Acute Laboratory Workup for Giant Cell Temporal Arteritis
Order ESR, CRP, complete blood count with platelet count, and comprehensive metabolic panel immediately when giant cell arteritis is suspected—these tests should never delay initiation of high-dose glucocorticoids (40-60 mg prednisone daily), which must be started within minutes to hours to prevent irreversible blindness.
Essential Laboratory Tests
Inflammatory Markers (Highest Priority)
Erythrocyte sedimentation rate (ESR) is essential and provides critical diagnostic information: an ESR >100 mm/h yields a positive likelihood ratio of 3.11, strongly supporting GCA, while an ESR <40 mm/h essentially rules out the disease with a negative likelihood ratio of 0.18. 1
C-reactive protein (CRP) is complementary to ESR and should always be obtained alongside it: a CRP ≥2.5 mg/dL supports the diagnosis, whereas a normal CRP (<2.5 mg/dL) makes GCA highly unlikely with a negative likelihood ratio of 0.38. 1, 2
Both ESR and CRP must be ordered together because they provide independent diagnostic information—approximately 4% of biopsy-proven GCA patients have both markers normal at diagnosis, so relying on a single test risks missing cases. 2, 3
Pre-treatment values are critical: obtain these inflammatory markers before starting glucocorticoids whenever possible, as ESR and CRP begin declining within days of steroid initiation, with CRP normalizing faster than ESR (67% normal by 2 weeks versus 56% for ESR). 1, 4
Complete Blood Count with Differential
Platelet count >400 × 10³/µL provides a positive likelihood ratio of 3.75 for GCA and should be specifically evaluated. 1, 5
Hemoglobin level should be assessed, as anemia (hemoglobin <12 g/dL) is present in approximately 54-87% of GCA patients and paradoxically serves as a negative predictor for severe ischemic complications including vision loss. 5, 6
White blood cell count may show leukocytosis in approximately 28% of patients, though this finding has limited diagnostic specificity. 6
Comprehensive Metabolic Panel
Alkaline phosphatase is elevated in approximately 25% of GCA patients and reflects systemic inflammatory activity. 6
Albumin level is decreased (hypoalbuminemia) in roughly 28% of patients as part of the acute-phase response. 6
Baseline metabolic panel, hemoglobin A1C, fasting lipid profile, prothrombin time, and partial thromboplastin time are reasonable for cardiovascular risk stratification before initiating long-term glucocorticoid therapy. 2
Critical Interpretation Guidelines
When Laboratory Results Argue Against GCA
ESR <40 mm/h combined with CRP <2.5 mg/dL makes GCA extremely unlikely (negative likelihood ratios of 0.18 and 0.38 respectively) and should prompt consideration of alternative diagnoses. 1, 2
However, do not withhold treatment based solely on normal inflammatory markers if clinical suspicion remains high—approximately 4% of biopsy-proven cases present with normal ESR and CRP, and these patients more commonly have polymyalgia rheumatica symptoms with fewer constitutional features. 3
When Laboratory Results Support GCA
ESR >100 mm/h strongly supports the diagnosis but paradoxically correlates with lower rates of permanent vision loss (only 22.6% of patients with permanent visual loss had ESR >100 mm/h at diagnosis). 6
The combination of elevated ESR and elevated CRP provides an odds ratio of 3.06 for biopsy-proven GCA, while concordantly normal values yield an odds ratio of 0.49 against the diagnosis. 3
Common Pitfalls to Avoid
Never delay glucocorticoid initiation while awaiting laboratory results—untreated GCA carries a 14-50% risk of permanent, irreversible vision loss, and treatment must begin immediately based on clinical suspicion alone. 2
Do not rely on a single inflammatory marker—CRP is slightly more sensitive than ESR (86.9% versus 84.1% sensitivity), but obtaining both tests maximizes diagnostic accuracy and provides complementary information. 3
Do not use isolated elevation of inflammatory markers without clinical symptoms to escalate therapy or make treatment decisions—clinical correlation is essential. 7
Remember that glucocorticoid treatment rapidly affects laboratory values—if pre-treatment values cannot be obtained, proceed with treatment anyway and arrange temporal artery biopsy within 2 weeks, as histopathologic changes remain detectable for 1-2 weeks after steroid initiation. 1, 2, 7