Ankylosing Spondylitis: Understanding Abnormal Lab Findings and Diagnostic Markers

What labs are abnormal with ankylosing spondylitis?

Ankylosing Spondylitis (AS) is a chronic inflammatory disease primarily affecting the spine and sacroiliac joints. While no single laboratory test definitively diagnoses AS, several markers commonly show abnormalities, reflecting systemic inflammation and genetic predisposition, and aiding in diagnosis and disease monitoring.

Understanding Ankylosing Spondylitis and Diagnostic Markers

Ankylosing Spondylitis is a type of inflammatory arthritis that, over time, can cause some of the vertebrae in your spine to fuse, leading to a rigid spine. It's part of a group of diseases called spondyloarthropathies. Diagnosis relies on a combination of clinical symptoms, physical examination, imaging studies (like X-rays and MRI), and specific laboratory tests that can indicate inflammation or genetic predisposition. It's crucial to understand that lab results alone are rarely sufficient for a definitive diagnosis but serve as important pieces of the diagnostic puzzle.

Primary Inflammatory Markers

These tests indicate the presence of inflammation in the body, which is a hallmark of AS.

• Erythrocyte Sedimentation Rate (ESR)

  • What it measures: The rate at which red blood cells settle in a test tube over a specific period. Inflammation causes red blood cells to clump together and fall faster.
  • Why it's abnormal in AS: Elevated ESR indicates systemic inflammation, a common feature in active AS.
  • Considerations: While often elevated, ESR can be normal even in patients with active AS, especially those with less severe peripheral joint involvement or primarily axial disease. It's also a non-specific marker, meaning many other conditions can cause an elevated ESR.

• C-Reactive Protein (CRP)

  • What it measures: A protein produced by the liver in response to inflammation. It's an acute-phase reactant.
  • Why it's abnormal in AS: Elevated CRP levels are a more sensitive indicator of systemic inflammation than ESR and are often elevated during AS flares or active disease.
  • Considerations: Like ESR, CRP is non-specific and can be elevated due to various inflammatory conditions, infections, or tissue damage. However, persistently high CRP can help monitor disease activity and treatment effectiveness.

Genetic Marker: HLA-B27 Antigen

This is a specific protein found on the surface of white blood cells.

• What it is: Human Leukocyte Antigen B27 (HLA-B27) is a gene that plays a significant role in the immune system.
  • Why it's abnormal in AS: While not everyone with HLA-B27 develops AS, approximately 90-95% of people with AS (especially those of European descent) test positive for this antigen. This strong genetic association suggests a predisposition to the disease.
  • Considerations: A positive HLA-B27 test alone is not diagnostic for AS. Many healthy individuals (about 8% of the general population) also carry the HLA-B27 gene and never develop AS. Conversely, a small percentage of individuals with AS may test negative for HLA-B27. Therefore, it's used as supportive evidence, particularly when combined with characteristic symptoms and imaging findings.

Other Potentially Abnormal Labs and Their Significance

While less directly diagnostic for AS, these tests may show abnormalities related to the disease's effects, complications, or medication side effects.

• Complete Blood Count (CBC)

  • Anemia: Some individuals with chronic inflammatory conditions like AS may develop anemia of chronic disease, characterized by mild to moderate normocytic (normal cell size) anemia. This results from the body's altered iron metabolism in response to persistent inflammation.

• Liver Function Tests (LFTs)

  • Elevated Liver Enzymes: These tests might show mild elevations, often not directly due to AS itself but potentially as a side effect of medications used to manage AS, such as nonsteroidal anti-inflammatory drugs (NSAIDs) or disease-modifying anti-rheumatic drugs (DMARDs), or biologic agents. Rarely, severe extra-articular manifestations could impact liver function.

• Kidney Function Tests (Creatinine, Blood Urea Nitrogen - BUN)

  • Generally Normal: Kidney function tests (measuring creatinine and BUN) are typically normal in AS. However, long-term use of certain medications (e.g., NSAIDs) can rarely affect kidney function, or in very rare cases, AS can be associated with specific kidney pathologies like IgA nephropathy.

• Urinalysis

  • Microscopic Findings: A urinalysis is usually normal in AS. However, in rare instances of kidney involvement (e.g., secondary amyloidosis or IgA nephropathy, which can be associated with AS), it might reveal microscopic hematuria (blood in urine) or proteinuria (protein in urine).

• Autoantibodies (e.g., Rheumatoid Factor, Anti-CCP Antibodies)

  • Typically Negative: One of the distinguishing features of AS and other spondyloarthropathies is the typical absence of common autoantibodies found in other autoimmune diseases. For instance, Rheumatoid Factor (RF) and Anti-Cyclic Citrullinated Peptide (Anti-CCP) antibodies are generally negative in AS. This helps differentiate AS from rheumatoid arthritis.

The Holistic Diagnostic Approach

It is critical to reiterate that no single lab test confirms or rules out Ankylosing Spondylitis. The diagnosis is a clinical one, made by a rheumatologist who integrates all available information:

• Patient History: Characteristic inflammatory back pain, morning stiffness, and other systemic symptoms.
• Physical Examination: Assessment of spinal mobility, posture, and tenderness.
• Imaging Studies: X-rays or MRI of the sacroiliac joints and spine, which can reveal characteristic inflammation, erosion, or fusion.
• Laboratory Findings: The results of the tests discussed above, used to support the clinical picture, rule out other conditions, and monitor disease activity.

Conclusion

Abnormal lab findings in Ankylosing Spondylitis primarily include elevated inflammatory markers like ESR and CRP, and the presence of the HLA-B27 genetic marker. While these tests provide valuable insights into the inflammatory process and genetic predisposition, they are part of a larger diagnostic framework. Understanding these markers helps clinicians confirm suspicion, monitor disease progression, and tailor treatment strategies, ultimately empowering individuals with AS to manage their condition effectively.