'Sweater with dress slacks/chinos' is very much business casual. Even here in the East, where the dress code is taken up a notch. As you're seeing patients, though, your practice may have a different definition of what is acceptable. Ask 'em. Early in a job, most people dress a step higher than the stated dress code, 'cause it's the safe route. You'll find out what's kosher really quick.

As for the suits? You said 'interview' in the title. People hear that word, they usually go to professional dress, period. If you've already signed an offer, and this is just HR-related stuff, there's generally no need for a suit.