The platelet count is a test that determines the number of platelets in a person's sample of blood. Platelets, also called thrombocytes, are tiny fragments of cells that are essential for normal blood clotting.
A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (newborns)
Platelets, also called thrombocytes, are tiny fragments of cells that are essential for normal blood clotting. They are formed from very large cells called megakaryocytes in the bone marrow and are released into the blood to circulate. The platelet count is a test that determines the number of platelets in a person's sample of blood.
When there is an injury to a blood vessel or tissue and bleeding begins, platelets help stop bleeding in three ways. They:
- Adhere to the injury site
- Clump together (aggregate) with other platelets
- Release chemical compounds that stimulate further aggregation of other platelets
These steps result in the formation of a loose platelet plug at the site of the injury in a process called primary hemostasis. At the same time, activated platelets support the coagulation cascade, a series of steps that involves the sequential activation of proteins called clotting factors. This secondary hemostasis process results in the formation of strands of fibrin that weave through the loose platelet plug, form a fibrin net, and compress to form a stable clot that remains in place until the injury has healed. When the clot is no longer needed, other factors break the clot down and remove it.
Each component of primary and secondary hemostasis must be present, activated at the right time, and functioning properly for adequate clotting. If there are insufficient platelets, or if platelets are not functioning normally, a stable clot may not form and a person may be at an increased risk of excessive bleeding.
Platelets survive in the circulation about 8 to 10 days, and the bone marrow must continually produce new platelets to replace those that degrade, are used up, and/or are lost through bleeding. Determining the number of platelets in blood with a platelet count can help diagnose a range of disorders having to do with too few or too many platelets.
How is the test used?
A platelet count is used to detect the number of platelets in the blood. The test is included in a complete blood count (CBC), a panel of tests often performed as part of a general health examination.
Platelets are tiny fragments of cells that are essential for normal blood clotting. A platelet count may be used to screen for or diagnose various diseases and conditions that can cause problems with clot formation. It may be used as part of the workup of a bleeding disorder, bone marrow disease, or excessive clotting disorder, to name just a few.
The test may be used as a monitoring tool for people with underlying conditions or undergoing treatment with drugs known to affect platelets. It may also be used to monitor those being treated for a platelet disorder to determine if therapy is effective.
A platelet count may be performed in conjunction with one or more platelet function tests, which assess the function of platelets, and other tests that evaluate coagulation such as PT and PTT. If results are not within the normal interval, a number of other tests may be performed to help give clues as to the cause. Sometimes a blood smear may be done in follow up to examine the platelets under a microscope. This would help to determine, for example, whether platelets might truly be low in number or have clumped together during testing.
When is it ordered?
A platelet count is often ordered as a part of a complete blood count (CBC), which may be done at the time of a routine health examination.
- Unexplained or easy bruising
- Prolonged bleeding from a small cut or wound
- Numerous nosebleeds
- Gastrointestinal bleeding (which can be detected in stool samples)
- Heavy menstrual bleeding
- Small red spots on the skin called petechiae—may sometimes look like a rash
- Small purplish spots on the skin called purpura, caused by bleeding under the skin
Testing may also be done when it is suspected that an individual has too many platelets. An excess of platelets can cause excessive clotting or sometimes bleeding if the platelets are not functioning properly. However, people with too many platelets often have no signs or symptoms, so the condition may be found only when a platelet count is done as part of a health check or for other reasons.
What does the test result mean?
A low platelet count, also called thrombocytopenia, and accompanying signs and symptoms may be caused by a number of conditions and factors. The causes typically fall into one of two general categories:
- Disorders in which the bone marrow cannot produce enough platelets
- Conditions in which platelets are used up (consumed) or destroyed faster than normal
Examples of conditions causing a low platelet count include:
- Idiopathic thrombocytopenia (ITP), also known as immune thrombocytopenic purpura, is the result of antibody production against platelets.
- Viral infections such as mononucleosis, hepatitis, HIV or measles
- Certain drugs, such as aspirin and ibuprofen, some antibiotics (including those containing sulfa), colchicine and indomethacin, H2-blocking agents, hydralazine, isoniazid, quinidine, thiazide diuretics, and tolbutamide, are just a few that have been associated with drug-induced decreased platelet counts.
