The hemoglobin (Hgb) test measures how much hemoglobin your red blood cells contain.

Hgb is a protein produced by your bone marrow that’s stored in red blood cells. It helps red blood cells transport oxygen from your lungs to your body through your arteries. It also transports carbon dioxide (CO2) from around your body back to your lungs through your veins. Hgb is what makes red blood cells look red.

Abnormally high or low Hgb can cause symptoms like exhaustion, dizziness, or shortness of breath. Your doctor may suggest an Hgb test if you’re experiencing these symptoms. You may have an underlying condition that needs to be diagnosed.

Let’s learn more about why you may need an Hgb test, what the typical ranges are for Hgb, what symptoms to watch out for, and what can cause abnormal Hgb levels.

The Hgb test uses a sample of your blood to determine hemoglobin levels. To take a sample, your doctor or a nurse extracts blood from a vein by pricking your finger or inserting a needle with an attached tube into the crease of your arm. The sample is then stored in the tube to be analyzed later at a lab.

The needle may cause brief discomfort, but the insertion usually lasts less than a minute. If you’re sensitive to getting blood drawn or the sight of blood, have someone come with you.

The Hgb test may be ordered as part of a complete blood count (CBC) test. A CBC test also measures other important components of your blood, such as white blood cells and platelets. Abnormal levels of any of these cells can indicate underlying conditions or blood disorders.

Here are a few other reasons your doctor may order an Hgb test:

  • You have parents or other family members who have blood disorders, such as sickle cell anemia.
  • You have an infection.
  • You don’t have enough iron in your diet.
  • You’ve lost a lot of blood after surgery or a traumatic injury.
  • You are pregnant.
  • You have a medical condition that can affect your Hgb levels.

You don’t need to fast for the Hgb test specifically. You may need to fast, avoiding food or liquids with calories for about 12 hours, if your doctor plans to test the chemistry of your blood at the same time. You should drink plenty of water, however.

Your age and gender both affect your Hgb levels. Typical healthy Hgb levels are as follows:

CategoryHgb level, in grams per deciliter (g/dL)
infants11 to 18
young children11.5 to 16.5
adult males13 to 16.5
adult females (nonpregnant)12 to 16
adult females (pregnant)11 to 16

For men, Hgb levels below 13 g/dL are considered low. For women, Hgb levels below 12 g/dL are considered low if not pregnant. This threshold may change with certain conditions. It can also vary depending on the lab, so be sure to check your lab’s reference range. For children these levels may also vary due to age, especially in infants under 6 months old.

Low Hgb is also known as anemia, which means that you don’t have enough red blood cells in your body. With anemia, a blood test will also show that you have a low red blood cell count and may have low hematocrit, the volume of red blood cells to other components in your blood.

Anemia can have many causes, so symptoms can vary widely. Common anemia symptoms can include:

Low Hgb levels can be caused by any condition that affects your body’s ability to create red blood cells or conditions that lower red blood cells in your bloodstream.

Possible causes of low Hgb include:

  • lack of iron in your diet, which makes it harder for your bone marrow to produce Hgb
  • lack of folate or vitamin B-12, which can lead to your body producing fewer red blood cells than are needed
  • severe blood loss after surgery or a major injury
  • internal bleeding from stomach ulcers, stomach or colon cancer, or internal injuries
  • sickle cell anemia, a genetic condition that causes red blood cells to be abnormally sickle-shaped and able to carry less Hgb
  • hypothyroidism, which means that the thyroid gland doesn’t produce enough thyroid hormones
  • splenomegaly, or an enlarged spleen from infection, liver conditions, or cancer
  • bone marrow conditions, such as leukemia, that prevent your bone marrow from producing enough red blood cells
  • chronic kidney disease, in which your kidneys don’t function properly (resulting in a deficiency of erythropoietin, a hormone that stimulates red blood cell production in your bone marrow)

Other causes can include:

  • donating blood too often
  • heavy bleeding during your period
  • alcohol misuse
  • chronic health problems such as autoimmune diseases or cancer

High Hgb is known as polycythemia. This means you have too many red blood cells. Polycythemia vera is a cancer of the blood in which your bone marrow overproduces red blood cells.

With polycythemia, a blood test also shows that you have a high red blood cell count and high hematocrit.

Common symptoms of high Hgb levels include:

  • itchiness
  • headache
  • dizziness
  • getting easily bruised or bleeding
  • sweating more than usual
  • painful joint swelling
  • abnormal weight loss
  • yellow tint to the eyes and skin (jaundice)
  • feeling exhausted
  • purple or reddish tint to the skin

High Hgb may result from your body needing to store more Hgb in red blood cells due to your environment, a condition that affects your heart or lung function, or lifestyle choices.

Possible causes of these types of high Hgb levels include:

  • living at high altitudes where there’s not as much oxygen in the air, such as in the mountains
  • smoking tobacco products, including cigarettes or cigars
  • chronic obstructive pulmonary disease (COPD), a condition that inflames the lungs and blocks air from getting into your lungs
  • heart or lung diseases that affect your ability to breathe, your lungs’ ability to pass oxygen into your bloodstream, or your heart’s ability to pump normally
  • taking erythropoietin unnecessarily, such as to enhance high-level physical performance

Other causes include:

  • being severely dehydrated
  • heart failure
  • cancer of the liver or kidneys

Your doctor may recommend a Hgb test if you have symptoms of abnormal Hgb levels or if you’re pregnant. The earlier you notice the symptoms of abnormal Hgb levels and have the cause diagnosed, the more likely you are to have successful treatment.

See your doctor if you’re experiencing any symptoms of high or low Hgb. If you have a family history of blood disorders or conditions that can affect bone marrow or red blood cell production, you will likely need regular Hgb tests along with a CBC to monitor how these health problems may be affecting your blood cells.