There are no single tests that can confirm the diagnosis of ankylosing spondylitis. It is diagnosed mainly on the basis of symptoms as well as some tests.
Pain of ankylosing spondylitis is usually typical, worsening after rest usually in the second half of the night or early morning.
Complete blood count including ESR (erythrocyte sedimentation rate)
ESR denotes the level of blood cells after an hour of standing in a tube. High ESR is usually a marker of an inflammatory condition.
ESR may be raised in several conditions and thus this test is not specific for ankylosing spondylitis.
Blood tests also reveal mild anemia called normochromic normocytic anaemia meaning there is little change in appearance of Red blood cell size or colour.
A raised alkaline phosphatase level may be present in severe disease. Above normal serum IgA (Immunoglobulin A) levels are common.
This is yet another marker of inflammation and is raised in several inflammatory conditions. However, only 50–70% of patients with active disease will have an increased level of C reactive protein and a raised ESR.
Imaging studies like an X ray of the spine and lower back is then performed. X rays reveal damage to the joints at the base of the spine and formation of tiny new bones between the vertebrae (bones that form the spine). These new bones can press of surrounding structures and cause pain.
An X ray is followed up with a magnetic resonance imaging (MRI) scan to look at details of the damage to the spine, vertebrae and the sacroiliac joints (joints at the base of the spine).
MRI also reveals damage and inflammation of the ligaments in the spinal column. An ultrasound scan is sometimes performed to detect inflammation of the tissues.
These imaging studies can often confirm ankylosing spondylitis since they reveal characteristic inflammatory changes.
However, during initial stages of the disease, damage to the spine may not yet be visible making diagnosis difficult. To confirm a diagnosis of the condition some features must be present and these include presence of inflammation of the sacroiliac joints on X ray in a person with:-
at least three months of lower back pain that gets better with exercise and becomes worse with rest
restricted movement of the lower spine or the lumbar spine
restricted movement of the chest for breathing according to normal as per age and sex
In persons with all 3 features without sacroiliac inflammation the diagnosis of “probable ankylosing spondylitis” is made. An MRI can help detect the condition early.
The HLA-B27 gene is present in about 90–95% of white patients with ankylosing spondylitis in central Europe and North America. This is one of the confirmatory tests.