What Is Hematocrit Test: Levels, High-Low Range, Chart and Symptoms

‘Hematocrit it is the relationship between plasma and the figurative elements of the blood (platelets, red and white blood cells).

Since the corpuscular part of the blood is mostly made up of erythrocytes or red blood cells, the hematocrit can also be defined as the percentage ratio between the red blood cells (erythrocytes) and the liquid part of the blood, called plasma.
When performing a venous sampling, it is sufficient to add an anticoagulant substance to the sample to assist the progressive sedimentation of the corpuscular particles.

Then evaluating the relationship, in terms of percentage height, between the figured elements and the total blood volume, the hematocrit value is obtained.
To accelerate the sedimentation process, the test tube can be centrifuged, obtaining, thanks to the sedimentation action of the centrifugal force, a clear separation between the figured elements, which settle at the bottom of the test tube, and the straw yellow liquid that overlies them (plasma). .
The number that expresses the percentage of cells accumulated at the bottom is called hematocrit.


What is Hematocrit?

L’hematocrit (abbreviated with Ht or HCT) expresses the relationship between the liquid part of the blood (plasma) and the corpuscular parti.e. the one occupied mainly by red blood cells.

Hematocrit is an index expressed as a percentage or fraction. For example, a hematocrit value of 35% means that there are 35 ml of red blood cells in 100 ml of blood.

Hematocrit values ​​that are too high or low can be a sign of some diseases.

Because it is measured

What is the Hematocrit used for?

Hematocrit reflects both the number of red blood cells and their volume (mean corpuscular volume or MCV).

This exam is listed as part of routine tests or when the doctor suspects that the patient has anemia (low hematocrit) o polycythemia (high hematocrit).

Additionally, the hematocrit is useful for evaluating the hydration status.

Hematocrit: when prescribed

Hematocrit is prescribed as generic control to evaluate the state of health, generally together with the hemoglobin test or as part of blood count test.
This index can also be used to evaluate and monitor at regular intervals those pathological conditions that involve changes in red blood cells, such as anemia or polycythemia.

The hematocrit signals that there is a problem with red blood cell production and/or their survival, but does not indicate the underlying cause. For this reason, increases or decreases in hematocrit must be interpreted in conjunction with other parameters, such as hemoglobin value, reticulocyte count, and/or red blood cell indices (MCV, MCH, and RDW).

In general, the hematocrit reflects the result of the red blood cell count and hemoglobin test.

Normal Values

Hematocrit normal values

Under normal conditions, the hematocrit value is slightly higher in men who, thanks to a greater secretion of testosterone, have a higher concentration of red blood cells in the blood.

In this regard, let us remember that red blood cells are produced in the bone marrow, under the stimulating action of erythropoietin, a hormone produced at the renal level in response to various stimuli (including testosterone).

Normal Hematocrit (Hct) values

MEN 38-52%

WOMAN 36-46%


newborns: 45-75%
1 month: 30-55%
6 months: 34-46%
from 1 to 4 years: 33-44%
10 years: 36-43%

In addition to the stimulating action of testosterone, the kidney is particularly sensitive to oxygen levels in the blood. When these are scarce, there is a greater secretion of erythropoietin (even 1,000 times higher) which, by increasing the synthesis of new erythrocytes, ensures more effective transport of oxygen to the tissues.
This explains, for example, the natural increase in hematocrit in response to prolonged stay at altitude. Variations in this parameter, commonly detected in classic blood tests (complete blood count), can be linked to numerous other factors, some physiological, others pathological. Let’s see the most common ones.

High Hematocrit – Causes

Possible Causes of High Hematocrit

1) Dehydration

  • Excessive sweating: due to dehydration the liquid part of the blood is reduced; consequently the ratio between figured elements and plasma increases and with it the hematocrit.
  • It should be noted that dehydration can be induced not only by excessive sweating, but also by the use of diuretics, burns, vomiting, diarrhea and diabetes (glycemic values ​​above 180 mg/dl are accompanied by urinary elimination of glucose which, for osmotic gradient, brings with it significant quantities of water).
  • Cholera, a disease that causes massive water loss through feces, also causes significant increases in hematocrit.
  • In all these cases the hematocrit value does not correspond to an actual increase in circulating erythrocytes (therefore we speak of hemoconcentration). Falsely high values ​​of the corpuscular part are therefore recorded, despite the fact that a normal number of erythrocytes are present within it.

