Hemoglobin (Hb)

Hemoglobin (Hb)Synonyms: Hgb, Hb

What is HGB?

Hemoglobin is an iron-containing red pigment. Blood is red because it contains hemoglobin that binds oxygen.

Apart from giving blood its red color, hemoglobin can form unstable bonds with oxygen enabling red blood cells to deliver oxygen to all body organs and tissues.

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The air contains oxygen. When it gets into the lungs, oxygen molecules are taken up by hemoglobin that flows through the body’s blood vessels transporting this oxygen to all cells of the body.

When an oxygen molecule is delivered to its cell, hemoglobin removes a molecule of carbon monoxide from it. Then the red blood cell moves back to the lungs carrying the carbon monoxide molecule. The carbon monoxide is released from the body during exhalation, and a new oxygen molecule is picked up. After that, the red blood cell moves further to deliver this oxygen to a new cell.

HGB in Pregnancy

Hemoglobin level is especially important during pregnancy. Hemoglobin is necessary for the efficient supply of nutrients and oxygen to the fetus. Until birth, the baby’s circulatory system is connected to the mother’s circulatory system. In the woman’s body, an additional placental circle of blood circulation is formed. In order to provide the mother’s body and placenta with enough oxygen, the woman’s total circulating blood volume increases. The plasma volume increases by 40-50% while red blood cell levels increase by only 20-30%, which leads to a relative decrease in the hemoglobin level in the blood. Such condition is called physiological anemia of pregnancy

HGB in children

In a fetus, hemoglobin begins to be produced in the womb from 10 to 12 weeks of pregnancy. This hemoglobin is different from adult hemoglobin and is called fetal hemoglobin (HbF). HbF has a stronger oxygen affinity and gives the fetus better access to oxygen.

Low hemoglobin leads to oxygen starvation and, as a result, reduced immunity and developmental delay in children. Increased hemoglobin is also dangerous. This condition is fraught with blockage of blood vessels.

Units of Measurement

The hemoglobin level can be expressed in international units:

  • g/L
  • mmol/L

Conventional units:

  • g/dL (gram per deciliter)
  • g/100mL (gram per 100 milliliters)
  • g% (gram-percent)
  • mg/mL (milligrams per milliliter)

Conversion factors:

  • g/dL = g/100mL = g%
  • g/L / 10 => g/dL
  • g/L = mg/mL
  • 1 g/L = 0,06206 mmol/L
  • 1 g/dL = 0,6206 mmol/L

Reference Range

The reference ranges for hemoglobin concentration can differ slightly from lab to lab. According to the World Health Organization (WHO), a normal level of hemoglobin in adult male is 138 to 172 g/L (13.8-17.2 g/dL). A normal hemoglobin level for a non-pregnant adult woman is 121 to 151 g/L (12.1-15.1 g/dL).

HGB Level in Pregnancy

During pregnancy, blood volume increases by roughly 50%. But plasma (liquid part of the blood) increases more than red blood cells. This results in a physiological anemia of pregnancy.

According to WHO criteria, the minimum concentration of hemoglobin in the blood of a healthy pregnant women is 110 g/L (11 g/dL) in the first half of pregnancy and 105 g/L (10.5 g/dL) in the second. Iron deficiency is the most common cause of anemia in pregnant women worldwide.

HGB Level in Children

The maximum concentration of hemoglobin in the blood of a child is observed in the first days of his life (reaches 240 g/L in the first 3 days). Then the hemoglobin level gradually decreases.

Hemoglobin rates for children:

Age Reference interval
0-2 weeks120-200
2-6 months100-170
6 months – 1 year95-140
1-6 years95-140
6-18 years100-155

High HGB

A significant increase in hemoglobin is dangerous because it increases the viscosity of the blood. There is a risk of blood clots and blockage of blood vessels.

Hemoglobin is found in red blood cells, so a high level of hemoglobin usually indicates an increased level of red blood cells.

Symptoms of High Hemoglobin

An increased hemoglobin concentration in the blood in most cases is detected by chance. Symptoms of high hemoglobin are usually associated with the underlying disease that caused this increase.

Symptoms of high hemoglobin:

  • weakness, fatigue
  • itchiness
  • dizziness, headache
  • sweating
  • increased blood coagulation can lead to edema, numbness of limbs, temporary loss of vision, thrombosis

In some cases, a high level of hemoglobin may be normal. It is observed among people living at high altitudes (there is not enough oxygen in the air, and the body, increasing the level of hemoglobin, increases its content in the blood). A higher level of hemoglobin (compared with normal) can be observed in people who smoke or with dehydration.

