For initial investigations to support a diagnosis of giant cell arteritis (GCA), blood tests are recommended in primary care before or immediately after commencing high-dose glucocorticoids NICE CKS. These include a full blood count, C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR) NICE CKS. Typically, the CRP level is elevated, and the ESR is often greater than 50 mm/hour, although it can be normal at presentation or during a flare NICE CKS. Other common blood test abnormalities include normochromic normocytic anaemia and an elevated platelet count NICE CKS.
Depending on the clinical situation, other initial investigations may be indicated to exclude alternative diagnoses or identify risks for glucocorticoid-related adverse effects NICE CKS. These can include baseline urea and electrolytes, HbA1c, calcium, and liver function tests NICE CKS. If the ESR is disproportionately elevated compared to CRP, serum protein electrophoresis and urine Bence–Jones protein/serum-free light chains may be considered NICE CKS. Screening tests for serious infection risk (e.g., urine dipstick, chest X-ray, latent tuberculosis tests) and osteoporosis risk (e.g., TSH, vitamin D, bone density test, DEXA) may also be relevant NICE CKS.
It is crucial not to delay referral to a specialist or the initiation of glucocorticoid treatment while waiting for laboratory results if GCA is strongly suspected NICE CKS. Definitive diagnosis relies on further assessment and investigation in secondary care, typically involving imaging such as temporal and axillary artery ultrasound, or a temporal artery biopsy NICE CKS. All people with suspected GCA should have at least one of these confirmatory diagnostic tests NICE CKS.