HEMATOCRIT: normal, high and low values

What is hematocrit

What does hematocrit mean? L'hematocrit (also called Ht or HCT) is one of the parameters that can commonly be found in a blood test and represents the percentage of the volume of circulating blood (expressed in ml) occupied by the volume of erythrocytes (also expressed in ml). Therefore, within this value, the volumes occupied by are not counted White blood cells is platelets. The latter component is, in fact, analyzed through its specific parameter, the plateletcrit (or PCT).

Normally, this parameter is expressed by using a fraction or, more commonly, as a percentage.

Why and when the hematocrit is measured: indications for the exam

The HCT, as already mentioned above, is part of the set of values found within a very common blood count.

Variations in HCT values indicate the existence of an alteration in production or survival of Red blood cells within the blood circulation, without however clarifying the cause of this problem in any way. Only thanks to the interpretation of other parameters, normal values of hemoglobin  (Hgb) most of which are also present in the blood test, allows us to understand what is the underlying process that alters the hematocrit percentage.

Normal hematocrit values

The hematocrit varies according to the age and sex of the subject. The values in children vary enormously up to 10 years of age, starting from a normal range between 45 and 75% in infants and reaching the range of 36-43% in children of 10 years.

The reference values of the hematocrit in adults are:

  • Man = 39 - 50 %;
  • Female = 35 - 45 %.

The difference between male and female individuals lies in the greater secretion of testosterone by the former. This factor, in fact, induces the greater production of the hormone, called erythropoietin or EPO, responsible for the production of red blood cells within the bone marrow.


Normal, high and low hematocrit values.

High hematocrit (high HCT)

What are the high hematocrit values? A hematocrit value higher than 50% for men and higher than 45% for women is defined as high.

Causes of high hematocrit

The possible causes of high hematocrit I'm:

  • Dehydration. In this case we are not witnessing a real increase in circulating red blood cells, but a concrete loss of liquids from the body (it will therefore be said that we are facing a condition of "haemoconcentration"). Among the possible losses of liquids from the body, we remember abundant vomiting, diarrhea (especially in pathological processes such as cholera, in which large volumes of water are lost through the faeces), burns, incorrect use of diuretics, diabetes (in which there is urinary excretion of glucose, which brings with it large amounts of fluids);
  • Polycythemia vera and conditions of polyglobulism. This is a disease of the erythrocyte component of the blood, which is produced in an aberrant way, thus raising both the absolute value of red blood cells and the hematocrit;
  • Some lung diseases. The absorption of oxygen in them is not adequate to meet the body's demands and will lead to a stimulation of erythropoietin in an attempt to compensate for this lack;
  • Acute renal failure and kidney tumors. In these two conditions, there is an increase in the production of the hormone stimulating erythrocyte synthesis;
  • Some congenital heart diseases, which do not allow the correct distribution of blood in the periphery. The body will "read" this condition as a situation of low peripheral oxygenation and will induce the production of new "vectors" for oxygen (red blood cells) to make up for this lack. In fact, the hematocrit may be completely normal. Among others, we remember the condition of cor pulmonale;
  • Sleep disorders, for example Obstructive Sleep Apnea Syndrome (OSAS - also referred to as Pickwick's Syndrome or Disease). In it, major obesity causes the upper airways to collapse, causing apneas and, consequently, hypoxia and hypercapnia. To cope with this, the body stimulates the production of erythropoietin, to increase the amount of circulating erythrocytes and, therefore, also of the hematocrit;
  • Often the hematocrit value is increased to improve sports performance through increased and improved oxygen transport to peripheral tissues.

High hematocrit symptoms

This situation creates a greater density e blood viscosity. If taken to the extreme and for a long time, it can create important pumping difficulties in the heart: the thick blood it will need more force to be pushed into the peripheral capillaries, causing the heart to exert a greater effort than normal. If the effort is prolonged and continuous, ahigh blood viscosity may expose the subject to important risks for his health due to the resulting hypertension, such as haemorrhagic conditions (especially in the brain), situations of increased genesis of venous thrombus (called "thrombophilia"). Other symptoms of high hematocrit, also not very specific to the problem, can be dizziness, continuous headache and other neurological symptoms.

Low hematocrit (low HCT)

What are the low hematocrit values? Low hematocrit is defined by a value below 39% in men and 35% in women.