2) Other possible causes

  • Stay at altitude above 2,500 meters for at least a week (see: training and altitude)
  • Acute renal failure
  • Polycythemia or absolute polyglobulia (increased number of red blood cells, with normal plasma component)
  • Taking doping drugs: testosterone and derivatives; erythropoietin and derivatives (generically defined as epoetins) of the second generation (Aranesp® and Nespo®: darbepoetin alfa) and third generation (Mircera®: Cera)
  • Lung diseases
  • Congenital cardiovascular diseases

Why a high hematocrit is dangerous

Too thick blood encounters greater difficulties in its path. The heart, consequently, must contract with greater force to overcome peripheral resistance and impart considerable pressure to the blood.

For this reason, a hematocrit that is too high can worsen pre-existing cardiac pathologies and overcome the resistance of the vessels, causing, for example, a very dangerous cerebral hemorrhage.

A high hematocrit also favors the formation of blood clots (thrombi), which can occlude important vessels, with all the negative consequences of the case (heart attack and stroke in the most serious cases).

Low Hematocrit – Causes

Possible Causes of Low Hematocrit

  • Pregnancy (from the third trimester) and prolonged aerobic training: these conditions are associated with hypervolemia, that is, an increase in the total volume of blood present in the body.
  • In these cases the hematocrit value is misleading, as it is lower than normal, even though the blood contains an adequate quantity of red blood cells. It should be noted that in athletes a reduction in hematocrit values ​​due to an increase in the liquid part of the blood improves performance levels. In fact, for the same number of circulating red blood cells, a more fluid blood will encounter less resistance along its path, with a consequent increase in systolic volume and blood flow to the tissues.
  • Iron deficiency
  • Vitamin B12 deficiency
  • Folic acid deficiency
  • Leukemias
  • Malignant tumors
  • Anemias (hemolytic a. due to autoimmunity or red blood cell defects; aplastic anemia; sickle cell anemia, etc.)
  • Liver cirrhosis
  • Serious infections
  • Chronic renal failure
  • Acute or chronic bleeding from the digestive tract or other organs (such as the bladder or uterus) or following severe trauma (blood loss causes a decrease in both plasma and corpuscular levels)
  • Decreased hemoglobin production (e.g. thalassemia)
  • Exposure to toxins and radiation
  • Chronic inflammatory diseases

Low hematocrit: associated symptoms

Pallor, weakness, headache, decreased vision, malaise and easy fatigue: these are just some of the symptoms classically associated with the pathological reduction in the number of circulating red blood cells.

See also:
Low Red Blood Cells: What Does It Mean?

How to measure it

Hematocrit: what does the test consist of?

Usually, the analysis is performed using automated tools that measure various parameters.
To measure hematocrit, the patient must have blood drawn from a vein in the arm or through a finger or heel prick (infants).

The test tube containing the blood sample is then centrifuged, obtaining a clear separation between the red blood cells, which settle at the bottom, and the plasma above them. The hematocrit expresses the percentage quantity of cells accumulated on the fundus.


Hematocrit test: is there any preparation required?

To undergo the blood test useful for evaluating the blood count it is not necessary to be fasting. In fact, the test takes place on the corpuscular part of the blood, i.e. on the cells, for which the fact of having consumed food or drink has no influence.

However, if in addition to the blood count, you need to carry out some other blood test that requires fasting (such as determining blood sugar or cholesterol levels), you will need to abstain from food and drink. The general practitioner who prescribes the tests will still be able to provide useful information in the case.
It should be remembered that a recent blood transfusion can distort the hematocrit result.

Interpretation of Results

High hematocrit: what does it mean?

When the hematocrit is high, it means that the blood is thicker; this can have repercussions on the heart, which has a harder time pumping it into circulation. Furthermore, the greater density of the blood favors the formation of clots, which can predispose to a heart attack or stroke.
Conditions associated with higher than normal hematocrit values ​​include dehydration, polycythemia vera, acute renal failure and some lung diseases.

Low hematocrit: what does it mean?

There are numerous reasons why the hematocrit value may be low. These range from iron deficiency anemia (due to iron deficiency) to hemorrhages, from prolonged aerobic training to chronic renal failure.

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