I was thrown off because Pathoma/Dr. Sattar and pixorize say it is critical to start steroids ASAP to avoid permanent damage to the ophthalmic artery and blindness in patients suspected of having Giant Cell (Temporal) Arteritis.
This means not waiting for diagnostic confirmation.
However a 99% sensitivity does not equate to a 99% certainty of GCA/TA. So additional testing is the best answer choice even though her elevated ESR gives us strong suspicion of GCA/TA.
Sensitivity of 99% means that almost all TA will have high ESR. (Sn = tp/(tp+fn) -> low False negative). HOWEVER, a specificity of just 60% means high ESR can indicate MANY OTHER disease processes. In a test with low specificity, more tests should be done to confirm diagnosis.