What is the usual range of C-Reactive Protein (CRP) in patients with Giant Cell Arteritis (GCA)?

Usual Range of CRP in Giant Cell Arteritis (GCA)

In patients with Giant Cell Arteritis (GCA), C-reactive protein (CRP) is typically elevated above the reference range, with a sensitivity of approximately 90% for diagnosing GCA. 1

Diagnostic Value of CRP in GCA

• CRP is elevated in >95% of GCA cases at diagnosis, making it one of the traditionally used markers to assess disease activity 1
• A pretreatment elevated CRP level shows a sensitivity of 90.1% (95% CI, 76.3%-96.3%) for diagnosing GCA 1
• A normal CRP level has a negative likelihood ratio of 0.40 (95% CI, 0.29-0.56), meaning a normal CRP should significantly lower suspicion for GCA 1
• CRP levels less than 2.5 mg/dL have a negative likelihood ratio of 0.38 (95% CI, 0.25-0.59), suggesting that values below this threshold make GCA less likely 1

CRP Compared to ESR in GCA Diagnosis

• While both markers are useful, CRP is considered a more sensitive marker than ESR for a positive temporal artery biopsy diagnostic of GCA 2
• There is clinical utility in obtaining both CRP and ESR in the evaluation of patients with suspected GCA 2
• CRP tends to decrease more rapidly than ESR with treatment, with 67% of patients showing normal values after two weeks of corticosteroid therapy 3

Atypical Presentations and Pitfalls

• Approximately 4% of patients with biopsy-proven GCA may have normal CRP and ESR at diagnosis 4, 2
• In cases where traditional acute phase reactants are normal but clinical suspicion remains high, high-sensitivity CRP testing may detect inflammation not captured by standard CRP assays 5
• Patients with normal inflammatory markers at diagnosis may present with different clinical features compared to those with elevated markers:
  • More likely to have polymyalgia rheumatica symptoms 2
  • Less likely to have constitutional symptoms, anemia, and thrombocytosis 2

Clinical Implications

• A CRP level in the reference range should downgrade the level of suspicion for GCA, but does not completely exclude the diagnosis 1
• When clinical suspicion is high despite normal CRP, temporal artery biopsy or appropriate imaging should still be pursued 4
• Features that should upgrade suspicion for GCA despite normal or only mildly elevated CRP include:
  • Jaw claudication 1
  • Limb claudication 1, 6
  • Temporal artery abnormalities 1
  • Anterior ischemic optic neuropathy 1, 7

Monitoring Disease Activity

• CRP is useful for monitoring disease activity and response to treatment in GCA 1, 3
• CRP typically normalizes more quickly than other acute phase reactants during treatment, making it valuable for early assessment of treatment response 3

Remember that while CRP is an important diagnostic marker in GCA, the diagnosis should be confirmed either histologically through temporal artery biopsy or by appropriate imaging techniques such as vascular ultrasound, MRI, or PET scans 1.