1/100,000 people per year in US, primarily 40s-50s year of age
Dysfunction of activated neutrophils
>50% associated with systemic disease:
IBD
Polyarthritis, sero-negative and -positive
Leukemia
Less commonly psoriatic arthritis, osteoarthritis, RA, spondyloarthopathy, hepatitis, primary biliary cirrhosis, multiple myeloma, SLE, Sjögren Syndrome
IBD
Polyarthritis, sero-negative and -positive
Leukemia
Less commonly psoriatic arthritis, osteoarthritis, RA, spondyloarthopathy, hepatitis, primary biliary cirrhosis, multiple myeloma, SLE, Sjögren Syndrome
Pyoderma gangrenosum
Systemic diseases
Erythema infectiosum
Neutrophilic dermatoses 
Cutaneous larva migrans
Calciphylaxis - very important to differentiate due to treatment
Purpura fulminans
Arterial/venous insufficiency