Platelet Count

The platelet count is used to check the number of platelets that are in blood. This may be used to diagnose conditions which cause there to be too many or too few platelets, to monitor the effects of certain drugs known to affect platelets, as part of a general health screen, during the investigation of suspected bleeding or clotting disorders and to diagnose or monitor diseases that affect the bone marrow.

In general, a platelet count may be requested for the following reasons:

•  As part of the general screening panel looking at blood cell characteristics during full blood count analysis
•  When trying to diagnose the cause of unexpected or prolonged bleeding, or in cases of unexplained or easy bruising
• For investigating the cause of petechiae or purpura (small red/purple spots in the skin caused by bleeding from small blood vessels)
• For monitoring purposes where patients are on drugs known to affect platelet count
• During the investigation of suspected blood or bone marrow diseases
• During the investigations of an unprovoked blood clot

The result is compared to a “normal” range, which represents the platelet count found in 95% of healthy individuals. In adults the platelet count is usually between 150 and 450 x 10⁹/L, which means there are 150,000 to 450,000 platelets per microlitre of blood.

A low platelet count (less than 150x10⁹/L) may also be referred to as ‘thrombocytopenia’. Thrombocytopenia may be caused by either of two different processes that cause a reduction in the count; the bone marrow is not producing enough platelets, or the bone marrow is producing normal amounts but the platelets are being consumed (used) or destroyed faster than they should in the blood.

Reduced platelet counts can be found in the following conditions:

• Immune thrombocytopenia –an immune condition that causes platelets to be destroyed faster than they should be due. This is one of the most common causes of thrombocytopenia, diagnosis involves ruling out any other potential cause of a low platelet count
• B12 or folate deficiency – severe deficiency of these B complex vitamins is associated with anaemia and, if severe, can also result in reduced platelet and white cell counts
• Drugs/medication – some medications (such as penicillin, gold used to treat arthritis, furosemide, valproic acid and others) can affect platelet production. Heparin (an anticoagulant drug) can cause a sudden fall in platelet numbers, this is a rare condition called heparin-induced thrombocytopenia or HIT 
• Infection and viruses – several infections and viruses can result in a reduced platelet count (e.g. parvovirus, cytomegalovirus, infectious mononucleosis)
• Liver disease – established liver disease is associated with a low platelet count, among other changes to blood cells and proteins
• Autoimmune disorders – such as lupus, can cause increased destruction of platelets
• Gestational thrombocytopenia – up to 1 in 10 women experience a fall in platelet count to below the normal range during pregnancy. If no underlying cause is found then a diagnosis of gestational thrombocytopenia (low platelets caused by pregnancy) is made
• Chronic bleeding – where there is long-term bleeding, such as from a stomach ulcer, platelet counts may be found to be low
• Leukaemia or lymphoma – cancers of the white blood cells affect the function of the bone marrow and platelet numbers will be reduced
• Rare inherited platelet disorders – such as Bernard-Soulier syndrome, Glanzmann Thrombasthenia and MYH9-related thrombocytopenias are associated with a reduction in circulating platelets and an increased platelet size (giant platelets)
• Bone marrow disorders/invasion – other diseases affecting the bone marrow (such as myelodysplasia, aplastic anaemia, or other haematological conditions), or invasion of the bone marrow by other cancer cells (metastasis) can affect platelet production
• Chemotherapy and radiotherapy – chemotherapy and/or radiotherapy will affect the function of the bone marrow and result in reduced production of all blood cells, including platelets
• Systemic disorders – some serious disorders can result in multiple abnormalities of the blood cell counts and characteristics. Haemolytic uraemic syndrome (HUS), thrombotic thrombocytopenic purpura (TTP) and disseminated intravascular coagulation (DIC) are examples of syndromes in which patients will have reduced platelet counts alongside other clinical symptoms and blood cell changes

The degree of thrombocytopenia (how low the platelet count is), the presence or absence of associated bleeding symptoms and the individual clinical situation will determine the course of action to be taken. Thrombocytopenia may be monitored closely by regular blood tests if the count is only slightly reduced and there are no symptoms. Treatment of the underlying cause might be considered for those who are at risk of bleeding. For patients with very low platelet counts (<20 x 10⁹/L), or at a high risk of bleeding, a platelet transfusion can be used to increase the numbers of platelets.

