It may take a long time before a sure diagnosis of ankylosing spondylitis is established. This is partly because the disease usually begins gradually and the symptoms are often fluctuating with periods of minor or no symptoms. There are no biochemical tests that can confirm the diagnosis except that the presence of the tissue type HLA B27 supports it.
According to the international accepted criteria for ankylosing spondylitis a definite diagnosis requires changes of the sacroiliac joints by radiography. However, radiography can only show structural damage of the joints and/or bones. Their occurrence may first be visible several years after the apperance of symptoms.
Computer tomography (CT) is superior to radiography for visualisation of the sacroiliac joints. It can therefore detect minor and hence earlier changes than by radiography. However, CT is like radiography based on X-rays and can only visualise structural joint or bone changes.
Magnetic resonance imaging (MRI) is superior to CT and radiography for detecting early sacroiliitis because MRI can show active inflammation before the occurrence of structural damage. It is based on magnetic fields and radiowaves and can give information about tissue composition.
In later stages of the disease the diagnosis is often based on radiography and cross sectional imaging (MRI or CT) is usually used only to estimate the disease activity or detect complications such as spinal fractures.