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  • artefectual results are common, and unexpected significantly low platelet counts should be re-checked
  • be alert to the possibility of TTP or heparin-induced thrombocytopenia, as a delay in diagnosis can result in serious morbidity or mortality
  • patients with counts < 20,000 or those having surgery may warrant platelet transfusion


  • thrombocytopenia is defined by an excessively low concentration of platelets in the peripheral blood < 150 x 10^9/L
  • platelets are formed by fragmentation of megakaryocytes, most of which occurs in the lungs and not the bone marrow as was thought 1) and circulate in the blood for 7-10 days
  • bleeding time is generally not prolonged until the platelet count is below 100,000/µL
  • platelet counts from 20,000/µL to 50,000/µL, petechiae and ecchymoses are observed following mild trauma
  • platelet counts below 20,000/µL pose significant risks of major spontaneous haemorrhage
  • platelet counts below 10,000/µL, the risk of spontaneous mucocutaneous bleeding (gingival bleed, epistaxis, menorrhagia, petechiae and ecchymoses) and life threatening, spontaneous intracranial haemorrhage or gastrointestinal bleeding increases rapidly
  • the risk of bleeding is also dependent upon presence of functional platelet defects


  • artefactual or pseudothrombocytopenia
    • accounts for ~15% of cases in ambulatory patients
    • this is caused by in vitro clumping of platelets when ethylenediaminetetraacetic acid (EDTA) is used as an anticoagulant
  • dilutional
  • 5% of the normal population will fall outside the “normal range” by definition

increased destruction

    • counts rarely fall below 40,000/µL
  • immune-mediated:
      • esp. well young women
      • acute
      • chronic
      • Evans syndrome: ITP with autoimmune haemolytic anaemia
    • heparin-induced thrombocytopenia (HIT)
      • Type 1 HIT is a modest transient decrease in platelet counts that occurs within the first 2 to 3 days after heparin initiation and returns to normal spontaneously, even with continuation of heparin
      • Type 2 HIT is less common, seen in about 0.3 to 5% of patients caused by antibodies against platelet factor 4-heparin complex, usually occurs 4 to 14 days after heparin initiation. Abnormal thrombosis, sometimes catastrophic, is the predominant manifestation of HIT and bleeding is uncommon. Cease heparin and do not use LMW heparin. - see also thrombosis with thrombocytopenia syndrome (TTS / VITT)
    • drug-induced antibodies
      • GPIIb/IIIa inhibitor related
        • unlike other drug-induced thrombocytopenias, GPIIb/IIIa inhibitor-induced thrombocytopenia develops within 24 hours of exposure
    • post-transfusion purpura
      • rare, mostly in women, it presents as severe thrombocytopenia 5 to 10 days after red cells or platelet transfusion
    • HELLP in pregnant patients
    • gestational thrombocytopenia occurs in 5% near delivery
  • non-immune:
    • cardiac valves
    • TTP and HUS
      • also have microangiopathic hemolytic anemia (schistocytes, elevated LDH, and indirect hyperbilirubinemia)
      • may also have neurologic abnormalities, fever, and renal disease
      • may be related to clopidogrel or ticlopidine
      • caused by deficiency (due to autoantibodies) of von Willebrand factor-cleaving protease known as ADAMTS13 leads to accumulation of large multimers of von Willebrand factor which cause spontaneous platelet aggregation and thrombi
      • emergent plasma exchange is the cornerstone of TTP treatment
    • Kasabach Merrit syndrome

decreased production

Mx of incidental thrombocytopenia

  • exclude artefactual cause ⇒ repeat full blood examination (FBE)
  • if within 4-20 days of an Astra-Zeneca Covid-19 vaccine then investigate for possible thrombosis with thrombocytopenia syndrome (TTS / VITT)
  • if neutropenia and/or anaemia as well then ?aplastic anaemia
  • is patient pregnant?
    • LFT's ⇒ ?HELPP vs gestational vs ITP
  • history - PH ITP, drugs, heparin, chemotherapy, cancer, etc.
  • examination - evidence of bleeding, splenomegaly,
  • if febrile, search for underlying illness eg. sepsis, TTP, viral infections, typhoid, etc
    • platelet clumps ⇒ artefactual
    • giant platelets ⇒ hereditary thrombocytopenia
    • schistocytes or spherocytes ⇒ look for haemolysis (LDH, bilirubin) and if confirmed:
      • Direct Coombs
        • +ve: Evans syndrome
        • -ve: check for DIC, if negative then ?TTP
    • suspicious smear for primary marrow disorder ⇒ bone marrow Bx
      • nucleated red cells
      • bilobed neutrophils
      • macrocytosis
      • blasts
    • isolated thrombocytopenia

indications for platelet transfusion

  • platelet count < 10,000/µL
  • platelet count < 20,000/µL plus presence of bleeding, fever, infection, platelet function defect, or coagulopathy
  • platelet count < 50,000/µL prior to minor procedures, in actively anticoagulated patients or in the presence of active bleeding
  • platelet count < 75,000/µL prior to general surgery
  • platelet count < 100,000/µL prior to neurologic or ophthalmologic surgery2)
thrombocytopenia.txt · Last modified: 2021/05/11 08:08 by gary1