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Background Giant cell arteritis (GCA) usually affects the temporal arteries but can result in systemic vasculitis that can be seen on Positron Emission Tomography-Computed Tomography (PET-CT). Currently PET-CT is not used first line to diagnose cranial-GCA and temporal artery biopsy (TAB) remains the most common investigation to assess for this. There have been few studies performed to assess the use of PET-CT as an initial diagnostic tool in cranial-GCA. One study performed PET-CT for suspected GCA in their cohort of 64 patients. They found, when compared with TAB, global GCA assessment by PET-CT had a sensitivity of 92%, specificity of 85%, positive predictive value of 61% and negative predictive value of 98%. They concluded PET-CT had good diagnostic accuracy when compared with TAB and can be used as a first-line test to assess for GCA and rule out lower-risk GCA. Another study used PET-CT in steroid-naïve patients and also showed a high accuracy (sensitivity 64% and specificity of 100%) in diagnosing cranial artery inflammation. Methods We present two cases where the patient had symptoms of headache, constitutional symptoms and raised inflammatory markers, however PET-CT did not suggest GCA. The patients were subsequently re-referred later with ongoing symptoms and TAB confirmed GCA. Results Case 1: A 71-year-old woman presented with a one-month history of intermittent right sided headache and rigors. Her inflammatory markers were elevated (CRP of 115mg/L and ESR of 57mm/hr). Suspecting GCA, prednisolone 30mg od was commenced. Upon rheumatology review, it was felt her symptoms were atypical for GCA. Prednisolone was discontinued, and a PET-CT scan was arranged. This was performed ten days later and did not show any features of vasculitis. Six weeks later, the patient presented to ophthalmology reporting visual aura, ongoing headache with elevated inflammatory markers. Prednisolone 60mg od was commenced and a TAB was arranged. This was reported as partially treated GCA. Case 2: A 73-year-old gentleman had a prolonged admission for pyrexia of unknown origin. Headache was present throughout admission and rheumatology opinion was sought. A PET-CT did not show vasculitis. The patient was discharged when pyrexia improved however his inflammatory markers remained consistently high (CRP of 82mg/L and ESR 106mm/hr). One-month post-discharge, the patients GP contacted rheumatology concerned about ongoing headache and scalp tenderness. Prednisolone 40mg od was commenced and a TAB was arranged which reported features consistent with GCA. Both patients responded well to steroids. Conclusion As demonstrated, patients with biopsy proven GCA do not always exhibit vasculitis on imaging. A negative PET-CT did not reliably exclude GCA in our patients suggesting its use as a diagnostic investigation for GCA needs to be approached with caution. A TAB should also be performed when there is ongoing suspicion of GCA. Disclosures S. Khalid None. J. Maxwell Honoraria; BMS, Pfizer, Lilly, Abbvie. Other; Talks, BMS, Pfizer, Lilly, Abbvie.

P36 Never say never: two cases of PET-CT negative but biopsy proven giant cell arteritis