- Heparin-induced thrombocytopenia (HIT) results in low platelets when a person who is on or received heparin therapy develops an antibody. (For more on this, see the article on HIT Antibody)
- Leukemia, lymphoma, or another cancer that has spread (metastasized) to the bone marrow—people with cancers often experience excessive bleeding due to a significantly decreased number of platelets. As the number of cancer cells increases in the bone marrow, normal bone marrow cells are crowded out, resulting in fewer platelet-producing cells.
- Aplastic anemia—a condition in which the production of all blood cells is significantly reduced
- Long-term bleeding problems (e.g., chronic bleeding from stomach ulcers)
- Sepsis, especially that caused by a serious bacterial infection with Gram-negative bacteria
- Autoimmune disorders, such as lupus, where the body's immune system produces antibodies that attack its own organs or tissues, causing increased destruction of platelets
- Chemotherapy or radiation therapy, which may affect the bone marrow's ability to produce platelets
- Platelet consumption may be observed in various diseases and conditions. For example, disseminated intravascular coagulation (DIC), thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) can result in fewer circulating platelets in the blood.
- Exposure to toxic chemicals, such as pesticides, arsenic, or benzene
If the platelet count falls below 20,000 per microliter, spontaneous bleeding may occur and is considered a life-threatening risk. A person with a very low count may be given platelets through a transfusion. See Blood and Blood Components in the Blood Banking article for more details.
A high platelet count may be referred to as thrombocytosis. This is usually the result of an existing condition (also called secondary or reactive thrombocytosis) such as:
- Cancer, most commonly lung, gastrointestinal, ovarian, breast or lymphoma
- Anemia, in particular iron-deficiency anemia and hemolytic anemia
- Inflammatory conditions such as inflammatory bowel disease (IBD) or rheumatoid arthritis
- Infectious diseases such as tuberculosis
- If an individual has had their spleen removed surgically
- Use of birth control pills (oral contraceptives)
Some conditions may cause a temporary (transitory) increased platelet count. These may include:
- Recovery from significant blood loss such as from trauma or major surgery
- After physical activity or exertion
- Recovery from excess alcohol consumption and vitamin B12 and folate deficiency
Rarely, thrombocytosis is caused by a bone marrow disorder. An example is thrombocythemia, also called primary or essential thrombocythemia, a rare myeloproliferative disorder in which the bone marrow produces an extremely high number of platelets. Often there are no signs and symptoms and the condition is discovered when testing is done for a health check or for other reasons.
Individuals who have this condition may be at risk of excessive clotting (thrombosis) due to the excess platelets, but they may have bleeding problems, as the platelets may not function normally. This disorder is often associated with a mutation in the gene called JAK2. A test for this mutation should be performed if a health practitioner suspects that an individual has the disorder. More than half of the people with essential thrombocythemia have the JAK2 mutation. People with other myeloproliferative or myelodysplastic disorder, such as chronic myeloid leukemia, polycythemia vera or certain subtypes of myelodysplastic syndrome, may also have markedly higher platelet counts.
Is there anything else I should know?
Some people have platelets that tend to "pool" or collect (sequester) in their spleen, resulting in a low platelet count. However, these individuals typically do not experience any signs or symptoms related to this condition.
Living in high altitudes, strenuous exercise, and having recently delivered a baby (post partum) may cause increased platelet numbers. Drugs that may cause increased platelet counts include estrogen and birth control pills (oral contraceptives).
Mildly decreased platelet counts may be seen in women before menstruation. Up to 5% of pregnant women may have a lower platelet count at term.
Inherited disorders caused by genetic defects in platelets include Glanzmann's Thrombasthenia, Bernard-Soulier disease, Chediak-Higashi syndrome, Wiskott-Aldrich syndrome, May-Hegglin syndrome, and Down syndrome. The occurrence of these genetic abnormalities, however, is relatively rare.
Are there signs or symptoms of high or low platelet levels that I should pay attention to?
My platelet count is low. How can I increase it?
