Platelet count

Abnormal platelet counts or abnormally shaped platelets are associated with bleeding disorders or bone marrow diseases, such as leukaemia. These conditions require the determination of the number of platelets present and/or their ability to function correctly.

A platelet count is often requested as a standard part of a full blood count which may be done as part of an annual 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.

In an adult, a normal count is about 150-400 x 10⁹ per litre of blood.

If platelet levels fall below 20 x 10⁹/L, spontaneous bleeding may occur and is considered a life-threatening risk. Patients who have a bone marrow disease, such as leukaemia or other 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 occur 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 attacks its own organs) can cause the destruction of platelets. Patients undergoing chemotherapy may also have a decreased platelet count.

Certain drugs, such as paracetamol, quinidine, sulpha 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 haemolytic 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).

Living in high altitudes, strenuous exercise, and being post partum may cause increased platelet levels.

Decreased levels may be seen in women before menstruation.

Drugs that may cause increased platelet levels include oestrogen and oral contraceptives.

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.