To treat people with Wegener's granulomatosis most effectively, doctors must diagnose the disease early. There are no blood tests that a doctor can use to diagnose Wegener's granulomatosis, but blood tests are important to rule out other causes of illness and to determine which organ may be affected.

Most blood tests can only suggest that a person has inflammation somewhere in the body. Anemia (low red blood cell count), elevated white blood cell count and platelet count, and an elevated sedimentation rate are commonly found in people with Wegener's granulomatosis. If the kidneys are involved, a health care provider can see red blood cells and structures called red blood cell casts in the urine when viewed under a microscope, and the blood tests measuring kidney function may show abnormalities.

X-ray results can be very helpful in diagnosing Wegener's granulomatosis. People with lung problems will have abnormal chest x-rays. CT (computed tomography) scans in people with sinus problems may show thickening of the sinus lining.

Many people with active Wegener's granulomatosis have a blood test that shows the presence of a specific type of antibody called antineutrophil cytoplasmic antibodies (ANCA) (an antibody is a disease-fighting protein). Although a positive ANCA test is useful to support a suspected diagnosis of Wegener's granulomatosis, in most instances health care providers do not use it by itself to diagnose this disorder. The ANCA test may be negative in some people with active Wegener's granulomatosis. Currently, the only clear-cut way to diagnose Wegener's granulomatosis is by performing a biopsy (removing a tiny piece of tissue) of an involved organ (usually the sinuses, lung, or kidney). A health care provider will examine tissue from the organ under the microscope to confirm the presence of vasculitis and granulomas (a specific type of inflammation), which together are features of Wegener's granulomatosis. A biopsy is very important both to confirm the presence of the disease and also to make sure other disorders that may have similar signs and symptoms are not present.