What to know about hemoglobin levels

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Each hemoglobin protein contains four iron atoms and can therefore carry four molecules of oxygen. Hemoglobin attaches to red blood cells and delivers oxygen throughout the body — everywhere that blood flows. Every one of the body’s billions of cells needs oxygen to repair and maintain itself.

Hemoglobin also plays a role in helping red blood cells obtain their disc-like shape, which helps them move easily through blood vessels.

How are hemoglobin levels tested?

A blood test can assess hemoglobin levels. This is called a hemoglobin test. Doctors may take a blood sample from the person and send it to a laboratory for testing.

Doctors may measure hemoglobin levels as part of a complete blood count (CBC) test.

A low level of hemoglobin in the blood means there is also a low level of oxygen. This can result in a condition called anemia. Tests usually express hemoglobin, or Hb, in grams per deciliter (g/dL) of blood.

In the United States, doctors diagnose anemia when the test finds hemoglobin levels to be less than 13.5 g/dL in males and less than 12.0 g/dL in females. The results for children can vary with age.

Learn more about the CBC test here.

High hemoglobin levels could signal a rare blood disorder called polycythemia. In polycythemia, the body makes too many red blood cells, causing the blood to be thicker than usual. This can lead to clots, heart attacks, and stroke. It is a serious lifelong condition that can be fatal without treatment.

Dehydration, smoking, or living at high altitudes may also cause high hemoglobin. It may also signal other conditions, such as lung or heart disease.

Low hemoglobin levels usually indicate that a person has anemia. There are several kinds of anemia:

• Iron-deficiency anemia: This is the most common type that can occur when a person does not have enough iron in their body, and it cannot make the hemoglobin the body needs. Blood loss usually causes it, but it can also happen when the body does not absorb enough iron. This can happen, for example, when someone has had gastric bypass surgery.
• Pregnancy-related anemia: This is a kind of iron deficiency anemia, which occurs because pregnancy and childbirth require a significant amount of iron.
• Vitamin-deficiency anemia: This happens when there are low levels of nutrients, such as vitamin B12 or folic acid — also called folate — in a person’s diet. These anemias change the shape of the red blood cells, which makes them less effective.
• Aplastic anemia: This is a disorder where the immune system attacks blood-forming stem cells in the bone marrow, resulting in fewer red blood cells.
• Hemolytic anemia: This can be the result of another condition, or it can have a genetic cause. It occurs when the red blood cells break up in the bloodstream or the spleen.
• Sickle cell anemia: This is a genetic condition where the hemoglobin protein is abnormal. It means the red blood cells are sickle-shaped and rigid, which stops them from flowing through small blood vessels.

Other conditions, such as kidney disease and chemotherapy for cancer, can cause anemia when they affect the body’s ability to make red blood cells.

Low hemoglobin in infants

Newborns can have temporary anemia when they are 6–8 weeks old. This occurs when they run out of the red blood cells they are born with, but their bodies have not made new red blood cells. This condition will not affect the baby adversely unless they are ill for some other reason.

Babies can also have anemia from breaking down cells too quickly, which results in yellowing skin, a condition known as jaundice. This often occurs if the mother and baby have incompatible blood types.

Learn more about infant jaundice here.

The results of a hemoglobin test will indicate if a person’s hemoglobin levels are within the normal reference range.

Different labs have slightly different guidelines, but in general, normal hemoglobin levels are as follows:

• 13.5 to 18.0 g/dL in males
• 12.0 to 15.0 g/dL in non-pregnant females
• 11.0 to 16.0 g/dL in children — this range can vary with age
• greater than 10.0 g/dL during pregnancy

If a person’s levels fall below these thresholds, they have anemia. However, low hemoglobin alone does not always diagnose the cause of anemia.

Some potential causes of anemia include:

• iron deficiency
• chronic disease
• thalassemia, a blood disorder that affects red blood cell production
• lead poisoning
• hypothyroidism
• liver disease
• some medications

Some factors, such as diet, activity level, certain medications, or menstrual periods, may affect the results of this test.

A doctor may ask questions about a person’s health history, medications, and symptoms to interpret the results. People may require further testing to rule out certain diseases or determine the underlying cause of any abnormal results.

Typical symptoms of low hemoglobin include:

• weakness
• shortness of breath
• dizziness
• fast, irregular heartbeat
• pounding in the ears
• headaches
• cold hands and feet
• pale or yellow skin
• chest pain

Older people or people who lack iron in their diets can be at risk of developing anemia.

People who perform vigorous exercise are also at greater risk, as exertion can lead to a breakdown of red blood cells in the bloodstream. Someone who is menstruating or pregnant may also be at increased risk of developing anemia.

People who have chronic health conditions, including autoimmune conditions, liver disease, thyroid disease, and inflammatory bowel disease, may have lower hemoglobin levels, which increases the chances of developing anemia.

Hemoglobin levels increase when a person needs more oxygen in their body. Consequently, someone who has lung or kidney disease, smokes, or is dehydrated, may be at risk of increased hemoglobin levels.

While many types of anemia are not preventable, eating iron-rich foods, such as beef, dark green leafy vegetables, dried fruits, and nuts can prevent anemias caused by iron or vitamin deficiencies.

Meat and dairy are good sources of vitamin B12. Citrus juices, legumes, and fortified cereals contain folic acid.

The American Society of Hematology recommends against taking an iron supplement unless a doctor advises doing so.

Also, smoking cessation — for those who smoke — and drinking plenty of water can help avoid high hemoglobin levels.

Find out about good sources of iron here.

The outlook for anemia depends on its cause.

Iron-deficiency anemia, vitamin-deficiency anemia, and anemia from blood loss have a good outlook with treatment. People with iron deficiency typically need treatment for at least 3 months to rebuild their iron stores.

Sickle cell disease, polycythemia, and some other chronic diseases that can cause anemia, such as kidney disease, will require ongoing management and treatment.

Untreated anemia can cause:

• preterm labor, birth complications, and anemia in the baby if a person is pregnant
• heart disease, including heart attack
• restless leg syndrome
• multiorgan failure and, eventually, death

Older people and people with comorbidities are more vulnerable to serious complications.

Hemoglobin is an iron-rich protein in red blood cells that helps to transport oxygen throughout the body.

High hemoglobin levels could be due to polycythemia. This is where the body makes excess red blood cells and can lead to clots, heart attacks, and stroke without treatment.

Anemia is a group of medical conditions that undermine the body’s ability to get oxygen. This is because people with anemia do not have enough hemoglobin or red blood cells to distribute oxygen throughout the body.

It is important to diagnose the cause of anemia and treat it. Dietary changes, nutritional supplements, blood transfusions, symptom management, and treatment for underlying medical conditions may help. People with symptoms of anemia, a history of anemia, or at high risk of anemia should consult a doctor for regular anemia testing.

Read the article in Spanish.