A doctor who suspects central nervous system (CNS) or peripheral nervous system (PNS) vasculitis will gather a comprehensive medical history of the individual, perform a physical examination, order laboratory tests (primarily blood tests), and recommend any other tests that seem appropriate. Electromyography (EMG) and nerve conduction studies identify blocks and loss of nerve supply to muscle due to vasculitic nerve damage. Studies of the brain blood vessels, such as MR or CT angiograms can sometimes identify narrowing in the larger blood vessels. Direct injection of the brain blood vessels may be needed to look for narrowings consistent with vasculitis in medium-sized brain arteries.
However, the diagnosis of vasculitis often requires evidence that there is ongoing inflammation. Inflammatory cells may be found in the spinal fluid. Often, there is a need to conduct a tissue biopsy to examine blood vessels under a microscope. A definitive diagnosis is important because the treatment usually requires powerful immune-suppressive drugs. In addition, it is important to make sure that an infection is not causing the inflammation.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.