Background: In recent years, temporal artery ultrasound (TAUS) has become a reliable alternative to temporal artery biopsy (TAB) for diagnosing Giant Cell Arteritis (GCA). There is also accumulating evidence that a substantial subset of Polymyalgia Rheumatica (PMR) patients have sonographic evidence of temporal artery inflammation in the absence of cranial symptoms. Early identification will help with earlier and more appropriate escalation of immunosuppressive therapy. Study objectives were to compare the diagnostic performance of TAUS with that of TAB and to define the prevalence of subclinical temporal artery inflammation in those presenting with PMR. Methods: In August 2020, we established rapid-access GCA/PMR clinics in Tallaght University Hospital (TUH) and Cork University Hospital (CUH). ACR classification criteria for GCA and EULAR classification criteria for PMR were used as inclusion criteria. In most cases, patients were seen within 24 hours from referral. Referral sources included primary care physicians, emergency departments, acute medical units and TUH/CUH inpatients. All study participants had vascular ultrasound performed of both temporal arteries (all 3 branches) and both axillary arteries. TAB was performed where possible. Results: 123 patients have been assessed in our clinic over 1 year. 100 were referred with a working diagnosis of GCA, of whom 57 ultimately had GCA diagnosed clinically. 23 had been referred with a working diagnosis of PMR, of whom 6 had sonographic evidence of temporal artery inflammation. 49 patients with suspected GCA had TAB performed. Using clinical criteria as the reference standard, US and TAB demonstrated the following diagnostic performance: Conclusion: Ultrasound is a far more sensitive test than TAB for diagnosing GCA. In our model, hospital admission would have been avoided entirely in 50% of patients owing to the diagnostic accuracy and timing of ultrasound. Our data also adds to the evidence that suggests a significant proportion of pure-PMR patients have underplaying GCA at diagnosis. We propose that vascular ultrasound should be performed as routine in all PMR patients at baseline. Disclosures: None to declare. (n=57) TAUS TAB Sensitivity 89% 41% Specificity 91% 100% Positive Predictive Value 93% 100% Negative Predictive Value 87% 16%