I would stick to the IRT on Broadway. Do remember that two the stations for Columbia are local stops. So you have to be on a local train. Express stops where you can switch to a local train are 59th, 72nd and 96th.
Coulumbia's medical school is at the 168th street stop at Presbyterian Hospital on the IRT line too.
The B and C stop on 110th, 116th and 125th and Central Park South/Fredrick Douglas Blvd.
The A express train stops at 125th and Fredrick Douglas Blvd.