Generally, there are no lifestyle changes that you can make that would increase your platelet count. Treatment for a low platelet count usually involves addressing the underlying condition that is causing it. If your condition is mild and your platelet count is only slightly low, you may not require any treatment. If it is caused by a drug, your healthcare provider may switch you to a different one. If it is due to an autoimmune disorder, your practitioner may prescribe a drug that helps to suppress the immune system. People with serious conditions and/or platelet counts that are significantly decreased may be at risk of excessive bleeding, so they may be transfused with platelets.
My report includes mean platelet volume (MPV) and platelet distribution width (PDW). What are they?
Mean platelet volume (MPV) and platelet distribution width (PDW) are calculations performed by automated blood analyzers. MPV reflects the average size of platelets present in a person's sample of blood while PDW reflects how uniform the platelets are in size. These calculations can give the doctor additional information about platelets and/or about the cause of a high or low platelet count. Larger platelets are usually relatively young and more recently released from the bone marrow, while smaller platelets may be older and have been in circulation for a few days.
A high number of large platelets (high MPV) in a person with a low platelet count suggests the bone marrow is producing platelets and releasing them into circulation rapidly. Conversely, the MPV may be low in people with low platelet counts due to a disorder affecting production by the bone marrow. A normal PDW indicates platelets that are mostly the same size, while a high PDW means that platelet size varies greatly, a clue that there may be a disorder affecting platelets.
Often, abnormal results will prompt additional testing. Under certain conditions, platelets may clump together and falsely appear to be low in number and/or large in size so a blood smear may be performed to examine platelets directly using a microscope.
My report mentions "giant platelets." What are they?
"Giant platelet" is a term used to describe platelets that are abnormally large, i.e., as large as a normal red blood cell. These may be seen in certain disorders such as immune thrombocytopenic purpura (ITP) or in rare inherited disorders such as Bernard-Soulier disease. However, as mentioned in the previous question, a direct examination with a blood smear may be necessary to determine whether the platelets are truly giant or platelets have clumped together during testing. If platelets are clumping, repeat testing may be performed using a different collection tube containing a different anticoagulant that prevents or minimizes platelet clumping.
My complete blood count (CBC) report includes a result for immature platelet fraction (IPF). What is it?
IPF is the relative number of immature platelets (also called reticulated platelets) in the blood. Platelets are produced in the bone marrow and are normally not released into the bloodstream until they have matured. When platelet numbers in the blood are low (thrombocytopenia), it stimulates the bone marrow to produce platelets faster. When the need is great and when production cannot keep up with "demand," then an increased number of immature platelets will be released into the bloodstream.
This IPF test result would be one of the values reported when blood is evaluated using an automated hematology analyzer. The IPF may be used to help a healthcare provider determine the likely cause of a person's thrombocytopenia, that is, decrease in production by the marrow (IPF is low) versus increased loss of platelets in the blood (IPF is higher). Lab test results including platelet count and IPF can also help determine if a person needs a platelet transfusion and help monitor bone marrow recovery, such as after a bone marrow transplant. Other uses are being studied and the test's ultimate clinical utility has not yet been well determined.
If my platelet count is abnormal, what follow-up tests might my doctor order?
If the cause of the abnormal result is not apparent and cannot be determined from your medical history and physical examination, your healthcare provider may choose to order additional tests. Depending on the suspected cause and results from a CBC and blood smear, various follow-up tests may be performed. A few examples include:
• Tests for infectious diseases including bacteria and viruses
• Tests for kidney failure
• Tests for liver disease
• In unexplained, serious cases, a bone marrow biopsy
On This Site
Tests: Complete Blood Count, Platelet Function Tests, Blood Smear, von Willebrand Factor, HIT Antibody, Coagulation Factors, PT and INR, PTT, JAK2 Mutation
Conditions: Bleeding Disorders, Aplastic Anemia, Myeloproliferative Neoplasms, DIC, Leukemia, Lymphoma, Autoimmune Disorders, Sepsis
Elsewhere On The Web
American Society of Hematology: Blood Basics
Platelet Disorder Support Association
Platelets on the Web
MedlinePlus: Platelet Disorders
Mayo Clinic: Thrombocytopenia, Thrombocytosis
National Heart, Lung and Blood Institute: What are Thrombocythemia and Thrombocytosis?