Despite thorough inquiry, including medical history, physical examination, laboratory testing, and radiologic techniques, the underlying causes of the inflammation cannot always be determined. Rheumatologists are frequently confronted with individuals whose condition is referred to as inflammation of unclear aetiology (IUO). The differential diagnosis of IUO is varied, and the study of these patients is difficult and time-consuming. The purpose of the study was to determine the diagnostic role of positron emission tomography/computed tomography (PET/CT) in the evaluation of patients with IUO. The study included 97 adult patients with IUO who had never been diagnosed with an infectious, inflammatory, or neoplastic illness. The relevant data were gathered between January 2015 and June 2018, with a six-month follow-up period. After thorough laboratory and radiologic examinations failed to establish a definitive diagnosis, the patients were examined utilising PET/CT. At the time of the follow-up, 47 of the 97 patients had inflammatory illnesses, 30 had malignancies, and 10 had infections. Despite thorough research, ten patients remained undiagnosed during the follow-up. PET/CT helped diagnosis in 59 cases but was ineffective in 38. PET/CT was positive in 30 of the 47 patients with inflammatory illnesses whose ultimate diagnosis was an inflammatory rheumatic disease, including 19 with large-vessel vasculitis, 7 with polymyalgia rheumatica, and 4 with seronegative arthritis or other uncommon miscellaneous diseases. PET/CT had a sensitivity of 67%, with specificity and diagnostic accuracy of 100% and 71%, respectively. 

The study of the underlying aetiology of IUO is time-consuming and difficult. PET/CT may aid in the faster identification of the final diagnosis by finding an obscure inflammatory location; it may minimise the number of needless biopsies, diagnostic time, anxiety, work loss, morbidity, and death.

Reference:https://journals.lww.com/jclinrheum/Abstract/2021/09000/Diagnostic_Role_of_18F_Fluorodeoxyglucose_Positron.1.aspx