How is a platelet count performed?

Answer 1

Platelet counts use a freshly-collected blood specimen to which a chemical called EDTA has been added to prevent clotting before the test begins. About 5 mL of blood are drawn from a vein in the patient's inner elbow region.

Answer 2

The Test

1. How is it used?
2. When is it ordered?
3. What does the test result mean?
4. Is there anything else I should know?

How is it used?

Bleeding disorders or bone marrow diseases, such as leukemia, require the determination of the number of platelets present and/or their ability to function correctly.

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When is it ordered?

A platelet count is often ordered as a part of a complete blood count, which may be done at an annual physical examination. It is almost always ordered when a patient has unexplained bruises or takes what appears to be an unusually long time to stop bleeding from a small cut or wound.

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What does the test result mean?

In an adult, a normal count is about 150,000 to 450,000 platelets per microliter of blood.

If platelet levels fall below 20,000 per microliter, spontaneous bleeding may occur and is considered a life-threatening risk. Patients who have a bone marrow disease, such as leukemia or another cancer in the bone marrow, often experience excessive bleeding due to a significantly decreased number of platelets (thrombocytopenia). As the number of cancer cells increases in the bone marrow, normal bone marrow cells are crowded out, resulting in fewer platelet-producing cells.

Low number of platelets may be seen in some patients with long-term bleeding problems (e.g., chronic bleeding stomach ulcers), thus reducing the supply of platelets. Decreased platelet counts may also be seen in patients with Gram-negative sepsis.

Individuals with an autoimmune disorder (such as lupus or idiopathic thrombocytopenia purpura (ITP), where the body's immune system creates antibodies that attack its own organs) can cause the destruction of platelets.

Certain drugs, such as acetaminophen, quinidine, sulfa drugs, digoxin, vancomycin, valium, and nitroglycerine, are just a few that have been associated with drug-induced decreased platelet counts. Patients undergoing chemotherapy or radiation therapy may also have a decreased platelet count. Up to 5% of pregnant women may experience thrombocytopenia at term.

Platelet consumption may be observed in renal diseases. Thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) are seen in renal failure and can result in fewer circulating platelets in the blood. Similarly, a condition known as splenic sequestration, where platelets pool within the spleen, can also cause a platelet decrease.

More commonly (up to 1% of the population), easy bruising or bleeding may be due to an inherited disease called von Willebrand's disease. While the platelets may be normal in number, their ability to stick together is impaired due to a decrease in von Willebrand's factor, a protein needed to initiate the clotting process. Many cases may go undiagnosed due to the mild nature of the disease. Many cases are discovered when a patient has to have surgery or a tooth extraction or when delivering a baby. However, some cases are more severe and can be aggravated by use of certain drugs, resulting in a life-threatening situation.

Increased platelet counts (thrombocytosis) may be seen in individuals who show no significant medical problems, while others may have a more significant blood problem called myeloproliferative disorder. Some, although they have an increased number of platelets, may have a tendency to bleed due to the lack of stickiness of the platelets; in others, the platelets retain their stickiness but, because they are increased in number, tend to stick to each other, forming clumps that can block a blood vessel and cause damage, including death (thromboembolism).

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Is there anything else I should know?

Living in high altitudes, strenuous exercise, and being post partum may cause increased platelet levels. Drugs that may cause increased platelet levels include estrogen and oral contraceptives.

Decreased levels may be seen in women before menstruation.

Other inherited disorders caused by defective platelets or decreased/absent proteins that activate the 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.

Answer 3

There are three methods used to count platelets; hemacytometer, voltage-pulse counting, and electro-optical counting.