Platelets (also called thrombocytes) are disc-shaped fragments of cells (without nucleus) with a diameter between 2 and 3 micrometers (µm), present in the blood of each individual. They result from the fragmentation of larger cells, called megakaryocytes, which are found in the bone marrow.
In a healthy adult individual, 150,000 to 400,000 platelets are normally present in one microliter of blood. Their average life is short, about 10 days, and at the end they are eliminated by macrophages, "scavenger" cells present in the liver and spleen. Every day 30,000 to 40,000 "new" platelets are produced per microliter; if necessary, the bone marrow can increase their production.
Despite their small size, the internal structure of platelets is extremely complex and allows them to profoundly change their shape in relation to the degree and stage of activation. Furthermore, platelets are extremely fragile and this allows, in a few seconds, the contents of their granules rich in enzymes and other substances necessary to carry out a specific function in the blood clotting process are released into the blood.
In fact, they form the so-called "Platelet plug" which allows, in the early stages of coagulation, to contain bleeding by closing the lesions present in the blood vessel wall. Subsequently, to complete the repair process, the so-called enzymes also intervene coagulation cascade, proteins present in an inactive form in the liquid part of the blood (plasma).
The platelet count is performed by means of an examination, the blood count (commonly used abbreviation for "blood count"). This is a laboratory analysis which consists of a normal blood sample from a vein in the arm. Fasting is not necessary.
Platelet defects may relate to their number (quantitative defects) or their functionality (qualitative defects).
The qualitative defects (platelet disorders) can be caused by diseases present at birth (congenital) such as, for example, Bernard-Soulier syndrome, Glanzmann's thrombasthenia, Wiskott-Aldrich syndrome, etc) or by taking drugs or certain therapies (heparin, antibiotics, etc).
Quantitative defects are related to the "increase or decrease (thrombocytopenia) in the number of platelets circulating in the blood. Platelet values higher than normal (high platelets) can be determined by various conditions such as tumors, iron deficiency, inflammation, removal spleen (splenectomy), trauma and the use of vitamin B12; lower values (low platelets) can be caused by various diseases, such as aplastic anemia, viral infections, blood cancers (leukemia and lymphoma), malaria or vitamin B12 deficiency and the use of drugs such as antibiotics, barbiturates, diuretics, non-steroidal anti-inflammatory drugs (NSAIDs).
Platelets play an important and essential role in blood clotting processes. When their number is particularly small, spontaneous bleeding or even minimal trauma can occur.Usually, the risk of spontaneous bleeding is very low if the number of platelets is greater than 40,000 platelets per microliter of blood; it is frequent, but not constant, for lower values, and becomes constant, and rather severe, when the platelets drop below 10,000 per microliter. If spontaneous bleeding occurs with values greater than 40,000 platelets per microliter, other possible causes must also be considered, such as, for example, a weakness of the vessel walls, a defect in platelet function or clotting factors.
In some cases it is necessary to transfuse platelets. Platelet transfusion therapy can be used:
- for preventive purposes (prophylactic) to avoid bleeding episodes
- for therapeutic purposes, to stop the bleeding in progress
Prophylactic transfusion may be necessary in case of a severe reduction in the number of platelets (thrombocytopenia) or before some major surgery (especially if significant bleeding is expected and if platelet values are lower than normal).