Giant Cell Arteritis and Autoimmune Antibodies
There are no specific autoimmune antibodies that can diagnose giant cell arteritis (GCA)—the disease lacks a diagnostic serologic marker. 1
Why No Antibody Test Exists
- GCA is diagnosed through a combination of clinical presentation, elevated inflammatory markers (ESR/CRP), and confirmation by temporal artery biopsy or vascular imaging—not by autoantibody testing. 1
- Unlike other autoimmune conditions, no specific autoantibody has been identified that reliably indicates GCA. 1
- The pathophysiology involves T-cell-mediated granulomatous inflammation of medium and large vessels, but this does not produce a characteristic autoantibody signature. 2
What Laboratory Tests Are Used
Inflammatory Markers (Non-Specific but Highly Sensitive)
- Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are elevated in >95% of GCA cases at diagnosis. 1
- ESR >100 mm/h has a positive likelihood ratio of 3.11 for GCA. 1
- The absence of ESR >40 mm/h has a negative likelihood ratio of 0.18, making GCA highly unlikely. 1
- The absence of CRP ≥2.5 mg/dL has a negative likelihood ratio of 0.38, also strongly arguing against GCA. 1
- However, inflammatory markers can be normal in <5% of active cases, so high clinical suspicion should still prompt biopsy or imaging. 1
Other Laboratory Findings
- Platelet count >400 × 10³/μL has a positive likelihood ratio of 3.75. 1
- Anemia is present in 87.4% of patients but has low specificity (positive likelihood ratio = 1.27). 1
Diagnostic Gold Standard
- Temporal artery biopsy remains the gold standard for GCA diagnosis, showing arterial wall thickening, mononuclear inflammatory infiltrates, and occasionally multinucleated giant cells. 1
- Biopsy specimens must be ≥1 cm in length to minimize false-negatives from skip lesions (present in ~10% of cases). 1
- Biopsy should be obtained within 2 weeks of starting glucocorticoids, as histopathologic changes remain detectable for 1–2 weeks after treatment begins. 1
Alternative Diagnostic Imaging
- Temporal artery ultrasound showing a "halo sign" (circumferential wall thickening) has 88% sensitivity and 97% specificity for GCA. 1
- MR/CT angiography or FDG-PET can detect large-vessel involvement and assess disease activity. 1
Critical Clinical Pitfall
- Do not delay high-dose glucocorticoids (prednisone 40–60 mg daily) while awaiting biopsy or searching for a non-existent antibody test—untreated GCA carries a 14–50% risk of permanent vision loss. 1
- Treatment must be initiated immediately based on clinical suspicion (new headache in patients >50 years, jaw claudication, visual symptoms, temporal artery abnormalities) plus elevated ESR/CRP. 3, 1