Thrombocytopenia: mechanisms
- Mechanism
A low platelet count comes from poor production, quick destruction, trapping in a big spleen or dilution after massive transfusion.
What it is
Thrombocytopenia is the commonest cause of generalized bleeding. Four mechanisms explain almost all cases: decreased production (marrow failure, infiltration, megaloblastic anaemia, alcohol, HIV, MDS), decreased survival (immune destruction by autoantibodies, alloantibodies or drug-dependent antibodies, or consumption in DIC and thrombotic microangiopathies), sequestration in an enlarged spleen, and dilution after massive transfusion. Bleeding risk rises as counts fall, with spontaneous bleeding and intracranial haemorrhage at the lowest levels.
Step by step
- Production falls when stem cells, megakaryocytes or the thrombopoietin response fail.
- Survival falls when antibodies opsonize platelets for splenic macrophages or when platelets are consumed in microthrombi.
- A large spleen pools a big share of the circulating platelets.
- Massive transfusion dilutes the remaining platelets.
- Low counts weaken the primary plug: petechiae, mucosal bleeding and, at very low counts, intracranial bleeding.
Mechanisms of thrombocytopenia
| Mechanism | Examples | Marrow megakaryocytes |
|---|---|---|
| Decreased production | Aplastic anaemia, leukaemia, metastases, B12 or folate deficiency, alcohol, HIV, MDS, chemotherapy | Low (normal or high when ineffective) |
| Decreased survival | ITP, drug-induced, HIT, alloimmune, DIC, TTP, HUS, dengue | Normal or increased |
| Sequestration | Hypersplenism from portal hypertension, lymphoma, storage disease | Normal |
| Dilution | Massive transfusion of stored blood | Normal |
Clinical notes
- Petechiae in dependent areas, epistaxis and gum bleeding; intracranial bleeding is the most feared complication.
- A falsely low count from platelet clumping in EDTA tubes should be excluded by smear review.
Test yourself
Think of the answer, then tap the question to see it.
How does the marrow help separate production from destruction thrombocytopenia?
Few megakaryocytes suggest poor production; normal or increased megakaryocytes suggest peripheral destruction.
Why does massive transfusion cause thrombocytopenia?
Stored red cell units contain few functional platelets, so the patient's platelets are diluted.
What share of platelets does a normal spleen hold?
About a third; much more in splenomegaly.
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Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Platelet Disorders | MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Epidemiological trends of dengue disease in Colombia (2000-2011): a systematic review (PLoS neglected tropical diseases, 2015).
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Representative model, for educational purposes only.