Diagnosis of exclusion
No specific criteria
CBC, CMP, UA, hepatitis profile, coags, VDRL
Autoimmune labs: ANCA, antiphospholipid antibody
Tissue cultures (comprehensive, all organisms)
Coags
Anti-phospholipid antibody
Referral for heme malignancy workup
Referral for IBD workup
CXR if systemic disease suspected
Coags
Anti-phospholipid antibody
Referral for heme malignancy workup
Referral for IBD workup
Pyoderma gangrenosum
Systemic diseases
Erythema infectiosum
Neutrophilic dermatoses 
Cutaneous larva migrans
Calciphylaxis - very important to differentiate due to treatment
Purpura fulminans
Arterial/venous insufficiency