Diagnostic performance of 18 F-FDG PET-CT for large vessel involvement assessment in patients with suspected giant cell arteritis and negative temporal artery biopsy Abstract Objective The purpose of our study was to assess the diagnostic performance of 18F-FDG PET-CT for large vessel involvement in patients with suspected giant cells arteritis (GCA) and a negative temporal artery biopsy (TAB). Methods We conducted a retrospective study in a cohort of patients with suspected GCA and negative TAB who underwent an 18F-FDG PET-CT. Ten vascular segments were studied using a visual score and a semi-quantitative method based on SUVmax ratio with respect to liver uptake. The diagnosis of GCA was established during a mean follow-up of 42 months, based on the presence of clinical symptoms, laboratory results, and imaging data compatible with GCA, good response to corticosteroid therapy, and no differential diagnosis after a follow-up of at least 18 months. Results We included 63 patients (30 men and 33 women, aged 67 ± 12 years). 18F-FDG PET-CT showed large vessel involvement in 22 patients, 14 of whom were finally diagnosed with GCA. Forty-one patients were 18F-FDG PET-CT negative, 9 of whom were finally diagnosed with GCA. Overall, 18F-FDG uptake by large vessel yielded 61% sensitivity, 80% specificity, 64% positive predictive value, 78% negative predictive value, and 73% diagnostic accuracy. A significant number of patients were treated by corticosteroids before 18F-FDG PET-CT. However, corticosteroid therapy did not impact significantly the diagnostic performance, although there was a trend to a lower sensitivity in patients receiving corticosteroid therapy for more than 3 days. Conclusions 18F-FDG PET-CT is a useful imaging technique to assess large vessel involvement in patients with suspected GCA and negative TAB.