What is the best initial test for diagnosing polyarteritis nodosa?

The best initial test for diagnosing polyarteritis nodosa (PAN) is abdominal angiography. Polyarteritis nodosa is a medium-vessel vasculitis that primarily affects systemic arteries and can lead to significant complications involving multiple organ systems.

Abdominal angiography is particularly useful because it allows for the visualization of vascular changes associated with PAN, such as the presence of aneurysms, stenoses, or occlusions in the major arteries, especially in the renal, mesenteric, and hepatic areas. This imaging modality can identify the characteristic findings of vasculitis, helping to confirm the diagnosis.

Blood cultures are not typically useful in diagnosing polyarteritis nodosa, as it is not an infectious process but rather an autoimmune condition. Muscle biopsy may be pertinent in other conditions but is less specific for PAN and may not reveal findings that confirm the diagnosis. A skin biopsy could potentially show signs of vasculitis; however, the systemic involvement of PAN and its vascular manifestations make abdominal angiography the more direct and informative initial test to establish the diagnosis effectively.