A high platelet count (more than 450x10⁹/L), ‘thrombocytosis’, is due to overproduction of platelets by the bone marrow which can be caused by certain bone marrow disorders or might simply be a side-effect of another condition (reactive thrombocytosis).

The platelet count may be increased as a reactive thrombocytosis in association with:

• Infection – in response to many infections
• Inflammation – chronic conditions such as Crohn’s disease, or other inflammatory processes
• Recent trauma – if there has been tissue damage, or after having an operation
• Poor spleen function – or in patients who have had their spleen removed
• Bleeding or recent blood loss – as a reactive process in response to bleeding
• Iron deficiency anaemia – especially where the anaemia is due to chronic blood loss (e.g. from a stomach ulcer)
• Cancer – thrombocytosis is commonly found in association with lung, breast and ovarian cancer
• Drugs/medication – e.g. corticosteroids can stimulate platelet production
• Rebound phenomenon – after treatment with growth factors or during recovery following chemotherapy.

The platelet count may also be increased as the consequence of a bone marrow disorder; this is sometimes called ‘primary thrombocytosis’. Essential thrombocythaemia (ET) is a condition where an acquired defect in bone marrow cells results in an abnormally high number of platelets whose production is uncontrolled. A high platelet count along with a high white cell count and red cell count may also be found in a similar condition affecting the bone marrow called polycythaemia vera (PV). These conditions, which can be referred to as ‘myeloproliferative neoplasms’, might not cause any symptoms and are often picked up incidentally when a full blood count is performed as a routine check or for another reason. Although in some people there may be no clinical signs or symptoms, there are risks associated with having abnormally high platelet counts, including blood clots and risk of bleeding. PV is associated with mutations in a gene called JAK2. ET is associated with mutations in the JAK2, MPL or CALR genes. If PV or ET is suspected then tests to look for mutations in these genes may be carried out.

Different types of bone marrow disorders such as chronic myeloid leukaemia (CML), myelodysplasia (MDS) and other myeloproliferative syndromes can also be linked to high platelet counts.

Platelets are cells that have a short life-span in the blood (5-10 days), which means the count can rise and fall quickly in response to disease or trauma. Short-term increases in platelet counts are known to be associated with strenuous exercise, childbirth and extreme stress.

The platelet count will be falsely reduced if there are errors in taking the blood and it is allowed to clot in the sample tube before it reaches the laboratory. However, low and high counts are investigated thoroughly in the laboratory with samples checked for clots and blood films looked at under the microscope to check the number count that is reported genuinely reflect the numbers of platelets present.

Sometimes, the EDTA present in the Full Blood Count sample tube causes the platelets to stick together and so the analysers give a falsely low reading. This is why blood films are checked to see if these platelet clumps can be seen. In these situations, a Citrated Full Blood Count is performed as the citrate anticoagulant doesn’t cause this problem. This is the only way to get an accurate platelet count in this situation. Some people have a tendency to form clumps and so they always need a citrated sample. No one knows why some people are affected and not others, but there is no known association with other conditions.

Living at high altitudes and drugs that include oestrogen (including oral contraceptives) can increase the platelet count. Alternately, slightly reduced platelet counts are seen in women prior to menstruation.

If an abnormal platelet count is detected then the results of the other blood cells analysed in the full blood count, as well as any other tests that have been performed will be taken into consideration and further investigations (blood tests) may be requested. This information, along with any clinical signs and symptoms will be considered by your healthcare professional who will be able to provide you with more information regarding the significance of the results.

Patients are fully investigated to determine what the cause of their bleeding disorder is. Treatment options will be different and depend on the specific diagnosis and the implications of the condition, but may include close monitoring and special precautions when invasive procedures are to be carried out (i.e. surgery), medication to prevent bleeding or clotting, replacement of missing coagulation factors or transfusion of platelets or other products.