Laboratory Testing

During the process of diagnosing ankylosing spondylitis (AS), several laboratory tests may be done. Some of these tests help rule out other conditions, and others provide doctors with helpful information to arrive at a diagnosis. There is no single laboratory or blood test available that can definitively diagnose AS, however. Diagnosis of AS is usually made primarily based on imaging tests, a physical examination, and a person’s symptom history.^1,2

HLA-B27 gene

HLA-B27 is a genetic marker that has been identified as playing a role in the cause of AS and can be detected using a blood test. However, not everyone with the HLA-B27 mutation develops AS, and it is believed that many people in the general population have the HLA-B27 marker but will not develop the condition. In addition, some people with AS do not have the HLA-B27 marker. The association between the presence of HLA-B27 and AS varies greatly among different ethnic groups, with the highest rate of HLA-B27 occurring in Caucasians.¹

Inflammatory markers

Inflammatory markers are certain proteins that show up in blood tests when there is an increase in inflammation in the body. Two inflammatory markers that may be tested in someone suspected of having AS are erythrocyte sedimentation rate and C-reactive protein²:

• Erythrocyte sedimentation rate, or ESR, measures the falling (sedimentation) rate of red blood cells (erythrocytes) in a blood sample. ESR is faster when there are an increase in inflammatory proteins in the blood.³
• C-reactive protein, or CRP, is an acute phase reactant, a protein that is produced by the liver and is found elevated during times of inflammation, such as what occurs with an injury, infection, or conditions that chronically increase inflammation.⁴

While inflammation is a natural process that occurs as part of the body’s immune system response, chronic inflammation can cause damage to surrounding tissues, such as the damage that can occur to joints in people with inflammatory arthritis conditions like AS. These inflammatory markers may be helpful in discovering if there is acute or chronic inflammation in the body. However, they are non-specific tests and do not tell what is causing the inflammation. The inflammation may be caused by AS, or it may be due to an infection, cancer, or autoimmune conditions.^2-4

Anemia of chronic disease

When someone has a chronic disease like AS, it may lead to anemia. This type of anemia is called anemia of chronic disease (ACD) or anemia of inflammation. ACD occurs when there are not enough red blood cells produced by the bone marrow. This usually occurs gradually, and while most cases of ACD are mild to moderate, symptoms may negatively affect a person’s quality of life. Symptoms of ACD include^5,6:

• Fatigue
• Pale skin
• Lightheadedness or dizziness
• Shortness of breath
• Fast heartbeat
• Chest pain
• Irritability

ACD can be diagnosed through patient symptoms and blood tests that measure the amount of red blood cells, hemoglobin, and levels of iron in the blood.⁶

Ruling out other conditions

Other laboratory tests that may be conducted are used to rule out other conditions, such as rheumatoid arthritis (RA), gout, or lupus. These tests may include⁷:

• Rheumatoid factor (RF), an antibody which often shows up in people with RA
• Uric acid, which is elevated in people with gout
• Antinuclear antibody (ANA), commonly found in people with lupus and some people with RA, scleroderma, Sjogren’s syndrome, or other conditions
• Anti-cyclic citrullinated peptide (anti-CCP), which may be present in people with RA
• Lyme serology, a test used to detect Lyme disease

Labs performed before taking medication

Prior to prescribing medications such as NSAID’s, SAARD and biologics physicians will need to perform certain tests to make sure that there is no underlying liver or kidney disease. Tests to rule out infections are also done prior to biologics and these include tests for tuberculosis and hepatitis.