Platelets are non-nucleated fragments of megakaryocyte cytoplasm, formed chiefly in the bone marrow. Each megakaryocyte can give rise to as many as 3,000 platelets. Approximately 20-30% of circulating platelets are sequestered in the spleen and the normal life span of a platelet is 8-14 days.
In the laboratory, the platelets are measured by electrical impedance in the Red Cell Channel using Sysmex analysers. Platelets are counted in the 0 - 40 fL range and so giant platelets may not be counted. Very rarely, small red cell fragments can be counted as platelets and falsely elevate the count. If the platelet distribution graph is abnormal or the platelet count is low or falling, then a count can be produced in the PLT-F channel on XN analysers using a specific fluorescent stain.
Platelet Function:
Normally, platelets do not adhere to blood vessel walls because the endothelial cells produce prostaglandin I2and nitrous oxide. Both of these substances are vasodilators and inhibit platelet aggregation. If the vessel is injured, the platelets will adhere to exposed collagen and microfibrils via glycoprotein receptor sites on the platelet membrane. Following adhesion, the platelets become more spherical, start putting out long psedopods and their cytoplasmic granules are forced towards the cell membrane. These granules produce many platelet aggregating agents, such as thromboxane A2, thus activating more circulating platelets. Platelet aggregation, as well as binding to fibrinogen, leads to the formation of the platelet plug. This, coupled with local vessel vasoconstriction, is usually sufficient to maintain haemostasis, but in larger vessels fibrin is also necessary to prevent the plug from being dislodged
Causes of a raised platelet count (thrombocytosis):
Infection/Inflammation
Iron deficiency
Aquired Haematologic - Essential thrombocythaemia, CGL, Polycythaemia rubra vera
Blood loss
Malignancy
Redistributional - post splenectomy, hyposplenism
Causes of a lowered platelet count (thrombocytopenia):
Bone marrow suppression/infiltration - Drugs, radiotherapy, leukaemia, aplastic anaemia, MDS, HIV, alcohol
Increased platelet consumption - autoimmune (ITP), drug-induced, neonatal (isoimmune) purpura, DIC, TTP, Heparin, viral/bacterial infection
Redistributional - Splenomegaly
Dilutional - Massive transfusion