Synonyms: HCT, Ht, Crit, packed cell volume (PCV), volume of packed red cells (VPRC), erythrocyte volume fraction (EVF)

What is hematocrit?

Hematocrit is the ratio of the volume of red blood cells to the volume of blood plasma.

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Red blood cells carry oxygen throughout the body. Having too much or too little red blood cells in the blood can be a sign of certain diseases.

Hematocrit levels may help determine the severity of anemia which may cause them to drop down to 15-25%. Hematocrit levels should not be measured after hemotransfusion or blood loss as they may be the cause of a falsely increased or decreased hematocrit level.

Units of Measure

Hematocrit (Ht) is expressed as a percentage (%) of the total blood volume or in liters per liter (L/L). That is the proportion of red blood cells in 1 liter of blood (e.g. 480 ml of cells in 1 liter of blood = 0.48 Liter/Liter = 48 %).

There is a linear relationship between hematocrit (HCT) and hemoglobin (HGB) concentration.


Normal Range

Normal hematocrit levels may vary slightly depending on age, gender, pregnancy, altitude where people live, the use of different testing methods.

The following hematocrit values are usually considered normal:

  • For men: 42%-54%
  • For women: 38%-46%

For high-altitude residents, hematocrit levels increase as the altitude where people live increases. This is a result of a lower concentration of oxygen in the atmosphere, that is why more red blood cells are needed to provide the body with the necessary amount of oxygen.

The hematocrit levels can be low or high for various reasons. But in some cases, hematocrit values can be calculated incorrectly (when there is a loss of blood, transfusion, in the mountains). According to the study, the change from supine to sitting position caused clinically significant increases in the hemoglobin, hematocrit and red blood cell count.

Reference values for hematocrit in pregnancy

During pregnancy, blood volume increases. But the plasma volume increases faster than the volume of red blood cells, which leads to physiologic anemia. There is a physiological decrease in the number of red blood cells (RBC), as well as hemoglobin (Hb) and hematocrit (Hct) levels. Different laboratories may have different hematocrit reference values.

Typical hematocrit levels during pregnancy are as follows:

  • 1st Trimester: 31%-41%
  • 2nd Trimester: 30%-39%
  • 3rd Trimester: 28%-40%

At 6 weeks of gestation, the blood plasma volume is already increased by approximately 10-15%. The blood becomes more liquid, and the concentration of cells in it decreases. The plasma volume increases to 30-50% greater than pre-pregnancy volume by term, but the red blood cell volume is only increased by 20-30%. Thus, hematocrit levels are usually lower compared to non-pregnant women. The risk of developing iron-deficiency anemia increases.

Reference values for hematocrit in children

Newborns and especially premature babies have higher hematocrit values compared to older children and adults. If the hematocrit from a peripheral venous sample is greater than 65%, polycythemia (high red cell count) is diagnosed.

High Hematocrit

A high hematocrit may reflect an absolute increase in the number of red blood cells or a decrease in plasma volume. An increase in hematocrit means that the blood thickens, which means that the risk of blood clots increases. This causes oxygen deficiency of the body and may cause a stroke or heart attack.

A higher than normal hematocrit can be caused by:

  • Dehydration. With sufficient fluid intake, the hematocrit returns to normal.
  • Heart and lung diseases. The body tries to compensate for the lack of oxygen in the blood, producing more red blood cells.
  • Smoking, living at high altitudes (compensation for low oxygen levels in the body)
  • Kidney tumors accompanied by increased formation of erythropoietin.
  • Polycythemia vera is a rare disease that causes abnormal production of red blood cells.

High hematocrit in pregnancy

High hematocrit levels in pregnant women indicate thickening of the blood, increasing the load on the cardiovascular system and the likelihood of blood clots. High hematocrit may be a sign of preeclampsia.

High hematocrit in children

An elevated hematocrit in a newborn (greater than 65%) can be caused, for example, by hypoxia (low oxygen level in the blood), heart or kidney problems, and perinatal asphyxia.

Low Hematocrit

Low hematocrit reflects a low number of circulating red blood cells in the blood and is an indicator of reduced oxygen transport capacity. A person with low hematocrit can experience shortness of breath, fatigue, paleness, irregular heartbeat, cold hands or feet. A low hematocrit indicates anemia.

A lower than normal hematocrit can be caused by:

  • Significant blood loss (due to trauma, chronic bleeding)
  • Nutritional deficiencies (iron, folic acid, vitamins B12, B6)
  • Increased destruction of red blood cells (hemolytic anemia, sickle cell anemia)
  • Bone marrow disorders (aplastic anemia, cancer, chemotherapy)
  • Kidney diseases can lead to a decrease in the production of erythropoietin, a hormone that stimulates the production of red blood cells by the bone marrow

Low hematocrit in pregnancy

A slight decrease in hematocrit (HCT) due to an increase in blood volume is normal during pregnancy. A low hematocrit level is a sign of anemia.

Approximately 90% of all cases of anemia in pregnancy are due to iron deficiency. Since the body doesn't get enough oxygen, pregnant women may experience fatigue, weakness, headache, shortness of breath, rapid heartbeat, pale skin. The heart and blood vessels work in an increased load mode, the work of other organs and systems is disrupted.

Low hematocrit in children

Low hematocrit levels (anemia) is a common disorder in children. It is important to consider age-related reference range because normal values of hematocrit vary greatly with age.

Acquired types of anemia, especially iron-deficiency anemia, are more common in toddlers, older children, and adolescents. In full-term infants under 6 months of age, iron deficiency is an unlikely cause of anemia.

Complete Blood Count (CBC) Interpretation