Causes of low hematocrit

The main ones causes of low hematocrit I'm:

  • Prolonged aerobic training, a condition that is associated with an increase in the volume of blood inside the body, which will therefore lower the hematocrit value;
  • Deficiencies in iron, vitamin B12 and folic acid. In all these cases, fundamental elements will not be available for the creation of new erythrocytes, which will consequently be missing;
  • Bleeding throughout the body (from heavy menstruation to actual bleeding due to traumatic or anticoagulant treatment);
  • Leukemia and malignant tumors, which can lower the production of red blood cells
  • Cirrhosis of the liver
  • Chronic renal failure, which leads to an inability of the kidney to produce erythropoietin, therefore to a poor production of red blood cells;
  • Blood dyscrasias, or diseases of the production of hemoglobin (Hb), for example α and β thalassemias, which will give rise to little or no functional red blood cells and, consequently, to anemia;
  • Exposure to toxins and radiation, which can depress production to varying degrees erythrocyte;
  • Chronic inflammatory diseases, which give rise to particular anemias in which iron will not be readily available in the bloodstream and therefore the production of red blood cells will be impaired.

Low hematocrit symptoms

THE symptoms of low hematocrit they are also based on the reduction, in many cases, of the number of circulating red blood cells, closely matching those typically associated with anemic conditions. Others include paleness, brittle hair, enlarged spleen, fainting, dizziness, weakness, decreased vision, easier and increased fatigue, shortness of breath, headache, cold hands and feet, chest pain. Effective remedies such as artificial erythropoietin can be used in these situations.

Exam preparation

The patient, before undergoing the blood sample, should be fasting from the previous evening, as there are parameters ofblood count or blood chemistry tests (glycaemia, value of cholesterol) that are influenced by nutrition. However, this is not the case with hematocrit, in which no alterations in this sense can be found. Therefore, as far as this value is concerned, fasting may not be necessary. Blood is taken from a vein, typically in the arm, in adults, in the heel, in newborns; the blood is collected in a test tube which will be centrifuged for five minutes, separating the corpuscle component of the blood from the plasma component. At that point, it is read by an automated tool, which will analyze all the parameters we need in hemometry (including MCV, MCH, RBC, RDW).

Factors affecting the measurement

There are conditions, in addition to the different secretion of testosterone, that can alter the normal hematocrit value in the blood sample analyzed. For example, during pregnancy, there is an increase in the aqueous component of the blood, ie the plasma, with respect to the corpuscle component, generating a so-called physiological “hemodilution”. Therefore, for these categories of patients, this parameter will be a few percentage points lower (this value varies from laboratory to laboratory and there is no agreement of opinions) compared to a conserved number of red blood cells.

In addition to the low hematocrit in pregnancy, there is a second example: patients who reside in the high mountains (or at a certain altitude above sea level). Erythropoietin, in addition to being stimulated by testosterone, can be produced in all those situations in which the levels of oxygen in the blood are low, in order to increase the production of new red blood cells, guaranteeing a transport of oxygen in a more effective to fabrics. In the mountains, the oxygen pressure in the air is lower than that found at sea level and therefore it will be more difficult for our body to extract it to the point of satisfying all metabolic requirements. Therefore the patient, since he will not be able to extract more oxygen, will improve his transport chain, increasing the hematocrit.

Smokers also have higher hematocrit values. Due to the smoke and the absorption of quantities of carbon monoxide higher than those present in the air, the body gradually begins to absorb ever smaller quantities of oxygen from the air, stimulating it to improve the oxygen transport system, increasing , through the stimulation of erythropoietin, the number and concentration of red blood cells in the blood.

In some sport, on the other hand, the hematocrit can be altered to significantly increase the athlete's performance: this is the case, for example, in cycling. In fact, in the regulation of this sport, there is a limit to the hematocrit found in the blood tests to which they must constantly undergo. In this way, it is possible to control any fraud (it is considered to all intents and purposes a "doping"), but above all it allows to effectively protect the health of all athletes. Famous was the case of Pantani, accused of taking doping substances following the finding of an altered hematocrit value in the blood tests prior to the penultimate race of the Giro d'Italia in 1999 and who, even today, has left doubts and perplexities about the veracity of this accusation.

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