If your blood will be tested only for hemoglobin, you can eat and drink before the test. If your blood also will be tested for other reasons, you may be told not to eat before the test. This is called fasting. It's done for a certain amount of time before your blood sample is taken. Your healthcare team will give you instructions.

A hemoglobin test is a blood test. It measures the amount of a protein in red blood cells called hemoglobin. Hemoglobin carries oxygen to the body's organs and tissues when you breathe in. Then it carries the waste gas carbon dioxide back to the lungs to be breathed out.

If a hemoglobin test shows that your hemoglobin level is lower than it should be, that's a sign of a condition called anemia. Causes of anemia include low levels of certain nutrients, blood loss and some long-term diseases.

If a hemoglobin test shows a higher than typical level, that also could be due to various causes. Together, these causes are named erythrocytosis. The causes of erythrocytosis can be due to issues that are not cancer. These include living at a high altitude, smoking, sleep apnea, inherited hemoglobin conditions and dehydration. Or erythrocytosis can happen due to a type of bone marrow cancer called polycythemia vera.

The healthy range for hemoglobin is:

• For men, 13.2 to 16.6 grams per deciliter.
• For women, 11.6 to 15 grams per deciliter.

Healthy ranges for children vary with age and sex. The range for a healthy hemoglobin level may differ slightly from one medical practice to another.

Lower than typical results

If your hemoglobin level is lower than is typical, you have anemia. There are many forms of anemia, each with different causes. The causes can include:

• A lack of certain nutrients. For example, you might have low iron levels, also called an iron deficiency. Or you might have low vitamin B-12 levels, also called a vitamin B-12 deficiency. It's also possible to have low folate levels, called a folate deficiency.
• Bleeding. You might have heavy menstrual bleeding or bleeding from the digestive system.
• Cancers that affect the bone marrow such as leukemia.
• Other conditions linked with anemia. These could include kidney disease, liver disease or low levels of thyroid hormones called hypothyroidism.
• A genetic condition called thalassemia. This causes low levels of hemoglobin but regular red blood cells.

If you've had anemia before, a hemoglobin level that's lower than typical may mean your treatment plan needs to be changed. Talk with your healthcare professional about what steps to take.

Higher than typical results

If your hemoglobin level is higher than is typical, it may be due to:

• Bone marrow cancer. With polycythemia vera, the bone marrow makes too many red blood cells.
• Causes that are not cancer. These include lung disease, dehydration due to extreme exercise, living at a high altitude, heavy smoking, burns and sleep apnea. Use of a performance-enhancing drug called erythropoietin also can cause a higher than typical hemoglobin level.

If you already have polycythemia vera, a higher than typical hemoglobin level may mean your treatment plan needs to be changed. Talk with your healthcare professional.

If your hemoglobin level is below or above the healthy range, your healthcare team determines what steps to take. Your healthcare professional might double-check the hemoglobin test results along with those of other tests. You may need more tests to find out the cause of your irregular hemoglobin levels.

Ask your healthcare team to help you understand exactly what your hemoglobin test results mean.

For a hemoglobin test, a member of your healthcare team takes a sample of blood. Often, this is done by inserting a needle into a vein in the arm or the top of the hand. For infants, the sample may be taken by pricking the heel or finger.

After the test, your healthcare team may have you wait in the office for a few minutes. This is done to make sure that you don't feel dizzy or lightheaded. If you feel fine, you can get back to your usual activities. The blood sample is sent to a lab to be checked.

You may have a hemoglobin test for various reasons, including:

• To check your overall health. Your healthcare professional may test your hemoglobin as part of a complete blood count (CBC) during a routine checkup. A CBC is done to look at your general health and to screen for a variety of disorders, such as anemia.
• To find the cause of certain symptoms. A hemoglobin test may be done if you have weakness, fatigue, shortness of breath or dizziness. These symptoms may point to anemia or polycythemia vera. A hemoglobin test may help find these or other medical conditions.
• To monitor a medical condition. If you have anemia or polycythemia vera, your healthcare professional may use a hemoglobin test to track your condition. The test results also can help guide treatment.