Giant cell arteritis (GCA) is the most common systemic vasculitis affecting adults in Europe and North America, typically involving the extra-cranial branches of the carotid arteries and the thoracic aorta, and characterized by lympho-histiocytic inflammation of the arterial wall, typically with giant cells [1,2,3,4]. It predominantly affects people of northern European descent, with a female-to-male ratio of 2-3:1 and a United States prevalence of 204 per 100,000 aged 50 years or older. Morbidity largely relates to vision loss, although aortic involvement can also lead to aneurysms and dissections [5,6]. Unlike some of the other autoimmune systemic vasculitides, there are no specific serologic markers of disease, and while certain non-invasive imaging techniques have recently been adopted as relatively specific diagnostic modalities, histopathologic demonstration of arteritis involving a biopsied temporal artery remains the reference standard for a diagnosis of GCA [7,8,9].

Despite the central role of tissue pathology in the diagnosis of GCA, a uniform system for the interpretation and reporting of histopathologic findings in temporal artery biopsy (TAB) specimens has yet to be proposed. While there are guidelines regarding the processing of TAB specimens [10], it has been a decade since their publication and thus they may not thoroughly incorporate our expanded knowledge of this disease, nor accurately correlate with current clinical guidelines regarding GCA [11,12].

With this in mind, the Society for Cardiovascular Pathology set out to establish consensus guidelines for the processing, interpretation, and reporting of TAB specimens, based on the existing literature; these guidelines are intended for use not only by the pathologists who report these cases, but also by other health care professionals involved in the management and study of GCA. This document will address TAB specimen adequacy in the setting of suspected GCA, macroscopic tissue description and processing to ensure a thorough representation of biopsied tissue, the appropriate use of histochemical and immunohistochemical stains in the histologic interpretation of TAB specimens, light microscopic criteria for the diagnosis of GCA, and standardized nomenclature for the pathologic reporting of TAB specimens (Table 1). This document will also discuss the interpretation of findings which currently fall into so-called grey-zone areas, including inflammation restricted to arterial adventitia or peri-adventitial vessels, and how certain histologic findings may or may not be helpful in the differentiation of GCA from other vasculitides and other acute and chronic injury patterns [13], [14], [15]. Theories and controversies regarding the pathophysiology, including the role of certain microbes as triggering agents in GCA, will also be discussed [16,17,18].