Platelet disorders
Also known as: thrombocyte disorders
Platelet disorders are conditions in which platelets are too few (thrombocytopenia) or function abnormally, impairing primary hemostasis. They typically cause mucocutaneous bleeding such as petechiae, purpura, epistaxis, and easy bruising.
Platelets carry out primary hemostasis: they adhere to exposed subendothelial collagen through von Willebrand factor and the GpIb receptor, activate, and aggregate with one another via GpIIb/IIIa bridged by fibrinogen. A defect anywhere in that sequence — or simply too few platelets — produces the same clinical signature. Laboratory work shows a prolonged bleeding time with a normal PT and PTT, since the coagulation cascade itself is intact.
Quantitative disorders reduce the count. Immune thrombocytopenic purpura involves anti-GpIIb/IIIa antibodies that mark platelets for splenic destruction. Thrombotic thrombocytopenic purpura results from deficient ADAMTS13, so ultralarge von Willebrand multimers persist and consume platelets in microthrombi, producing a microangiopathic hemolytic anemia with schistocytes. Hemolytic uremic syndrome presents similarly and is classically associated with Shiga toxin from enterohemorrhagic E. coli. Disseminated intravascular coagulation consumes platelets alongside clotting factors, so it differs from the others by also prolonging PT and PTT and raising D-dimer. Heparin-induced thrombocytopenia is drug-induced and, counterintuitively, causes thrombosis rather than bleeding.
Qualitative disorders leave the count normal but the platelets ineffective. Bernard–Soulier syndrome is a GpIb defect that impairs adhesion and produces unusually large platelets. Glanzmann thrombasthenia is a GpIIb/IIIa defect that impairs aggregation. Aspirin irreversibly inhibits cyclooxygenase and thromboxane A2 production, and uremia impairs platelet function through circulating toxins.
The clinical contrast worth internalizing: platelet disorders cause superficial mucocutaneous bleeding, whereas coagulation factor deficiencies such as hemophilia cause deep tissue bleeding into joints and muscles with delayed rebleeding.
USMLE Step 1 tests platelet disorders in hematology and oncology, most often by giving a lab panel — platelet count, PT, PTT, bleeding time, smear findings — and asking you to identify the disorder or the defective receptor.
Key takeaways
- Platelet disorders impair primary hemostasis and cause mucocutaneous bleeding: petechiae, purpura, epistaxis, and easy bruising.
- They classically prolong bleeding time while leaving PT and PTT normal, except in DIC, which prolongs both.
- ITP, TTP, HUS, DIC, and heparin-induced thrombocytopenia are the major quantitative causes.
- Bernard–Soulier disrupts GpIb-mediated adhesion; Glanzmann thrombasthenia disrupts GpIIb/IIIa-mediated aggregation.
- Deep joint and muscle bleeding points to a coagulation factor deficiency rather than a platelet disorder.