Causes of Increased Hemoglobin

Pathological causes of an increase in hemoglobin may include:

  • dehydration due to vomiting, diarrhea, or diuretics
  • diseases of the lungs and heart, especially chronic
  • some tumors
  • bone marrow disease (polycythemia vera)
  • anabolic steroid abuse

High HGB in Pregnancy

During pregnancy, the upper limit of hemoglobin is lower than in non-pregnant women due to physiological changes in the body.

An increase in hemoglobin concentration during pregnancy is dangerous for both the mother and the baby. This can lead to high blood viscosity, increased load on the woman’s cardiovascular system, blood clots.

The study showed that women with a high concentration of hemoglobin in the first trimester have an increased risk of developing hypertension.

High HGB in Children

A slightly higher than normal hemoglobin in children is quite acceptable. But if the hemoglobin level significantly exceeds the normal range, this situation requires a more detailed examination. A higher amount of hemoglobin in the blood is often associated with an increased level of red blood or insufficient plasma volume. Quite often, an increase in hemoglobin indicates loss of fluid (dehydration), which leads to a thickening of the blood. It may be caused by intense sweating, stress, insufficient drinking, the use of diuretics, and fever.

Many children with elevated hemoglobin have no symptoms. If high hemoglobin is caused by dehydration due to intestinal infection, the child will have nausea, diarrhea. In some children, high hemoglobin may cause increased fatigue, impaired appetite, sleepiness, increased blood pressure, headaches, and frequent bruising.


A decrease in the amount of hemoglobin (and/or red blood cells) in the blood is called anemia. There are many types of anemia. Low hemoglobin levels in most cases indicate iron deficiency in the body, aka iron deficiency anemia. In addition, low hemoglobin can be a sign of aplastic anemia, anemia due to a deficiency of vitamin B12 or folic acid, hemolytic anemia, sickle cell anemia. Anemia can also be caused by kidney disease and chemotherapy.

Symptoms of Low Hemoglobin

Low hemoglobin can cause the following symptoms:

  • fatigue, weakness
  • dizziness
  • headache
  • shortness of breath
  • fast, irregular heartbeat
  • chest pain
  • pale skin, dark circles under the eyes
  • cold hands and feet
  • hair loss, brittle nails

Causes of Low Hemoglobin

A low level of hemoglobin is usually associated with a disease or condition that results in a lack of red blood cells in the body. This can happen if:

  • The body produces less red blood cells than required (aplastic anemia, iron deficiency anemia, cancer, cirrhosis, chronic kidney disease, some medications).
  • The body destroys red blood cells faster than they are produced (enlarged spleen, thalassemia, sickle cell anemia, vasculitis)
  • Blood loss (from a stomach ulcer or duodenal ulcer, hemorrhoids, oncology, severe menstrual bleeding, frequent blood donation).

Risk factors for anemia include:

  • a diet that is low in iron, vitamins and minerals
  • high physical activity
  • heavy menstrual bleeding
  • pregnancy
  • chronic diseases
  • age

Low Hemoglobin During Pregnancy

Low hemoglobin during pregnancy is a common occurrence. Mild anemia is a consequence of normal physiological changes. During pregnancy, the amount of blood increases by 20-30%, which means that more iron is needed to produce enough hemoglobin.

More severe anemia can lead to an increased risk of anemia in a child at an early age. Anemia in the first and second trimester increases the risk of premature birth and low birth weight. Low hemoglobin increases the risk of bleeding during labor and complicates the fight against infections.

The most common types of anemia during pregnancy are:

  • Iron-deficiency anemia
  • Folic acid deficiency anemia
  • Vitamin B12 deficiency anemia

Inadequate diagnosis and underestimation of the health status of the expectant mother increases the risk of complications. That is why, it is necessary to check the hemoglobin level before the pregnancy in order to correct anemia and avoid further problems.

Factors contributing to low hemoglobin during pregnancy:

  • short intervals between pregnancies
  • heavy menstrual bleeding before pregnancy
  • toxicosis
  • multiple pregnancy

Low hemoglobin in children

In newborns, a decrease in hemoglobin may be seen at 6-8 weeks. This is because they are running out of red blood cells with which they were born, and new red blood cells have not yet been produced in sufficient quantities.

In most children, mild anemia is asymptomatic and is detected by a routine blood test.

The treatment of anemia in children depends on the type of anemia and its cause. In most cases, you need to change your diet or use dietary supplements and medicines. In complex cases, long-term treatment and blood transfusion may be required. Most problems caused by anemia can be prevented by diagnosing anemia at an early stage.

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