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Blood Film

Platelet Morphology



  • Platelets are very small non-nucleated cells with fine granules that derive from fragmentation of megakaryocytes.


  • 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.
    • Thrombocytosis
    • 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 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
    • Abnormal Platelet Size
    • 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.
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