Do blood tests rule out temporal arteritis? or could there be a false negative?

The typical blood tests you order when you are concerned about temporal arteritis are CBC, ESR and CRP. They are not specific to temporal arteritis instead the physician would be looking for blood tests signs of inflammation and anemia. But, if the test were all normal, it would likely not be temporal arteritis.

As previously answered inflammatory markers are usually elevated in temporal arteritis (ESR and/or CRP). There have been rare cases described with normal inflammation markers, estimated at about 5% from total confirmed cases.

The cited blood tests already documented are basic screening measures to assist in decision making, such as whether to do a temporal artery biopsy, or to start therapy with steroids, and these days, antivirals. None of this is perfect or 100 % and sometimes, even the biopsy is normal in presence of segmental arteritis.