Purpura
Formal Definition
Purple or red-violet discoloration of the skin or mucous membranes caused by extravasation of red blood cells into the dermis; does NOT blanch with pressure (unlike erythema), indicating hemorrhage rather than vascular dilation; classified as petechiae (<2 mm), purpura (2–10 mm), or ecchymoses (>1 cm bruising).
How It's Used on the Ward
"Non-blanching rash" or "purpuric spots" — the non-blanching quality is the critical finding that elevates a rash from benign to potentially life-threatening.
Example
""Toddler with fever and a rash: scattered non-blanching petechiae and purpura on trunk and legs — non-blanching confirmed with diascopy. This is meningococcemia until proven otherwise. IV ceftriaxone immediately, LP after stabilization.""
Clinical Context
Blanching vs non-blanching is the critical distinction: press a glass over the lesion (diascopy) or use fingertip pressure. Blanching = erythema (vascular dilation). Non-blanching = purpura (blood outside vessels). Differential: thrombocytopenia (platelet <50K = petechiae), vasculitis (IgA vasculopathy/HSP, meningococcemia), DIC, scurvy, trauma. Non-blanching petechiae in a febrile child = meningococcal sepsis until proven otherwise — give antibiotics before LP if unstable. TTP: thrombocytopenia + microangiopathic hemolysis + neurological symptoms.