However, they are currently thought of as two separate entities.
PAN is a vasculitis of the medium-sized arteries, sparing the smaller vessels and therefore does NOT cause a glomerulonephritis. Any renal compromise which results is generally due to ischemia from inflammation of the medium-sized renal vessels. PAN is often associated with hepatitis B infection.
In contrast, MPA is a vasculitis of the small-sized vessels, and therefore results in a pauci-immune glomerulonephritis. It is very commonly ANCA-positive.