The Function of Blood Platelets or Thrombocytes

And Causes of High or Low Values

Platelets, also known as thrombocytes, are special blood cells with an important function. Platelets control blood clotting, which means they are critical for healing wounds and stopping bleeding.

A normal platelet count is 150,000–450,000 platelets per microliter. A low platelet count can put you at risk for uncontrolled bleeding. It is also possible to have too many platelets in your blood. This leads to a potentially life-threatening condition related to abnormal clotting.

This article explains how platelets work in the body and how your healthcare provider can measure your platelet count to determine if you have too few or too many. It also covers possible disorders related to platelet counts.

Close up of scientists hands selecting a blood sample for medical testing
Andrew Brookes / Getty Images

What Platelets Do

Platelets get their name from their appearance. They look like tiny, colorless plates. When a blood vessel tears, platelets stick together to form a clot, plugging the tear to stop bleeding.

Platelets are one of three types of blood cells. (Red blood cells and white blood cells are the other types of blood cells.) Platelets form in the bone marrow from cells known as megakaryocytes.

Platelets form clots through a multi-step process that includes:

  1. Adhesion: This is the first step in which platelets rush to the site that's bleeding. For example, if you cut your finger and rupture a blood vessel, it will bleed. To stop the blood flow, platelets within that broken vessel start attaching to the site of the injury. They then send out chemical signals for more help.
  2. Aggregation: In the next step, more platelets answer the call and begin to connect to each other to form a clot.
  3. Coagulation: As platelets build up at the site of the wound, they seal up the blood vessel in what's called a coagulation cascade. A structural protein known as fibrin joins the platelets to knit the clot together. Fibrin is what forms the scab on a cut.

Aspirin and some nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit normal platelet function, which is why you may be asked to stop using them for a period of time before a surgery or procedure.

What Causes Low Platelets?

If the body doesn't have enough platelets circulating in the blood, you may develop a condition called thrombocytopenia. This occurs when your bone marrow makes too few platelets, which means you're at greater risk for bruising and prolonged bleeding that takes a long time to slow down. You may need medical treatment for this condition.

The following are some factors that may contribute to low platelet count:

  • Chemotherapy or radiation therapy: These treatments suppress or kill off the blood-producing cells in your bone marrow, leading to low platelet production.
  • Viral infections: Hepatitis C or human immunodeficiency virus (HIV) infections may attack bone marrow, affecting thrombocyte production.
  • Autoimmune conditions: Platelets may be affected by conditions such as lupus (an autoimmune disease that aaffects many different tissues and organs) or immune thrombocytopenic purpura (ITP, a condition of low platelets).
  • Pregnancy: Hemolysis, elevated liver enzymes, low platelet count syndrome, better known as HELLP, is a condition that may occur during pregnancy. It's a type of preeclampsia (characterized by high blood pressure) and may result in the breakdown of blood cells and platelets.
  • Medications: Anticoagulants such as Coumadin (warfarin) and Lovenox (heparin) may stop platelet production.

Other examples of conditions that may cause thrombocytopenia include having a mechanical heart valve, chronic alcohol use disorder, liver disease, severe sepsis (a life-threatening infection), and toxic exposures.​

What Causes High Platelets?

If the body has too many platelets in circulation, it may be related to one of two conditions:

  • Thrombocythemia: This occurs when the bone marrow makes too many platelets. If you have thrombocythemia, you may have other blood cell disorders.
  • Thrombocytosis: This is a high platelet count caused by another pre-existing condition.

Disorders that may contribute to high platelet count include the following:

  • Primary bone marrow disorder: Essential thrombocytosis is a condition in which the megakaryocytes (cells that make platelets) in the bone marrow produce too many platelets, increasing the risk of blood clots.
  • Chronic inflammation in the body: Inflammatory conditions such as rheumatoid arthritis (RA, an autoimmune disease attacking the joints) and inflammatory bowel disease (IBD, Crohn's, and ulcerative colitis, which affect the digestive tract) may result in high platelet counts because the bone marrow makes more white blood cells and platelets to combat cellular damage caused by inflammation.
  • Infection: Bone marrow cells increase the production of white blood cells and platelets to help fight infection.
  • Iron deficiency anemia: Reactive or secondary thrombocytosis may result when the body is undergoing a breakdown of red blood cells. The bone marrow cells go into overproduction to meet the body's needs.
  • Spleen removal: Up to one-third of platelets are stored in the spleen at any time. Removing this organ causes a greater number of platelets to stay in the bloodstream since they can't be stored in the spleen. This is usually a temporary condition.
  • Cancer: High platelet counts can also be seen in cancer, especially with gastrointestinal cancer as well as lymphoma, lung, ovarian, and breast cancer. This may be caused by inflammation related to cancer.

