It’s a little more difficult because the blood test is supposed to be performed during a flare up, so it can be negative in an MCAS case if given at the wrong time.

Primary care doctors don’t want to gate the diagnosis so they’ll send the referral over and hope that the specialist will do the work of filtering out the likely and unlikely cases.

The everything-is-MCAS people on the internet use the fact that it can be negative at times in MCAS patients as a wedge to justify ignoring negative test results. In practice it’s not that hard to give someone a standing order for the test and have them get the blood draw when their symptoms flare.