Platelet Disorders
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Direct answer
Immune thrombocytopenic purpura is isolated thrombocytopenia from antibodies against glycoprotein IIb/IIIa, with a normal spleen, mucocutaneous bleeding and increased megakaryocytes in an otherwise normal marrow; children recover after viral infection while adults run a chronic course. Thrombotic thrombocytopenic purpura is the opposite disease in appearance — a thrombotic microangiopathy from severe ADAMTS13 deficiency producing the pentad of thrombocytopenia, microangiopathic haemolytic anaemia, fever, neurological signs and renal impairment, treated as an emergency with plasma exchange; platelet transfusion is contraindicated because it feeds the microthrombi. Disseminated intravascular coagulation consumes platelets and clotting factors together, so its coagulation screen is abnormal while ITP's is normal.
What you must remember
- ITP: anti-GPIIb/IIIa-mediated destruction; children post-viral and self-limiting, adult women chronic; petechiae, wet purpura of the mouth, menorrhagia; steroids first line, intravenous immunoglobulin for rapid rise, splenectomy for refractory disease; marrow shows increased megakaryocytes.
- TTP: ADAMTS13 deficiency leaves ultra-large von Willebrand multimers that platelets clump on; platelet-rich hyaline microthrombi without significant fibrin and with normal coagulation screen; plasma exchange reduced mortality from the historic near-90 per cent to the teens.
- Typical HUS: Shiga toxin of enterohaemorrhagic Escherichia coli O157:H7 after bloody diarrhoea in children; supportive care, and antibiotics generally avoided as they may increase toxin release.
- Atypical HUS: complement dysregulation; relapsing course, treated with the anti-C5 monoclonal eculizumab.
- DIC: consumption of platelets, fibrinogen and factors together — prolonged prothrombin and activated partial thromboplastin times, raised D-dimer, schistocytes on film; treat the obstetric catastrophe, sepsis or malignancy driving it.
- Bernard-Soulier syndrome: absent glycoprotein Ib; thrombocytopenia with giant platelets; ristocetin-induced platelet aggregation absent.
- Glanzmann thrombasthenia: absent glycoprotein IIb/IIIa; normal platelet count, normal ristocetin response, absent clot retraction.
- Clinical logic: platelet-type bleeding is mucocutaneous — petechiae, epistaxis, gum bleeding — whereas coagulation-type bleeding is deep, with haemarthroses and delayed rebleeding.
One blood film, three thrombocytopenias
A count of 20 thousand platelets per microlitre arrives with a full blood film, and the schistocyte question splits the field. If fragmented red cells are present, the process is a thrombotic microangiopathy or disseminated intravascular coagulation, and the coagulation screen separates those two: normal prothrombin and partial thromboplastin times with a normal fibrinogen point to TTP or HUS, in which the insult is confined to the microvasculature; prolonged times with falling fibrinogen point to DIC, where the entire clotting system is being consumed. If the film shows no fragments, isolated thrombocytopenia with large platelets in an otherwise well patient is immune thrombocytopenic purpura, and in an unwell one, marrow infiltration or hypersplenism.
The treatment fork that examiners probe: the TTP patient must reach plasma exchange urgently, and the instinct to transfuse platelets for the low count — reasonable in almost every other thrombocytopenia — actively worsens the neurological and cardiac ischaemia, because transfused platelets are instantly recruited to the microthrombi. In haemolytic uraemic syndrome the child is managed with fluids and dialysis support; in disseminated intravascular coagulation from abruptio placentae, evacuation of the uterus cures the coagulopathy faster than any blood product; and in immune thrombocytopenia, marrow examination is reserved for adults with atypical features or before splenectomy, because the marrow in ITP is dysregulated, not diseased.
Where students slip
Two reflex errors recur. First, expecting the prothrombin time to rise in ITP — a platelet problem does not disturb the coagulation screen, and writing "prolonged PT in ITP" exposes a missing concept. Second, transfusing platelets in TTP from the same habit that serves most thrombocytopenic patients; the exam question is designed to catch exactly that reflex. A quieter trap is the ristocetin pair — Bernard-Soulier fails ristocetin because the von Willebrand receptor (glycoprotein Ib) is missing, while Glanzmann passes it because ristocetin acts upstream of the fibrinogen receptor; reversing the two is the single commonest answer-level error in this chapter.
Frequently asked questions
Which antibody defines immune thrombocytopenic purpura?
Antibodies against platelet glycoprotein IIb/IIIa, causing peripheral destruction with a compensatorily increased megakaryocyte mass in the marrow.
What is the pentad of thrombotic thrombocytopenic purpura?
Thrombocytopenia, microangiopathic haemolytic anaemia, fever, neurological abnormalities and renal impairment, from severe ADAMTS13 deficiency.
Why are platelet transfusions contraindicated in TTP?
Transfused platelets are immediately incorporated into the von Willebrand-mediated microthrombi, worsening cerebral and cardiac ischaemia.
Which organism causes typical haemolytic uraemic syndrome?
Shiga toxin-producing Escherichia coli O157:H7, after bloody diarrhoea; children are affected and supportive management is the mainstay.
How does Bernard-Soulier syndrome differ from Glanzmann thrombasthenia?
Bernard-Soulier lacks glycoprotein Ib with thrombocytopenia and giant platelets and fails ristocetin aggregation; Glanzmann lacks glycoprotein IIb/IIIa with a normal count and normal ristocetin response.
Why is fibrinogen low in disseminated intravascular coagulation but normal in TTP?
DIC consumes fibrinogen and clotting factors systemically; TTP is a platelet-localised microthrombotic process that leaves the coagulation screen normal.