Ankylosing Spondylitis: Key Blood Tests and Diagnostic Role

What blood test is done for ankylosing spondylitis?

While no single blood test definitively diagnoses Ankylosing Spondylitis (AS), several are crucial tools in the diagnostic process, primarily including the HLA-B27 genetic marker and inflammatory markers such as Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP).

Understanding Ankylosing Spondylitis and Diagnosis

Ankylosing Spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton, meaning the spine and sacroiliac joints. It can lead to severe pain, stiffness, and, in advanced cases, fusion of the vertebrae, resulting in a rigid, inflexible spine. As an autoimmune condition, AS involves the body's immune system mistakenly attacking its own tissues. Diagnosing AS can be challenging because its early symptoms often mimic other conditions, and there isn't one definitive test. Instead, diagnosis relies on a combination of clinical assessment, imaging studies, and specific blood tests.

Key Blood Tests for Ankylosing Spondylitis

Blood tests play a supportive role in identifying potential indicators of AS or ruling out other conditions. The most commonly utilized blood tests are:

HLA-B27 Gene Test

• What it is: The Human Leukocyte Antigen B27 (HLA-B27) is a specific protein found on the surface of white blood cells. It's a genetic marker, and its presence is strongly associated with AS and other spondyloarthropathies (a group of inflammatory diseases that affect the joints and entheses, where tendons and ligaments attach to bone).
• Significance: Approximately 90-95% of people with AS in Caucasian populations test positive for HLA-B27. However, it's crucial to understand that testing positive for HLA-B27 does not mean a person has AS or will definitely develop it. A significant portion of the general population (5-10%) also carries the HLA-B27 gene but never develops AS. Conversely, a small percentage of individuals with AS test negative for HLA-B27, particularly in non-Caucasian populations (referred to as "seronegative" AS).
• Role in Diagnosis: The HLA-B27 test is used to assess an individual's genetic predisposition when AS symptoms are present. It helps in the overall clinical picture but is not a standalone diagnostic test.

Inflammatory Markers

These tests measure levels of inflammation in the body, which can be elevated during active periods of AS.

• Erythrocyte Sedimentation Rate (ESR or Sed Rate):
  • What it is: This test measures how quickly red blood cells (erythrocytes) settle in a test tube over a specific period. When inflammation is present, certain proteins cause red blood cells to clump together and fall more rapidly.
  • Significance: An elevated ESR indicates general inflammation somewhere in the body. While it can be high in AS patients, it's a non-specific marker, meaning many other conditions (infections, other autoimmune diseases) can also cause an elevated ESR. Some individuals with active AS may have a normal ESR.
• C-Reactive Protein (CRP):
  • What it is: CRP is a protein produced by the liver in response to inflammation. It's considered a more sensitive and rapid indicator of acute inflammation than ESR.
  • Significance: Like ESR, an elevated CRP level suggests inflammation and can be high during AS flares. However, it is also a non-specific marker and can be raised due to various other inflammatory conditions, infections, or even obesity.

The Role of Blood Tests in AS Diagnosis

It is critical to emphasize that blood tests alone are insufficient for diagnosing Ankylosing Spondylitis. They serve as pieces of a larger puzzle that a rheumatologist assembles.

• Supporting Evidence: A positive HLA-B27 test, combined with persistent back pain characteristic of inflammatory arthritis (e.g., pain that improves with exercise, worsens with rest, and is worse in the morning), and evidence of sacroiliac joint inflammation on imaging (X-rays or MRI), strongly supports an AS diagnosis.
• Excluding Other Conditions: Blood tests can also help rule out other conditions that might present with similar symptoms, such as rheumatoid arthritis (e.g., by testing for rheumatoid factor and anti-CCP antibodies, which are typically negative in AS).
• Monitoring Disease Activity: Inflammatory markers (ESR and CRP) can also be used to monitor disease activity and the effectiveness of treatment over time, though clinical symptoms often provide more direct feedback.

What a Positive or Negative Result Means

• Positive HLA-B27: Indicates a genetic predisposition and increases the likelihood of AS if characteristic symptoms and imaging findings are present. It does not confirm AS on its own.
• Negative HLA-B27: Does not rule out AS. About 5-10% of AS patients are HLA-B27 negative. Other clinical and imaging findings remain crucial for diagnosis.
• Elevated ESR/CRP: Suggests systemic inflammation, which is common in AS, but is not specific to AS and requires further investigation to determine the cause.
• Normal ESR/CRP: Does not rule out AS. Some individuals with active AS may have normal inflammatory markers.

Beyond Blood Tests: A Holistic Diagnostic Approach

The diagnosis of Ankylosing Spondylitis is a comprehensive process that typically involves:

1. Clinical Assessment: A thorough medical history focusing on the type and duration of pain, stiffness, and other symptoms (e.g., enthesitis, uveitis, fatigue). A physical examination assesses spinal mobility, joint tenderness, and posture.
2. Imaging Studies:
   • X-rays: Used to visualize changes in the sacroiliac joints and spine, though early changes may not be visible.
   • Magnetic Resonance Imaging (MRI): More sensitive than X-rays, MRI can detect early inflammatory changes in the sacroiliac joints and spine that are not yet visible on X-rays.
3. Blood Tests: As discussed, to check for genetic markers and inflammatory indicators, and to rule out other conditions.

Early and accurate diagnosis is paramount for managing Ankylosing Spondylitis effectively, allowing for timely intervention with lifestyle modifications, exercise, physical therapy, and appropriate pharmacologic treatments to slow disease progression and improve quality of life. If AS is suspected, consulting a rheumatologist is essential.