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. (See Common Questions #3 and #4 for more on this.)

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.

It may be ordered when a person has signs and symptoms associated with low platelets or a bleeding disorder, such as:

• 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.

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
• Cirrhosis
• 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.

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.