Objectives: Diagnosing polymyalgia rheumatica (PMR) can be difficult as many conditions present with similar symptoms and findings. This study aimed to analyse how often the diagnosis of PMR changes during follow-up in a university hospital setting and to determine the most common clinical conditions initially misdiagnosed as PMR.

Method:All patients with a new primary diagnosis of PMR on at least one visit during the years 2016-2019 were identified from the hospital discharge register of Turku University Hospital, Finland. A diagnosis of PMR was confirmed if the patient met at least one of the five classification criteria, complete clinical follow-up (median 34 months) was compatible with PMR, and no other diagnosis better explained their condition.

Results: Of the patients initially diagnosed with PMR, 65.5% were considered to have PMR after further evaluation and clinical follow-up. The most common conditions initially diagnosed as PMR were inflammatory arthritides (34.9%), degenerative or stress-related musculoskeletal disorders (13.2%), infection (9.3%), malignancy (9.3%), giant cell vasculitis (6.2%) and other vasculitis (6.2%), and a wide range of other less common diseases. The diagnosis of PMR remained in 81.3% of patients who fulfilled the 2012 American College of Rheumatology/European League Against Rheumatism PMR classification criteria and in 45.5% of patients who did not.

Conclusions: Diagnosing PMR is challenging, even in a university hospital. One-third of the initial diagnoses of PMR changed during further evaluation and follow-up. There is a substantial risk of misdiagnosis, especially in patients with atypical presentation, and the differential diagnoses of PMR must be considered carefully.