What lab tests are useful for the clinical diagnosis of Giant Cell Arteritis?
The erythrocyte sedimentation rate (ESR) is the most useful laboratory test and tends to be higher in GCA than in other vasculitides. It is almost always >50 mm/hour (85%–90%), typically 80 to 100 mm/hour by the Westergren method. Although ESR is highly sensitive in GCA, specificity is poor. A minority of patients (10%–15%) will have a “normal” ESR. These patients typically have fewer systemic symptoms with an ESR of 40 to 50 mm/hour accompanied by an elevated C-reactive protein (CRP). Less than 5-10% will have both an ESR < 30mm/hr and a normal CRP.
Other nonspecific markers of systemic inflammation, including anemia, thrombocytosis, abnormal liver function tests (especially alkaline phosphatase), and an elevated CRP (frequently to very high levels, >10 mg/dL), are commonly seen. None of these tests is diagnostic. GCA has no associated autoantibodies, such as rheumatoid factor and antinuclear antibody.