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Hematocrit (Hct)
The share of blood volume taken up by red blood cells. Falls in anemia and blood loss, rises with dehydration and hypoxia.
Other names: Hematocrit, packed cell volume (PCV)Section: Blood cells
Updated: August 29, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortHematocrit is the percentage of blood made up of red cells. Typical adult values are 37–48% for men and 34–43% for women; newborns run higher at 45–61%. The percentage moves with both the red cell mass and the plasma volume, which is why dehydration raises it and pregnancy lowers it.
Reference ranges
Group
Reference range
Adult men
37–48%
Adult women
34–43%
Newborns
45–61%
Infants
32–42%
Common reference values from the MedlinePlus Medical Encyclopedia. Intervals vary between laboratories and with smoking and altitude — the range on the report applies.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
If a tube of blood is spun down, the red cells pack at the bottom and the straw-colored plasma stays on top. Hematocrit is simply the height of the red layer as a percentage of the whole column — a result of 42% means 42% of the blood volume is red cells.
The measurement depends on both the number and the size of red cells, so it moves together with the RBC count and hemoglobin. The three lines are reported together in the CBC and are read as a set: anemia shows up in all of them, and their proportions hint at the cause.
Because hematocrit is a ratio, it is sensitive to plasma volume as much as to red cells. Dehydration concentrates the blood and raises the percentage; pregnancy and overhydration dilute it and lower the percentage without any change in the actual number of cells.
Providers order the test — almost always within a CBC — for fatigue, suspected bleeding, chronic kidney disease, before and after surgery, during pregnancy, and to monitor treatments that can suppress the bone marrow.
When it is high
DehydrationThe most common cause: with less plasma water the same red cells take up a larger share of the volume. Severe diarrhea is the textbook example.
Chronic low oxygenLung disease, congenital heart disease, and cor pulmonale drive red cell production up, and the percentage follows.
SmokingCarbon monoxide exposure reduces oxygen delivery and the body compensates with more red cells.
Polycythemia veraA marrow disorder that overproduces red cells; a persistently high hematocrit without another explanation leads here.
Kidney tumorsRenal cell carcinoma and some cysts secrete erythropoietin and push red cell production up.
AltitudeThe percentage climbs naturally over the first weeks at high altitude and drifts back after descent.
When it is low
BleedingRecent injury or chronic hidden blood loss removes red cells faster than they are replaced.
Anemia of any causeToo few or too small red cells — iron, B12, folate, B6, or copper deficiency among the causes — lower the red cell share.
Bone marrow suppressionLeukemia, other cancers, medicine toxicity, radiation, and infection can all slow the marrow's output of new red cells.
Chronic kidney diseaseLess erythropoietin means a weaker production signal and a gradually falling hematocrit.
PregnancyPlasma volume expands faster than red cell mass, so the percentage dips — a normal physiological change that is still monitored.
OverhydrationExcess fluid dilutes the blood; the percentage falls while the actual cell count is unchanged.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Hydration on the test dayPregnancySmokingAltitude of residenceRecent bleeding or transfusionMedicines affecting the marrow
How to prepare
No special preparation is needed for a hematocrit test. It is one line of the CBC; fasting requirements would come only from other tests in the same draw. Because hydration shifts the percentage, the circumstances of the day — a stomach bug, a long flight — are worth mentioning.
When a retest makes sense
An off-range percentage is commonly rechecked after rehydration or recovery from the provoking event. In anemia treatment or polycythemia monitoring the test is repeated on the treating provider's schedule to track the trend.
Who usually reviews this result
A result outside the reference range is usually reviewed by: your GP or primary care doctor; a hematologist if a blood disorder is suspected. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: HCTHctPacked cell volumePCV
Units
Measured in: %
%=L/L
L/L = % × 0.01 (1% = 0.01 L/L)
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
Hemoglobin measures the oxygen-carrying protein concentration; hematocrit measures the volume share of red cells. As a rough rule the hematocrit percentage is about three times the hemoglobin number, and the two move together.
Why is my hematocrit high when I feel fine?
The commonest benign reason is dehydration on the day of the draw. Smoking and altitude do the same. A repeat test after normal drinking usually settles the question.
Does hematocrit change during pregnancy?
Yes — plasma volume grows faster than red cell mass, so the percentage drops. Prenatal CBCs track it to separate this normal dilution from true iron-deficiency anemia.
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