Summary
Platelets are tiny cell fragments that are the first responders to blood vessel injury — they rapidly aggregate to form a temporary plug at sites of damage, initiating the clotting cascade. Low platelet count (thrombocytopenia) increases the risk of bruising and bleeding; high platelet count (thrombocytosis) can increase the risk of blood clots. The platelet count is a routine component of the full blood count.
Platelets are produced by megakaryocytes in the bone marrow. They circulate for 8–10 days before being removed by the spleen. Unlike red and white cells, platelets have no nucleus.
Mild thrombocytopenia (100–150 × 10⁹/L) is common and often benign — particularly in viral illness, pregnancy (gestational thrombocytopenia), or as an incidental finding. Severe thrombocytopenia ( 600 × 10⁹/L) — requires investigation and treatment.
What It Is
Platelets (thrombocytes) are anucleate cytoplasmic fragments derived from megakaryocytes in the bone marrow. Each megakaryocyte produces approximately 1,000–3,000 platelets by shedding cytoplasmic protrusions. Platelet lifespan is 8–10 days; ~30% are sequestered in the spleen at any time.
Upon vascular injury, subendothelial collagen is exposed. Platelets adhere via the GPIb-IX-V receptor (binding von Willebrand factor) and become activated — releasing ADP, thromboxane A2, and serotonin from granules. These mediators recruit additional platelets (aggregation) and initiate the coagulation cascade, ultimately producing fibrin to stabilise the platelet plug.
Reference range: 150–400 × 10⁹/L. Counts 400 × 10⁹/L = thrombocytosis. Spontaneous bleeding risk is low above 50 × 10⁹/L; surgical bleeding risk increases below 80 × 10⁹/L.
Functions
Primary haemostasis
Platelets form the initial platelet plug at sites of vascular injury — the first step in stopping bleeding.
Coagulation cascade initiation
Activated platelets provide the phospholipid surface on which the coagulation factors assemble to produce fibrin.
Bleeding and clotting risk marker
Low platelets indicate bleeding risk; high platelets may indicate clotting risk — both require context and differential diagnosis.
Bone marrow function indicator
Thrombocytopenia alongside anaemia and low WBC (pancytopenia) signals bone marrow failure — an urgent haematological emergency.
Reference Ranges
Platelet Count
Measured in ×10⁹/L| Status | Range (×10⁹/L) | Range (cells/μL (×10³)) | What it means |
|---|---|---|---|
| Low | < 150 | < 150,000 | Thrombocytopenia — bleeding risk depends on severity and cause. Investigate. |
| Normal | 150–400 | 150,000–400,000 | Normal platelet count — adequate clotting function. |
| Elevated | > 400 | > 400,000 | Thrombocytosis — usually reactive (infection, inflammation, iron deficiency). Investigate if > 600. |
Mild thrombocytopenia (100–150 × 10⁹/L) is often benign and transient. Counts < 30 × 10⁹/L carry spontaneous bleeding risk. Platelet counts > 600 × 10⁹/L warrant investigation for essential thrombocythaemia. Always consider pseudothrombocytopenia from EDTA clumping.
Symptoms of Imbalance
Platelet abnormalities manifest as either bleeding (low) or clotting (high) complications.
- Easy bruising and prolonged bruising
- Petechiae (tiny red/purple pinpoint spots under the skin)
- Purpura (larger purple skin patches from small vessel bleeding)
- Prolonged bleeding from cuts
- Nosebleeds and gum bleeding
- Heavy periods in women
- Headache or confusion (rare — from intracranial bleeding in severe cases)
- Often asymptomatic in reactive thrombocytosis
- In essential thrombocythaemia: headaches, visual disturbance, burning pain in hands/feet (erythromelalgia)
- Blood clots: deep vein thrombosis, stroke, heart attack
- Splenomegaly
Causes of Imbalance
- Immune thrombocytopenic purpura (ITP) — autoimmune platelet destruction
- Viral infections (HIV, hepatitis C, EBV) — bone marrow suppression
- Chemotherapy and radiotherapy
- Medications (heparin-induced, quinine, valproate)
- Hypersplenism — enlarged spleen sequesters platelets
- Bone marrow failure (aplastic anaemia, leukaemia)
- Disseminated intravascular coagulation (DIC)
- Pseudothrombocytopenia from EDTA clumping
- Reactive (most common): iron deficiency, infection, inflammation, post-surgery, exercise
- Essential thrombocythaemia (myeloproliferative neoplasm)
- Other myeloproliferative disorders (polycythaemia vera, CML)
- Post-splenectomy
FAQs
Spontaneous bleeding (without injury) rarely occurs above 30 × 10⁹/L. Between 30–50 × 10⁹/L, bleeding risk is significant and minor procedures may cause excessive bleeding. Between 50–80 × 10⁹/L, most procedures can proceed with precautions. Above 80 × 10⁹/L, bleeding risk is generally acceptable for most clinical procedures. Critical spontaneous bleeding (particularly intracranial) becomes a real risk below 10–20 × 10⁹/L.
Yes — pseudothrombocytopenia is a laboratory artefact where platelets clump in the EDTA anticoagulant tube, giving a falsely low count. If your platelet count is unexpectedly low and you have no bruising or bleeding symptoms, your doctor should request a repeat test in a citrate tube and a blood film. Pseudothrombocytopenia is harmless and requires no treatment.
Reactive thrombocytosis (elevated platelets from infection, inflammation, or iron deficiency) rarely causes problems and the platelets function normally. Essential thrombocythaemia (a myeloproliferative disorder) with very high platelets (> 600 × 10⁹/L) does carry a clotting risk — this requires haematology review and treatment with aspirin (and cytoreductive therapy in high-risk patients).
Immune Thrombocytopenic Purpura (ITP) is an autoimmune condition where antibodies target platelet surface proteins, marking them for destruction by the spleen. It causes isolated thrombocytopenia (normal Hb and WBC) with bruising, petechiae, and bleeding. Acute ITP in children often follows viral illness and resolves spontaneously. In adults it is more chronic and may require treatment with steroids, IVIG, or thrombopoietin receptor agonists.
References
- Michelson AD. Platelets. 3rd ed. Academic Press; 2013. ISBN: 9780123878373.
- Provan D, et al. International consensus report on the investigation and management of primary immune thrombocytopenia. Blood. 2010;115(2):168–186. View source