Diagnosis of Ankylosing Spondylitis

To diagnose ankylosing spondylitis, the physician takes a personal and family history and performs a physical examination. Other diagnostic tests (e.g., blood tests, imaging tests) also may be performed.

The physical examination includes the Schober test, which is used to detect decreased mobility in the back. In this test, the physician makes a temporary mark on the patient's lower back and a mark 10 cm higher. The patient then bends forward at the waist with the knees straight (as if trying to touch the toes) and the physician remeasures the distance between the marks while the patient is bent over. If the distance between the marks is less than 15 cm, there is decreased spine mobility. In a modified Schober test, the first mark is placed 5 cm lower and a distance of less than 20 cm between the marks indicates decreased mobility.

The Gaenslen test may be performed to detect inflammation in the sacroiliac joint. In this test, the patient lies on the back with one hip over the edge of the examination table. The patient draws both knees to the chest and then extends the legs, allowing the hip and knee to drop below table level. Pain in the sacroiliac joint may indicate a diagnosis of AS.

During the physical examination, chest wall motion also may be measured to detect restriction of movement caused by fusion of the spine.

Laboratory Tests to Diagnose Ankylosing Spondylitis

There is no blood test used specifically to diagnose ankylosing spondylitis; however, a test for the HLA-B27 antibody may be performed. Presence of the antibody does not indicate a diagnosis of AS, but the absence of the antibody makes the diagnosis less likely.

In Caucasians, HLA-B27 is present in 8 percent of the general population and in 90 percent of patients with ankylosing spondylitis. In African Americans, the antibody is present in only 2 percent of the general population and 50 percent of patients with ankylosing spondylitis.

Other blood tests may be used to detect inflammation in the body. These tests include the following:

  • C-reactive protein (CRP) test
  • Erythrocyte sedimentation rate (ESR)
  • Immunoglobulin A (IgA)

Imaging Tests to Diagnose Ankylosing Spondylitis

AS must be present for a number of years in order for signs of the disease to appear on x-ray. However, once these changes occur, they are very specific for ankylosing spondylitis. For example, the lower third of the sacroiliac joint develops a jagged edge, similar to a postage stamp, and as the disease progresses, the joint widens and fuses completely.

X-rays of the spine often show "shiny corners" on the vertebrae, which are caused by erosion of the bone followed by formation of new bone (ossification). The deposition of new bone continues to increase until the edge of each vertebra bridges the disk and fuses together with the next vertebra.

Magnetic resonance imaging (MRI scan) and computed tomography (CT scan) can detect changes in the spine and the sacroiliac joint earlier than x-ray. MRI has become the preferred diagnostic test for ankylosing spondylitis, especially in younger patients and in women of childbearing age.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 31 May 2006

Last Modified: 01 Sep 2015