Sure, if pre-employment, one had the privilege of having a primary physician.

As someone with a pretty good job in software, I don't have a primary physician, because what's "in network" changes every time I switch companies.

Indeed, this is one of the reasons I chose to pass on a full time offer recently and negotiated to work under contract instead. I get to keep the same healthcare provider.

A diagnosis of IBS requires a physician, right? It’s surprising that a letter wasn’t obtained on diagnosis in the first place.

Any time my doctor says something affects my work, I ask her for a letter in my medical record and keep a copy for myself.

This seems pretty straightforward and odd that a letter didn’t exist in the first place.

Oh they had a primary physician?

Someone diagnosed them with IBS

Does not require a physician to diagnose. Diagnosis is based on casual observation and the cause as yet is unknown.