, your hematocrit will typically land around 42%. When one is abnormal, the other almost always is too, since they’re measuring the same basic thing from different angles.
Normal Ranges by Age and Sex
Normal hematocrit values differ depending on who you are:
- Adult males: 41% to 50%
- Adult females: 36% to 44%
- Infants: 32% to 42%
- Newborns: 45% to 61%
The gap between men and women is driven largely by testosterone, which stimulates red blood cell production, and by menstruation, which creates regular blood loss in premenopausal women. Newborns have strikingly high levels because they needed densely packed red blood cells to extract oxygen efficiently in the womb. Their hematocrit drops naturally over the first few months of life.
What Pregnancy Does to Hematocrit
During pregnancy, your blood volume expands significantly, but the liquid plasma portion increases faster than red blood cell production can keep up. This creates a natural dilution effect. A healthy nonpregnant woman typically has a hematocrit between 38% and 45%, but by late pregnancy that drops to around 34% in a single pregnancy and as low as 30% with twins or multiples.
Because of this expected drop, the thresholds for diagnosing anemia during pregnancy are lower than usual. In the first trimester, hematocrit below 33% is considered anemic. In the second trimester, the cutoff drops to 32%, then returns to 33% in the third trimester. A result that looks low on the standard reference range may be perfectly normal for someone who is 30 weeks pregnant.
What Low Hematocrit Means
A hematocrit below the normal range means you have fewer red blood cells relative to your blood volume, a condition broadly called anemia. The most common cause worldwide is iron deficiency, either from not getting enough iron in your diet or from chronic blood loss (heavy periods, gastrointestinal bleeding, frequent blood donation). Vitamin B12 and folate deficiencies can also impair red blood cell production and drive hematocrit down.
Chronic diseases play a role too. Kidney disease reduces production of the hormone that signals your bone marrow to make red blood cells. Inflammatory conditions like rheumatoid arthritis and inflammatory bowel disease can suppress red blood cell production over time. Bone marrow disorders directly interfere with the body’s ability to manufacture blood cells at all.
Overhydration can also produce a falsely low reading. If you’ve received large amounts of IV fluids, or if your body is retaining excess water, the extra plasma dilutes the red blood cells and makes the percentage appear lower than your actual cell count would suggest.
What High Hematocrit Means
A hematocrit above the normal range means either your body is producing too many red blood cells or the liquid portion of your blood has shrunk. The most common and least worrisome cause is simple dehydration. When you lose fluid through sweating, vomiting, diarrhea, or just not drinking enough, plasma volume drops and the red blood cell concentration rises. Rehydrating typically brings the number back to normal.
Living at high altitude is another common cause. At elevations above 5,000 feet, the thinner air contains less oxygen, so your body compensates by producing more red blood cells. This is a normal adaptation. Reference ranges are actually adjusted upward for people living at altitude: a healthy nonsmoking woman at sea level has a baseline around 35.7%, while the same woman at 6,000 to 7,000 feet would be expected to measure around 37.7%.
Smoking raises hematocrit through a similar mechanism. Carbon monoxide in cigarette smoke binds to hemoglobin and blocks it from carrying oxygen. Your body senses the reduced oxygen delivery and responds by ramping up red blood cell production. A large genetic study through the Copenhagen General Population Study found that current smokers had hematocrit levels roughly 1.6% to 2.3% higher than never-smokers, with the effect scaling to about a 3.3% increase per additional 10 cigarettes smoked per day.
More serious causes include polycythemia vera, a blood disorder where a genetic mutation causes the bone marrow to produce red blood cells uncontrollably. Inherited gene mutations can also drive overproduction. Persistently elevated hematocrit without an obvious explanation like altitude or dehydration typically warrants further testing.
How the Test Is Done
Hematocrit is measured from a standard blood draw, usually as part of a CBC panel. In most labs, an automated analyzer counts your red blood cells, measures their average size, and calculates the hematocrit mathematically. This is fast and reliable for most patients, though certain conditions (like unusually small red blood cells or a very high white blood cell count) can slightly skew the calculated result.
The older method involves spinning a thin tube of blood in a centrifuge until the red cells pack down to the bottom. A technician then reads the percentage directly by measuring how much of the tube the red cell layer occupies. This centrifugation method tends to read slightly high, about 1.5% to 3% above the true value, because a small amount of plasma gets trapped between the packed cells.
Point-of-care blood gas analyzers use a third approach, measuring the electrical conductivity of your blood and estimating hematocrit from that. This method can give misleading readings in patients receiving large-volume IV fluids or certain medications that change the chemical makeup of the plasma.
What Can Throw Off Your Results
Because hematocrit is a percentage, anything that changes the ratio of cells to liquid will shift the number, even if the actual number of red blood cells hasn’t changed. Dehydration is the classic example: you have the same red blood cells as yesterday, but less plasma, so the percentage goes up. Overhydration does the opposite.
Timing matters too. If you’ve just finished intense exercise, mild dehydration and plasma shifts can temporarily raise your hematocrit. Recent blood loss will lower it, though the drop may not show up immediately because both cells and plasma are lost together at first. The plasma portion gets replaced faster, so hematocrit often looks lowest a day or two after significant bleeding rather than right away.
If your result is borderline or unexpected, your doctor may want to repeat the test under more controlled conditions, or pair it with other measurements like iron levels, a reticulocyte count (which shows how fast your bone marrow is making new red blood cells), or a direct hemoglobin measurement to get the full picture.