The most useful investigation in supporting the clinical diagnosis of PMR/GCA is elevation of the erythrocyte sedimentation rate (ESR) or viscosity. Acute phase proteins, particularly CRP, are also elevated but in most cases are not more helpful than the ESR in either diagnosis or follow-up. The definitive investigation is the demonstration of giant cell arteritis histologically, usually from temporal artery biopsy. Elevation of alkaline phosphatase of liver origin is seen in one-third to one-half of patients and may lead to delay in diagnosis. Measurements of alpha 1-antichymotrypsin and IL-1 beta may be helpful in diagnosis and management but more studies are required.