Hematocrit: What do values ​​that are too high or too low mean?

Blood sample: The hematocrit value represents the flow properties of the blood.

Blood sample: The hematocrit value represents the flow properties of the blood. (Source: vitranc/getty-images-bilder)
The hematocrit (Hk, Hkt, Hct) is a blood value. It indicates how thick the blood is. What’s behind it when it’s too high or too low.

The hematocrit is one of the classic laboratory values ​​that is always recorded as part of a small or complete blood count. However, many people don’t know what this term means. If the hematocrit value is outside the norm, this may have harmless causes. But there can also be various illnesses behind it.

What is hematocrit (Hk)?

The hematocrit (also: Hct, Hkt or Hk value) provides information about whether the blood is thin or thick. Using the Hk value, doctors can determine what percentage of the blood consists of blood cells. The higher the value, the higher the solid content of the blood – and the more difficult it is to flow.

Blood is made up of liquid and solid components. The liquid part is the blood plasma, while the solid part includes the different blood cells. The blood cells include

  • the red blood cells (erythrocytes),
  • the white blood cells (leukocytes) as well
  • the blood platelets (thrombocytes).

At around 99 percent, red blood cells make up by far the largest proportion of blood cells. The hematocrit is therefore also a measure of how many red blood cells there are in the blood.

Hematocrit values: which ones are normal?

Which hematocrit value is considered normal varies depending on age and gender. Men have a higher value than women. The hematocrit level is lower in children than in adults. The hematocrit value also temporarily decreases in pregnant women in the first and second trimesters of pregnancy.

The Hk value is usually given in percent (or alternatively in liters per liter, l/l). It tells you what percentage the solid components of the blood make up. For example, with a Hk value of 45 percent, this means: 45 percent of the blood consists of cells and 55 percent of liquid blood plasma.

Hematocrit: normal values ​​in adults

Women Men
37-47% 40-54%

Hematocrit too high: what does that mean?

If the hematocrit is too high, the blood contains more blood cells than usual. The blood is correspondingly thicker and can no longer flow as well.

In the long term, a hematocrit value that is too high means that the heart has to pump more to transport oxygen-rich blood to all organs. The risk of a blood clot forming in a vessel also increases.

Possible causes of an increased hematocrit include:

  • dehydration of the body, e.g. B. due to severe diarrhea, increased urine output or burns
  • an increased number of red blood cells (secondary polyglobulia), e.g. B. due to diseases of the lungs, heart and kidneys, certain tumors or staying at high altitude
  • rare: the disease polycythemia vera (primary polyglobulia)

Increased hematocrit due to dehydration

If there is too little fluid in the body, dehydration occurs. The result: The blood becomes more viscous and the hematocrit value rises.

Dehydration can have various causes. These include, for example:

  • severe vomiting and/or diarrhea
  • increased urine output due to kidney disease or diabetes mellitus
  • diuretic medications (diuretics)
  • extensive burns

The hematocrit value can usually be brought back to normal by giving fluids. In any case, it is important to treat the underlying disease.

Secondary polyglobulia: Increased hematocrit due to too many red blood cells

If there are too many red blood cells in the blood (polyglobulia), this is usually the result of an illness. Experts then speak of secondary polyglobulia.

Polyglobulia is always accompanied by high hematocrit values. In addition, the number of red blood cells and hemoglobin in the blood is increased.

Possible causes of secondary polyglobulia are:

  • long-term smoking
  • chronic lung diseases such as COPD, asthma
  • congenital heart defects
  • Kidney diseases (e.g. cystic kidneys)
  • certain tumors (e.g. Wilms tumor, renal cell carcinoma)
  • a longer stay at high altitude
  • Blood doping

Secondary polyglobulia is usually due to a lack of oxygen (hypoxemia) in the body. One of the tasks of red blood cells is to transport oxygen from the lungs to every cell in the body. When there is a lack of oxygen, the kidneys release the hormone erythropoietin (Epo). This stimulates the production of more red blood cells in the bone marrow. In this way, the body tries to compensate for the deficiency.

In addition, certain tumors can stimulate Epo production. Then more red blood cells form without there being a lack of oxygen.

Polyglobulia can arise not only from various illnesses, but also from a long stay at high altitude. The reason: The oxygen content in the air is lower at high altitude than at low altitude, so the body produces more red blood cells.


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