Platelet count (PLT): low, high and what happens next

Platelets are small, disc-shaped cell fragments made in the bone marrow. Lab Tests Online UK explains that they are the first cells recruited to a site of injury and, when activated, they clump together to seal the blood vessel and form a clot. If there are too few, or if the platelets that are present do not function properly, clots may not form well; too many can raise the risk of clots that block vessels.

The platelet count is simply the number in a volume of blood. It is one line of the full blood count and does not reveal whether the cells work properly. When a function defect is suspected, specialised platelet function tests are used instead.

It is requested as part of the general screen of the full blood count, when someone has unexpected or prolonged bleeding, unexplained or easy bruising, petechiae or purpura (small red or purple spots in the skin), when drugs known to affect platelets are monitored, when blood or bone marrow disease is suspected, and during investigation of an unprovoked blood clot. These are the situations listed by Lab Tests Online UK.

No preparation is needed. Blood is taken from a vein in the arm. Short-term increases can follow strenuous exercise, childbirth and extreme stress, and oestrogen-containing medicines or high altitude can raise the count; slightly lower counts are seen in some women before menstruation.

The usual adult interval quoted by Lab Tests Online UK is 150 to 450 x10^9/L, which equals 150,000 to 450,000 platelets per microlitre. Individual laboratories print their own interval, so compare against your own report. A count below 150 is called thrombocytopenia and one above 450 thrombocytosis.

The degree matters. A slightly low count in someone with no bleeding is often simply watched with repeat blood tests. For very low counts (below 20 x10^9/L) or high bleeding risk, the same source says a platelet transfusion can be used to raise numbers.

Thrombocytopenia arises because the marrow is not making enough platelets or because they are used up or destroyed faster than normal. Lab Tests Online UK lists immune thrombocytopenia, severe B12 or folate deficiency, medicines (heparin can rarely cause a sudden fall), infections including parvovirus, cytomegalovirus and infectious mononucleosis, liver disease, autoimmune disorders such as lupus, gestational thrombocytopenia (up to 1 in 10 women have a fall below the usual range in pregnancy), chronic bleeding, leukaemia or lymphoma, rare inherited platelet disorders, other marrow disorders, chemotherapy and radiotherapy, and serious conditions such as haemolytic uraemic syndrome, thrombotic thrombocytopenic purpura and disseminated intravascular coagulation.

The NHS notes that glandular fever can occasionally lead to blood conditions such as low platelet levels. If you have spontaneous nosebleeds, bleeding gums, heavy bleeding, black or bloody stools, or a spreading rash of tiny red spots, seek medical advice promptly; use 999 or A&E for severe bleeding or a head injury while on a low count.

Most raised counts are reactive. Lab Tests Online UK lists infection, chronic inflammation such as Crohn's disease, recent trauma or surgery, poor spleen function or removal of the spleen, recent bleeding, iron deficiency anaemia (especially from chronic blood loss), cancer (commonly lung, breast and ovarian), medicines such as corticosteroids, and a rebound after chemotherapy or growth factors.

Less often the marrow itself over-produces platelets. Essential thrombocythaemia and polycythaemia vera, called myeloproliferative neoplasms, can be found incidentally on a routine count; tests for JAK2, MPL and CALR gene changes may be done. They carry risks of clots and bleeding, so haematology follow-up is important.

A sample that clots in the tube before it reaches the laboratory gives a falsely low count. Sometimes the EDTA anticoagulant in the full blood count tube makes platelets stick together, so analysers under-count them. Lab Tests Online UK explains that scientists check samples for clots and review blood films for clumps, and that a citrated full blood count is the accurate way to count in people who clump. No association with other conditions is known.

For an abnormal count your doctor will look at the other cell counts, medicines and symptoms, and may order a blood film, repeat count, B12 and folate, liver tests or tests for infection. Persistent unexplained abnormalities go to haematology. Keep a note of bruising, bleeding and any new medicines for that appointment.