A platelet count measures the average platelet level in a person’s blood. High or low platelet levels can increase the risk of clotting or excessive bleeding.

Platelets, also called thrombocytes, are fragments of larger cells made in the bone marrow called megakaryocytes. These fragments are crucial to wound healing.

The mean platelet count blood test is typically part of a complete blood count (CBC) test. A CBC reveals important information about the number of different blood cells in the body.

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A person’s platelet levels can change with age, and certain medical conditions can also affect them.

A platelet count that is too low or too high can lead to health complications. A low platelet count is known as thrombocytopenia, while a high platelet count is known as thrombocytosis.

Tests measure average platelet levels per microliter (mcL) of blood. Below are guideline platelet levels.

ResultPlatelet count
high platelet level (thrombocytosis)more than 450,000
normal platelet level150,000–450,000
low platelet level (thrombocytopenia)less than 150,000

However, some studies suggest that normal platelet levels should take into account other variables, such as a person’s age and sex.

A high platelet count can occur when something causes the bone marrow to make too many platelets. When the reason is unknown, it is called primary or essential thrombocytosis.

When excess platelets are due to an infection or other condition, it is called secondary thrombocytosis.

Higher risk of blood clots

A person’s blood clots more quickly when they have too many platelets.

Clotting is a natural protection against bleeding. The body produces more platelets during and following an injury.

However, because platelets cause blood clotting, they can also cause dangerous blood clots in the arms or legs. The blood clot may break off or travel to another area of the body.

The risk of a blood clot is higher in people confined to bed by illness or who cannot move their limbs.

Someone who has a high platelet count because of a recent injury but who must remain in bed may need monitoring to reduce the risk of blood clots as a result.

A low platelet count can make it difficult for the blood to clot, putting a person at risk of excessive bleeding. The cause may be an inherited tendency not to produce enough platelets, but the cause may also be unknown. Or it may be due to an underlying medical condition.

Higher risk of spontaneous bleeding

If the blood platelet count falls below 20,000 per mcL, a person can begin bleeding spontaneously. This is a medical emergency, and people who experience spontaneous bleeding may require a blood, or platelet, transfusion.

Low platelet count increases the risk of death in people who experience a traumatic injury.

Several factors can cause a person’s platelet levels to change. These include acute and chronic medical conditions and age.

Causes of high platelet counts

Some temporary conditions can cause a higher than normal platelet count. A doctor may order a retest a few days or weeks later if this happens. Some common reasons for high platelet levels include:

  • recovering from a recent injury
  • recovering from blood loss after surgery
  • recovering from excessive drinking or vitamin B12 deficiency
  • intense physical activity or exertion, such as from running a marathon
  • using birth control pills

If a person’s platelet count remains high, chronic medical conditions may be responsible. These may include:

  • Cancer: Lung, stomach, breast, and ovarian cancers, as well as lymphoma, can cause high platelet counts. Additional blood testing, imaging scans, or a biopsy can test for cancer.
  • Anemia: People with iron deficiency or hemolytic anemia may have high platelets. Further blood testing can detect most forms of anemia.
  • Inflammatory disorders: Diseases that cause an inflammatory immune response, such as rheumatoid arthritis or inflammatory bowel disease (IBD), can increase platelet count. A person will have other symptoms in most cases.
  • Infections: Some infections, such as tuberculosis, can cause high platelets.
  • Splenectomy: Removal of the spleen can cause a temporary increase in platelets.

Causes of low platelet counts

Common causes of low platelet volume include:

  • Viruses: Viruses such as mononucleosis, HIV, AIDS, measles, and hepatitis may deplete platelets.
  • Medication: Drugs, such as aspirin, H2-blockers, quinidine, antibiotics containing sulfa, and some diuretics may lower platelet count.
  • Cancer: Cancer that has spread to the bone marrow can harm the body’s ability to make new platelets. Lymphoma and leukemia are common culprits.
  • Anemia: A type of anemia called aplastic anemia reduces the number of all kinds of blood cells, including platelets.
  • Infection: A bacterial infection, especially the blood infection called sepsis, can reduce platelet count.
  • Autoimmune disorders: Autoimmune diseases such as lupus and Crohn’s disease lower platelet count by causing the body to attack its tissue.
  • Chemotherapy: Chemotherapy harms existing tissue and cancer cells, making it difficult for the body to produce platelets.
  • Poisoning: Exposure to some pesticides can damage platelets.
  • Cirrhosis: Liver cirrhosis, often due to excessive drinking, can reduce platelet count.
  • Chronic bleeding: Any disorder that causes ongoing uncontrolled bleeding, such as stomach ulcers, can deplete platelets.

Platelet count tends to decline with age. A platelet count that is lower than it once was or on the lower end of normal may not be a cause for concern in an older adult — especially if there are no other symptoms.

A platelet count test reveals the average number of platelets a person has per microliter of blood.

Doctors can perform the test on its own or as part of a CBC test. They will often perform a platelet count test if they suspect a disorder that affects platelet count.

The test involves drawing blood from a vein in the arm or hand.

Obtaining a sample of blood from a vein takes a few minutes and generally causes only minimal discomfort. Occasionally, some people may feel queasy or light-headed while the blood is drawn or shortly after. Taking slow deep breaths is usually enough to calm these feelings. Some people may develop a small mark or bruise.

A technician will put the blood sample into a machine that counts the number of platelets and produces a report of the findings.

Is it safe?

The test is safe, and complications are rare. People with bleeding disorders should tell their doctor about any history of bleeding issues. Most people find that the test is only a brief inconvenience and a source of mild discomfort.

When do you get the results?

The amount of time it takes to get the results varies.

Hospitals administering the test for emergencies or people about to undergo surgery often get the results almost immediately. It can take a few days to get the results when a doctor’s office orders the test from an outside lab.

Changes in platelet count may mean that a person has a chronic illness or an issue with the bone marrow.

It is generally not possible, however, to diagnose a medical condition based on platelet count alone. People should talk with a doctor about further testing if a blood test reveals low platelets.

It is advisable to inform the doctor about any other symptoms, which can help narrow down testing options.

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