Purple Spots on Arm: Differential Diagnosis from Petechiae to Purpura

Purple skin lesions (purpura) result from extravasated blood. Size, morphology, and context distinguish causes.

Classification by Size

• Petechiae: <3 mm, non-blanching, pinpoint.
• Purpura: 3 mm – 1 cm.
• Ecchymosis: >1 cm (bruise).

Differential Diagnosis

Category	Conditions	Key Features
Trauma	Bruises, ecchymosis	History of injury; evolves colors (red→blue→green→yellow)
Vasculitis	IgA vasculitis (Henoch-Schönlein), ANCA-associated, cryoglobulinemic	Palpable purpura (lower extremities); systemic symptoms (arthralgia, abdominal pain, renal)
Platelet Disorders	ITP, TTP, HIT, drug-induced (heparin, quinine), chemotherapy	Petechiae, mucosal bleeding; low platelets (except TTP – normal count but ADAMTS13 deficiency)
Coagulopathy	Warfarin, DOACs, liver disease, DIC, vitamin K deficiency, hemophilia	Ecchymosis, hemarthrosis, prolonged PT/INR, aPTT
Infections	Meningococcemia, Rocky Mountain spotted fever, endocarditis, sepsis, viral (EBV, CMV, parvovirus)	Fever, toxic appearance; petechiae/purpura; medical emergency
Nutritional	Scurvy (vitamin C deficiency)	Perifollicular petechiae, corkscrew hairs, gum bleeding, poor wound healing
Vascular Fragility	Solar purpura (actinic), senile purpura, Ehlers-Danlos, Cushing's, chronic steroids	Forearms/dorsum hands; minimal trauma; atrophic skin
Malignancy	Leukemia, lymphoma, myeloma, metastatic deposits	Pancytopenia, B symptoms, organomegaly
Autoimmune	SLE, APS, rheumatoid vasculitis	Multi-system; specific autoantibodies
Medication	Anticoagulants, antiplatelets, corticosteroids, antibiotics (beta-lactams), SSRIs	Temporal relationship; review med list

Special Patterns

• Palpable purpura: Vasculitis until proven otherwise.
• Purpura fulminans: Rapid progression, necrosis – meningococcemia, DIC, protein C/S deficiency.
• Schamberg's disease: Cayenne pepper spots (capillaritis), asymptomatic, chronic.

When to Seek Urgent Care

• Fever + petechiae/purpura → meningococcemia / sepsis (minutes matter).
• Rapidly expanding bruising, hypotension, confusion → DIC / TTP / adrenal crisis.
• New purpura on anticoagulant + head trauma → intracranial hemorrhage risk.

Initial Workup

• CBC with platelets, PT/INR, aPTT, fibrinogen, D-dimer.
• Metabolic panel (renal, hepatic), UA.
• Inflammatory markers (ESR, CRP).
• Peripheral smear (schistocytes → TTP/HUS; blasts → leukemia).
• Urgent hematology/dermatology referral if unclear.