Polyarteritis nodosa features inflammation of small and medium-sized arteries, most often in middle-aged males. Single organs or multiple systems may be involved, although usually the lungs are spared. The kidney is the organ most frequently involved (about 77% of cases), with hematuria the main sign. Peripheral nerves are affected almost as frequently, either with clinical or subclinical neuropathy. Hypertension is present in 70% of cases. Arthritis involving multiple joints has been reported in about 50% of patients. Laboratory studies usually show moderate leukocytosis with neutrophilic increase and immaturity of the ordinary type seen with infections. Mild anemia is also common. There often is an increase in serum gamma globulin. Diagnosis sometimes can be made by selective angiography of an affected artery. The main diagnostic procedure is biopsy. The most common specimen is muscle, and the usual region is the gastrocnemius because it is easy to reach. However, if biopsy is done, it should be from some muscle with a painful area if any is present; random samples give poor detection rates. The biopsy specimen should be generous, since the lesions of polyarteritis occur in small arteries. The incidence of positive single muscle biopsy specimens in fairly definite cases of polyarteritis ranges from 20%-40%; obviously, it is on the lower side in mild or questionable cases. Another difficulty is that occasionally patients with classic RA may have arteritis nearly identical to polyarteritis. SLE patients also may have arteritis, but the lesions tend to be in arterioles. Other more rare syndromes of the collagen-vascular group may create histologic difficulty. Therefore, the clinical picture has as much importance as the biopsy report. If the muscle biopsy is normal, some investigators obtain nerve conduction studies on the sural nerve; and if results are abnormal, biopsy the sural.

Patients with polyarteritis may have various autoantibodies. The ANA test result is positive in about 20%-25% of cases. There may be a positive Crithidia anti-DNA test result in about 10% of cases and a positive RA latex test result in about 15% of cases.