Platelets (thrombocytes) are small, disc-shaped blood cells produced in the bone marrow. Their primary job is to form the initial plug at the site of a blood vessel injury — the first step in stopping bleeding. A platelet count is a routine component of a full blood count and can be abnormal in a wide range of conditions.
Normal range and thresholds
A normal platelet count is typically 150–400 × 10⁹/L. Values below 150 × 10⁹/L are called thrombocytopaenia. Values above 400 × 10⁹/L are called thrombocytosis. The clinical significance of these findings depends heavily on context, the degree of deviation and associated symptoms.
Causes of low platelet count
- Viral infections — influenza, dengue, EBV can suppress platelet production transiently
- Immune thrombocytopaenic purpura (ITP) — antibodies destroy platelets; common cause of isolated low count
- Bone marrow disorders — leukaemia, aplastic anaemia, drug toxicity
- Liver disease and hypersplenism — the spleen sequesters platelets
- Haemolytic uraemic syndrome or TTP — platelet consumption by clot formation in small vessels
- Medications — heparin (HIT), chemotherapy, some antibiotics
Causes of high platelet count
- Reactive (secondary) thrombocytosis — the most common cause; platelets rise in response to infection, inflammation, iron deficiency or surgery
- Essential thrombocythaemia — a bone marrow disorder causing persistent elevation; raises clot risk
- Post-splenectomy — the spleen normally removes old platelets; without it, counts rise
When to act
A platelet count below 50 × 10⁹/L carries a significant bleeding risk; below 10 × 10⁹/L is a haematological emergency. Very high counts (above 1000 × 10⁹/L) can paradoxically raise both clot and bleeding risk. Any count outside the normal range should be interpreted by your doctor alongside your full blood count and clinical picture.