Purpuric lesions, probably more than any other skin lesion morphology, carry a grave prognosis not shared by most other eruptions. Bleeding or extravasation of red cells into the dermis often results from occlusion and/or destruction of blood vessel walls, both phenomena often resulting from a grave diagnosis. More than most other patients in dermatology, patients who present with purpura undergo extensive systemic evaluation in addition to skin biopsies. Other specialists, such as hematologists and rheumatologists, often participate in the care of these patients; hence, the important value of a dermatology consultation.

The pathogenetic mechanisms of purpura may be divided based on different criteria:

In the clinical evaluation of purpura, the following determinations are usually valuable:

Inflammatory purpura is synonymous with vasculitis. In other words, inflammation (erythema and edema) in a purpuric lesion is strong evidence that the patient has vasculitis. A closely related feature to inflammation is palpable infiltration of the lesion. In other words, palpable purpura has been equated to vasculitis and is used by some (usually older dermatologists) as synonymous with vasculitis, in contradistinction to nonpalpable purpura, which is often secondary to a bleeding diathesis or coagulation abnormalities.

When combined together, palpable infiltration and inflammation are almost invariably secondary to vasculitis with the prototype and most common form in the skin being leukocytoclastic vasculitis (LCV), which is also referred to as palpable purpura, immune complex vasculitis, Henoch-Schoeinlin purpura (HSP), and neutrophilic venulitis.

LCV presents acutely, often following an infectious illness or medication , strongly favors the lower legs, is rarely associated with ulcers secondary to skin necrosis, and most often resolves spontaneously. Chronic LCV raises suspicion for an underlying chronic infection, such as hepatitis or autoimmune connective tissue disorder, such as systemic lupus erythematosus and Sjogren syndrome.

Acute diffuse purpuric eruption is a dermatological emergency . Whether caused by an infection (such as Rocky Mountain spotted fever (RMSF), disseminated meningococcal infection) or disseminated intravascular coagulopathy (DIC; or its closely related disorder, purpura fulminans), a diffuse purpuric eruption carries a grave prognosis and sometimes death .