Diagnostic Role of 18F-Fluorodeoxyglucose Positron Emission Tomography for the Evaluation of Patients With Inflammation of Unknown Origin

J Clin Rheumatol. 2021 Sep 1;27(6):219-225. doi: 10.1097/RHU.0000000000001297.


Background: Sometimes, the underlying causes of inflammation cannot be established despite meticulous investigation, including medical history, physical examination, laboratory tests, and radiologic procedures. Rheumatologists are often faced with patients whose condition is known as inflammation of unknown origin (IUO). Differential diagnosis of IUO is diverse, and investigation of these cases is challenging and time-consuming.

Objective: The study aimed to assess the diagnostic role of positron emission tomography/computed tomography (PET/CT) in the evaluation of patients with IUO.

Methods: The study sample consisted of 97 adult patients with IUO who have not been previously diagnosed with an infectious, inflammatory, or malignant disease. The necessary data were collected from January 2015 to June 2018 with a 6-month follow-up period. The patients were screened using PET/CT after a specific diagnosis could not be established with detailed laboratory and radiologic evaluations.

Results: A final diagnosis was established at follow-up, and 47 (54%) of the 97 patients had inflammatory diseases, 30 (34.4%) had malignancies, and 10 (11.4%) had infections. Despite meticulous investigation, 10 patients were left undiagnosed in the follow-up. PET/CT aided diagnosis in 59 patients (60.8%), but it was not helpful in 38 patients (39.2%). PET/CT was positive in 30 (63%) of the 47 patients with inflammatory diseases, whose final diagnosis was inflammatory rheumatic disease, as follows: large-vessel vasculitis in 19 patients, polymyalgia rheumatica in 7 patients, and seronegative arthritis or other rare miscellaneous diseases in 4 patients. The sensitivity of PET/CT was 67% with a specificity and diagnostic accuracy of 100% and 71%, respectively.

Conclusions: Investigation of the underlying etiology of IUO is time-consuming and challenging. PET/CT may help identify the final diagnosis more quickly by locating an obscure inflammatory site; thus, it may reduce the number of unnecessary biopsies, diagnostic time, anxiety, work loss, morbidity, and mortality.

MeSH terms

  • Adult
  • Fever of Unknown Origin* / diagnosis
  • Fever of Unknown Origin* / etiology
  • Fluorodeoxyglucose F18*
  • Humans
  • Inflammation / diagnostic imaging
  • Positron Emission Tomography Computed Tomography
  • Positron-Emission Tomography


  • Fluorodeoxyglucose F18