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 your 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 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 the clotting process must be present, activated at the right time, and functioning properly for adequate clotting. If there is an insufficient number of platelets or if the platelets are not functioning normally, a stable clot may not form and you may be at an increased risk of excessive bleeding.
Platelets survive in the circulation about 8 to 10 days, so 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 determine the number of platelets in your blood. The test is included in a complete blood count (CBC), a panel of tests often performed as part of a general health exam.
A platelet count may be used:
- To screen for or diagnose various diseases and conditions that can cause problems with blood 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.
- As a monitoring tool if you have an underlying condition, are being treated for a platelet disorder, or are undergoing treatment with drugs known to affect platelets.
A platelet count may be performed in conjunction with tests that evaluate coagulation, such as PT and PTT. A blood smear may be done in follow up to examine the platelets using a microscope. This would help confirm whether platelets might truly be low in number or have clumped together, in addition to assessing their appearance with regards to size and granularity.
When is it ordered?
A platelet count is often ordered as a part of a complete blood count (CBC) when you have a routine health exam.
- Unexplained or easy bruising
- Prolonged bleeding from a small cut or wound
- Numerous nosebleeds
- Bleeding in the digestive tract (blood may be present in stool, which may be dark black)
- 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 you have too many platelets (thrombocytosis). This can cause excessive clotting or sometimes bleeding if the platelets are not functioning properly. However, you may have no signs or symptoms if you have increased platelets, so the condition may be found only when a platelet count is done as part of a health exam or for another condition.
What does the test result mean?
Your platelet count is interpreted by your healthcare practitioner within the context of other tests that you have had done (e.g., CBC) as well as other factors, such as your medical history.
A single low or high platelet count may or may not have medical significance. Generally, this is the case when the result is only slightly lower or higher than the reference (normal) range. Your healthcare practitioner may repeat the test and may look at results from prior platelet counts. On the other hand, a result outside the reference range may indicate a problem and warrant further investigation. Your healthcare practitioner will determine whether a result that falls outside of the reference range means something significant for you.
A low platelet count, also called thrombocytopenia, along with other certain 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 the bone marrow can produce them
(For additional details, read the article on Low Platelets.)
Examples of conditions and factors 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
- Use of 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; these are just a few that have been associated with drug-induced decreased platelet counts.
- Heparin-induced thrombocytopenia (HIT) results in low platelets when you are or have been treated with heparin and you develop 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
- Sepsis, especially sepsis caused by a serious Gram-negative bacterial infection
- 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
- Platelets may be used up quickly in serious conditions, such as disseminated intravascular coagulation (DIC), thrombocytopenic purpura (TTP), and hemolytic uremic syndrome (HUS).
- Bone marrow damage caused by exposure to toxic chemicals, such as certain pesticides or benzene
- Excessive alcohol drinking, especially in heavy drinkers who are low in iron, vitamin B12 or folate
- A number of congenital (genetic) syndromes that result in a decreased number of platelets
When a platelet count is below 50,000, bleeding is more serious if you're cut or bruised. If the platelet count falls below 10,000 to 20,000 per microliter, spontaneous bleeding may occur and is considered a life-threatening risk. If you have a very low platelet count, you may be given platelets through a transfusion. (See Blood and Blood Components in the Transfusion Medicine 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 cancer, gastrointestinal cancer, ovarian cancer, breast cancer, 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 you have had your spleen removed surgically
- Use of birth control pills (oral contraceptives)
Some conditions may cause a temporary 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 number of platelets but may also experience bleeding problems due to platelets not functioning normally. This disorder is often associated with a mutation in the gene called JAK2. A test for this mutation should be performed if a healthcare practitioner suspects that you have 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.
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 healthcare practitioner may prescribe a drug that helps to suppress the immune system. People with platelet counts that are significantly decreased may be at risk for excessive bleeding and should be evaluated to determine the underlying cause and may require a platelet transfusion.
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. These calculations can give your healthcare practitioner additional information about your platelets and the cause of a high or low platelet count. Larger platelets are usually younger platelets that have been released earlier than normal from the bone marrow, while smaller platelets may be older and have been in circulation for a few days.
- MPV reflects the average size of your platelets. A high number of large platelets (high MPV) and a low platelet count suggests your bone marrow is producing platelets and releasing them into circulation rapidly. An elevated MPV has been associated with poorer survival rates in certain cancers. Conversely, a low MPV indicates platelets are smaller than average and are older. This may be due to a disorder affecting production by the bone marrow. A low MPV has been associated with inflammatory bowel disease, chemotherapy, and aplastic anemia.
- PDW reflects how uniform the platelets are in size. 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 is platelet activation and has been associated with vascular diseases and certain cancers.
Often, abnormal results will prompt additional testing. Under certain conditions, platelets may clump together and falsely appear to be low in number and/or larger 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 or in rare inherited disorders such as Bernard-Soulier disease. However, it may be necessary for a laboratory professional to use a microscope to examine a blood smear to determine whether the platelets are truly giant or if the platelets clumped together during the testing process. 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 percentage of immature platelets (also called reticulated platelets) circulating in the blood. It is one of the values reported when blood is evaluated using an automated instrument. The IPF may be used to help your healthcare provider determine if platelet production is increasing in cases of a low platelet count.
Platelets are produced in the bone marrow and are normally not released into the bloodstream until they have matured. When your platelet count is low (thrombocytopenia), the bone marrow is stimulated to produce platelets faster. When the need is great and when production cannot keep up with "demand," then an increased number of immature platelets, often large platelets are released into the bloodstream.
- A low IPF indicates that the bone marrow is producing fewer platelets.
- An increased IPF indicates an increased loss of platelets in the blood, generally due to platelet destruction as seen in immune thrombocytopenia (ITP).
Lab test results including platelet count and IPF can also be used to help determine if you need a platelet transfusion and to 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 healthcare practitioner 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 laboratory tests. Depending on the suspected cause and results from a CBC and blood smear examination, various follow-up tests may be performed. A few examples include:
- CRP (preferred test) or ESR to detect inflammation
- Tests for autoantibodies that target platelets (e.g., HIT antibody)
- Tests for infectious diseases, including bacteria and viruses
- Tests for bleeding disorders, such as PT, PTT, fibrinogen
- Tests for kidney failure
- Iron studies or vitamin B12 and folate levels
- Tests for liver disease
- In unexplained, serious cases, a bone marrow aspiration and biopsy
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. Between 7% and 12% 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.