Is either ESR or CRP more sensitive or specific for the diagnosis of GCA?  

I typically obtain both an ESR and a CRP in the workup of new onset or relapsing GCA. The CRP may be slightly more sensitive than the ESR based on Kermani et al., PMID 22119103 which demonstrated a sensitivity of 86.9% and 84.1% respectively for CRP and ESR, for a positive TAB. There is discordance ...

A related question: if you have positive inflammatory markers and a high clinical suspicion for GCA, do you always biopsy? The reason I ask is because I recently had a case that I ended up treating despite negative bilateral biopsy, which made me call into question the utility of biopsy in this clin...

I get both as some patients only have an elevated CRP. They are sensitive but not specific. Anemia of acute inflammation and reactive thrombocytosis can also be seen but, again, are not specific to GCA. Also, be mindful of age adjusted ESR.