By and large, reactions to MSG are sensitivity reactions to a toxin, not allergic responses. That’s probably why the Glutes suggest seeing an allergist if you think you’re having a problem with MSG because allergists have no tests for sensitivities. The only test for MSG (or any other sensitivity) is a challenge — ingesting the substance and waiting to see what happens. So, most allergists will tell you that you’re not allergic to MSG, which would be true.
We’ve had reports from a handful of people who said they tested positive for an MSG-allergy but it’s not common.