Spondyloarthritis : Episode 2

Diagnostic Tools in Ankylosing Spondylitis



In this segment, John D. Reveille, MD, Linda and Ronny Finger Foundation Distinguished Chair in Neuroimmunologic Disorders, Division Director, Professor, Vice-Chair - Rheumatology and Clinical Immunogenetics at McGovern Medical School, University of Texas, discusses diagnostic tools used for assessing disease activity in ankylosing spondylitis.

There are several blood tests that can be used both in the diagnosis and the following of patients with this disease. The blood test that most people think about is the HLA-B27 blood test. HLA-B27 is a gene that is found in 7.5% of the white population, 4.4% of the Hispanic population, and 1.1% of the black population of the United States. Most people who have HLA-B27 do not develop ankylosing spondylitis, but up to 90% of patients who do develop AS will have HLA-B27. And so a lot of doctors will order the HLA-B27 test, by itself, to assist in the diagnosis, but it does not make a diagnosis by itself. One common mistake that I see is that doctors will get a B27 test, it will be positive, and then they will make a diagnosis of AS in a patient who has another kind of non-inflammatory back pain. That’s a mistake, because the most important way we make a diagnosis of ankylosing spondylitis is with imaging. The classic time honored one is with an AP pelvic X-ray where you can see radiographic sacroiliitis, and that is the key to making the diagnosis of ankylosing spondylitis. However, it takes 7 to 10 years from the time the inflammatory back pain starts until those X-rays, on the average, turn positive. And so here the use of MRI can be very useful in making an earlier diagnosis by demonstrating edema around the sacroiliac joints on MRI, which is indicative of inflammation. That usually is present at the very beginning.

Other blood tests that we use to help in the following of the patients are the erythrocyte sedimentation rate, or “sed rate,” and C-reactive protein. Both of these are less useful in diagnosis but helpful in following the patient. As of present, they’re the best biomarkers that we have to assess disease activity. Most recent studies suggest that having the C-reactive protein is better than the sedimentation rate. In fact, I’ve seen a number of patients who have very high C-reactive proteins with ankylosing spondylitis and completely normal sed rates. But these other patients, although less commonly, will have just an elevated sed rate. I tend to order both in my patients, and especially when one patient shows that they tend to be an elevated CRP person, I’ll stick with that or sed rate. The problem with these blood tests is they miss about 50% of active patients, so they’re an imperfect way of following the disease and clearly demonstrate a need for better biomarkers.

I can probably talk about using questionnaires to assess disease activity, then go into the ASDAS as a way of introduction. So, the way that physicians, for several years, used to gauge activity in ankylosing spondylitis and axial spondyloarthritis is an instrument that was developed in the 1990s called the Bath Ankylosing Spondylitis Disease Activity Index, or the BASDAI, which is six questions that the patient fills out, often in the waiting room while they’re waiting to be seen.

It is six visual analog scales between 0 and 10 cm, and generally a 4—on the average—between the scales. A 4 out of 10 or greater is regarded as disease activity. The problem with the BASDAI is that it isn’t very specific, in that patients with fibromyalgia will have very high BASDAI scores.

So, the assessments in the Spondyloarthritis International Society looked for a more objective measure, and they formulated the Ankylosing Spondylitis Disease Activity Score, or ASDAS. This takes into account three of the six questions from the BASDAI, as well as one patient global assessment on a visual analog scale of disease activity, and one objective test, either the C-reactive protein or the sed rate, and this will then factor in to a score. And actually you can get an app for your iPhone right off of the ASDAS website to help you calculate it on the spot that allows you to assess more accurately disease activity than the old BASDAI did previously. It’s standardly used in clinical trials, and works pretty well in the clinical setting.

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