Platelet Morphology
Overview
Platelets are very small non-nucleated cells with fine granules that derive from fragmentation of megakaryocytes.
Thrombocytosis
Thrombocytosis is an increase in the number of circulating platelets. This may be primary, in the context of myeloproliferative neoplasms, or secondary to another process.
Causes of Thrombocytosis
Primary
- Myeloproliferative neoplasms - essential thrombocythaemia, polycythaemia vera, chronic myeloid leukaemia, myelofibrosis
Secondary
- Infection
- Inflammation - autoimmune disease, major surgery, trauma, fracture, burns, pancreatitis, infarction
- Malignancy
- Post splenectomy
- Iron deficiency anaemia
- Acute haemorrhage
- Acute haemolysis
- Drugs - adrenaline, corticosteroids, vinca alkaloids, thrombopoietin
Thrombocytopaenia
Thrombocytopaenia refers to a reduction in the number of circulating platelets.
In a patient with new thrombocytopaenia, look for platelet clumping and fibrin strands which may indicate a factitiously low platelet count, or pseudothrombocytopaenia.
Causes of Thrombocytopaenia
Reduced Production
- Congenital thrombocytopaenia
- Bone marrow failure - aplastic anaemia, paroxysmal nocturnal haemoglobinuria
- Bone marrow suppression - cytotoxic chemotherapy, radiotherapy, alcohol abuse
- Bone marrow infiltration - leukaemia, multiple myeloma, lymphoma, metastasis, myelofibrosis
- Myelodysplasia
- B12 / folate deficiency
- HIV infection
Increased Destruction
- Idiopathic thrombocytopaenic purpura (ITP)
- Heparin-induced thrombocytopaenia (HIT)
- Drug-induced thrombocytopaenia - heparin, quinine, bactrim, vancomycin, rifampin, paracetamol, NSAIDs, glycoprotein IIb/IIIa inhibitors
- Immune thrombocytopaenia - SLE, antiphospholipid syndrome, rheumatoid arthritis, transfusion reaction
- Thrombotic microangiopathies - thrombotic thrombocytopaenic purpura (TTP), haemolytic-uraemic syndrome (HUS), disseminated intravascular coagulation (DIC)
- Infection - hepatitis C, EBV, CMV, H pylori, sepsis
- Pregnancy - gestation thrombocytopaenia, preeclampsia / HELLP syndrome
Splenic Sequestration
- Hypersplenism
Factitious
- Platelet clumping
- Haemodilution (e.g. massive transfusion)
Abnormal Platelet Size
Platelets are normally very small in size, though occasionally large or giant platelets may be seen. Platelet anisocytosis refers to a significant variation in the size of platelets on a blood film.
Look For
- Large platelets - larger than normal size, though smaller than the size of red cells
- Giant platelets - larger in size than red blood cells
Causes of Large and Giant Platelets
- Inherited giant platelet disorders - e.g. Bernard-Soulier
- Myeloproliferative neoplasms
- Myelodysplastic syndrome
- Factitious
Platelet Clumping
In a patient with new or worsening thrombocytopaenia, look for clumping of platelets which would result in a factitiously low platelet count. This is particularly seen at the tail of a blood film.
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