OBJECTIVES: To describe patients who underwent a temporal artery biopsy (TAB) for suspected Giant Cell Arteritis (GCA) but were given a different diagnosis. We focused on a subset of alternate diagnoses mimicking GCA with ominous consequences of a delayed diagnosis or undue glucocorticoid treatment. METHODS: This was a single-center retrospective study. All patients (n = 579) underwent a TAB for initially suspected GCA and were recruited from 2005 to 2023. Four groups were defined: GCA with a positive TAB (n = 248)
GCA with a negative TAB (n = 135)
rapid alternative diagnoses without ominous consequences (usual mimics, n = 177)
and alternative diagnoses with severe consequences (ominous mimics, n = 19). RESULTS: Of the 19 ominous mimics (10% of all mimics), 9 had major diagnostic delays leading to severe outcomes and 12 patients suffered severe side effects from glucocorticoid treatment. The ominous mimics had higher ACR/EULAR 2022 scores than the usual mimics (6.9 vs 4.5), but they had scores similar to those with GCA and a negative TAB. The mean time to an alternative diagnosis was 66 days for the ominous mimics, and the average diagnostic delay of 145 days was much longer than that in the other groups, with a high mortality rate of 68%. CONCLUSION: Misdiagnosing GCA have severe consequences in a few patients. Classification criteria often fail to differentiate mimics from true GCA. Clinician vigilance, histology proof, and imaging tools are critical for ruling out GCA and should be used more widely. Reducing diagnostic delay is essential for improving patient survival in severe mimics cases.