In addition, a temporary increase in the platelet count can happen after major surgery or trauma.

Testing and Your Platelets

Your doctor can assess the numbers, size, and health of platelets using a complete blood count (CBC) test.

What Is a CBC?

A complete blood count (CBC) is a test that gives information about different blood cells. First, blood is drawn by a doctor. Then, it's sent to a lab that checks levels for red blood cells, white blood cells, and platelets. A CBC helps doctors diagnose a number of medical conditions.

The CBC provides specific lab markers that refer to platelets. These markers include the following.

Platelet Count (PLT)

Just as it sounds, this is the actual number of platelets you have (per microliter of blood). Your range can vary from low to elevated:

  • Low range: Less than 150,000 platelets per microliter
  • Normal range: 150,000–450,000 platelets per microliter
  • Elevated range: 500,000–1 million platelets per microliter

A platelet count that is too low can cause uncontrolled bleeding, while a count that is too high can put you at risk for excessive blood clots.

A platelet count below 20,000 per microliter is a life-threatening risk. You might start to bleed spontaneously and seemingly without reason. The bleeding could be very hard to stop. If this occurs, you may be given a platelet transfusion.

It's important for your doctor to know your platelet count before and after surgery. These numbers are used to predict the risk of bleeding and clotting problems. It's also important to track the count if you're undergoing chemotherapy and radiation therapy because these treatments can interfere with platelet production in the bone marrow.

Mean Platelet Volume (MPV)

The mean platelet volume (MPV) is a measure of the average size of your platelets. Younger platelets are larger than older ones, so an elevated number means you're producing and releasing them rapidly. A low number means there's a slowdown in production in the bone marrow.

Platelets live in the bloodstream for about eight to 10 days.

Platelet Distribution Width (PDW)

PDW measure the variations in size among the platelets. This measurement can reveal conditions that affect the platelets.

Platelet Function Tests

Platelet function tests are a series of exams that determine if platelets are effectively forming clots. Among the things these tests look at are:

  • The time it takes for a clot to close a wound
  • How strong a blood clot is
  • How well platelets aggregate or clump together
  • How long it takes for bleeding to stop

These tests are usually done if there are symptoms or risk of excessive bleeding or if a doctor needs to monitor antiplatelet medications.


Platelets are tiny cells with a highly important function in the body: to stop bleeding. There is a wide range of normal in terms of platelet count. Your healthcare provider can usually determine whether there's a problem with a routine blood test. Although, additional tests that measure the size of the platelets and how well they function might also be needed.

Having too few or too many platelets is a symptom of another condition. Your doctor will need to do additional tests to understand the underlying problems so you can be properly treated and your excessive bleeding or problems clotting can be managed.

A Word From Verywell

It's important to be aware of extreme platelets levels, too high or too low. This is especially true if you're considering surgery or undergoing another procedure that may require healthy bleeding and clotting.

If you've already been diagnosed with very low or very high levels of platelets, make sure you communicate regularly with your healthcare provider about a safe plan of action.

Frequently Asked Questions

  • How are low blood platelets treated?

    Treatment is only necessary if thrombocytopenia is causing health problems. Treatment may include blood transfusion, which is a temporary fix; spleen removal; and medications that may include steroids and immunoglobulins.

  • How are high blood platelets treated?

    Many people who experience high blood platelets, or thombocytosis, do not require treatment but may be monitored regularly by their doctor. If symptoms are problematic, treatment may include daily low-dose aspirin to prevent blood clots, medications that reduce platelet production, and treating the underlying cause of the condition.

  • What do blood platelets look like?

    Under a microscope, blood platelets look like small plates when inactive. When activated, they look like an octopus as they grow small tentacles.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. University of Rochester Medical Center. Health Encyclopedia. What Are Platelets?

  2. National Heart, Lung, and Blood Institute. Thrombocytopenia.

  3. National Heart, Lung, and Blood Institute. Thrombocythemia and Thrombocytosis.

  4. Natioinal Heart, Lung, and Blood Institute. Thrombocythemia and Thrombocytosis.

  5. MedlinePlus. Platelet Tests.

  6. Johns Hopkins Medicine. What are platelets and why are they important?

Additional Reading

By Suzanne Dixon, MPH, RD
Suzanne Dixon, MPH, MS, RDN, is an award-winning registered dietitian and epidemiologist, as well as an expert in cancer prevention and management.