To investigate the effect of baseline clinical characteristics and glucocorticoid treatment on temporal artery biopsy (TAB) findings in patients with giant cell arteritis (GCA).

Individuals who developed GCA after inclusion in two population-based health surveys were identified through linkage to the local and the national patient registers. In addition, other patients diagnosed with GCA at the Departments of Internal Medicine and Rheumatology at an area hospital were included. A structured review of medical records and TAB pathology reports was performed. The presence or absence of giant cells, granuloma, fragmented internal elastic lamina, fibrosis and grade of inflammatory infiltrates were recorded.

In 183 cases with a confirmed clinical diagnosis of GCA, 139 were biopsied after start of glucocorticoids (median treatment duration 3 days; interquartile range 2-5). Patients with a positive TAB (77 %) had significantly higher C-reactive protein (CRP; p = 0.007) and erythrocyte sedimentation rate (ESR; p = 0.03) at the time of clinical diagnosis. A positive TAB tended to more common in women, but there was no difference in the proportion of patients with polymyalgia rheumatica or visual symptoms. Patients biopsied before or on the same day as initial treatment where more likely than those biopsied 1-3 days after treatment start to have positive biopsy [odds ratio (OR) 2.86; 95 % CI 1.06-7.70] as well as inflammatory infiltrates (OR 3.30; 95 % CI 1.15-9.49). There was no significant difference in the proportions of a fragmented internal lamina (p = 0.86), giant cells (p = 0.10), granuloma (p = 0.19), minor inflammatory infiltrates (p = 0.47), major inflammatory infiltrates (p = 0.09), or overall positive biopsy (p = 0.17) report by treatment duration comparing: ≤ 0 days, 1-3 days, 4-6 days, 7-28 days. Among those biopsied 7-28 days after start of treatment, 80 % of TABs were positive, and histopathology features were not substantially different from those biopsied after shorter glucocorticoid treatment.

Biopsies were more likely to be positive and have characteristic histopathologic features in patients with high CRP and ESR, and prior to start of corticosteroid treatment TABs taken 1-4 weeks after initiation of glucocorticoid treatment reveal changes consistent with GCA and therefore still yields clinically useful information for the diagnosis.