By Dr. Gensler
The following is a Q&A on blood work in spondyloarthritis with Dr. Lianne Gensler.
Spondylitis Association of America (SAA): Can you describe the following blood tests and what they look for? Also, how do they relate to the diagnosis of AS?
Lianne Gensler (LG): Erythrocyte Sedimentation Rate
This is a blood test for inflammation. Unfortunately, it is not high in all AS patients and even when it is high, it can be from other causes. Other causes of an elevation in the ESR include anemia, infection and cancer. That is not to say that if you have an elevation in your ESR in AS, you need to worry about infection or cancer.
How the Test Works
As the test implies, we calculate the rate of sedimentation of the red blood cell (or how fast it falls in a test tube). If there is a lot of inflammation, there is often a molecule called fibrinogen and this makes the red blood cells fall faster thereby increasing the rate!
One more pearl about the ESR: it goes up normally as we age and in woman. Therefore I would not use the quoted common reference range of 0 to 10 or 15mm/hr. There is a rule of thumb you can use to approximate what is an acceptable ESR for age and gender. Age (+10 for a woman)/2. In other words, a 40 year old woman should not have an ESR > 25mm/hr.
This is another blood marker of inflammation. Different laboratories use different tests and different reference ranges so don’t worry if the number changes a lot from lab to lab. Look at the reference range. Also remember other causes of an elevation in the CRP include infection and the high sensitive CRP has been associated with cardiovascular disease.
Many researchers believe the CRP may be better than the ESR in AS. My experience is that sometimes one is elevated and not the other (without clear predictability), sometimes both and often neither. I generally follow both as a measure for disease activity in patients.
This is a genetic test. In other words, it doesn’t change over time and you cannot become positive. In general, once it is tested, it should not need to be retested. HLA B27 is positive in 80 to 90 percent of AS patients. This is especially true in white people and less true in some other ethnic groups, especially African Americans. It is often ordered in the diagnostic stage of disease and may help your doctor decide that the probability of AS is higher or lower. It is not a diagnostic test however for two reasons.
Not everyone with AS has the gene (though most people do).
In the United States population, 7.5 percent of white people carry the gene, yet less than 5 percent of them develop AS. It is lower in other ethnic groups, except in some Native American patients, when it can be much higher.
Other HLA-B27 pearls:
SAA: What are rheumatoid factor and antinuclear antibodies? Do these have any association with AS?
LG: Neither of these tests is associated with AS and should not be ordered if the provider is thinking about AS only and not rheumatoid arthritis or connective tissue disease. In general these diseases do not co-exist. Rheumatoid Factor (RF) is an antibody test found in Rheumatoid Arthritis, but also in other diseases (both rheumatologic (i.e. Sjogren’s syndrome) and non-rheumatologic (i.e. Hepatitis C diseases). The anti-nuclear antibody (ANA) is an antibody test seen in lupus, but also in other rheumatologic (i.e. systemic sclerosis) and non-rheumatologic (autoimmune thyroiditis) diseases. Neither of these tests is expected to be positive in AS. Unfortunately, there are a lot of providers that do not understand the differences and order a panel of rheumatologic tests that may include all of the above. There are also labs that allow for “panels” of tests to be done. In the future, we are moving towards better quality healthcare including appropriate laboratory testing.
SAA: Are there any other blood tests that may be used to help diagnose AS or that you personally have felt helpful in diagnosis?
LG: No (not yet).
SAA: In Dr. Muhammad Asim Khan's book, Ankylosing Spondylitis: The Facts, he states that, "...less than 70% of people with AS have a raised ESR value, even when there is active inflammation." Can you briefly discuss why this may be?
LG: It is not clear why AS patients don’t always have as much inflammation in the blood as diseases like rheumatoid arthritis. One reason may be that the inflammation is local to the sacroiliac joints and spine and therefore the blood measurement is not picking up this more remote process.
SAA: Is there any one blood test that can definitively diagnose AS on its own? (Note: We have had numerous members contact us under the assumption that the HLA-B27 test is actually diagnostic).
LG: No. There is no one blood test that gives a diagnosis of AS to a patient. The diagnosis is made based on several factors:
This does not mean you need all of these features, but your doctor will take these “pieces of the puzzle” and use something called “clinical reasoning” to decide whether you do or do not have AS.
Dr. Gensler is the Director of the Ankylosing Spondylitis Clinic at University of California San Francisco (UCSF). She is an Assistant Professor of Medicine in the division of Rheumatology and sees patients in addition to teaching and performing research in spondyloarthritis.
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