This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
It`s Harper Tracy. Her first name is Harper, and her last
name is Tracy.
Confusion over this issue arose when Harper introduced
herself  to Mark as "Harper Tracy", and Mark, misunderstanding,
introduced himself by saying "Greene, Mark."
There`s also a certain amount of confusion due to the fact
that Carter is called by his last name -- so why isn`t Harper? The
fact is that some characters are routinely called by their last
names, and some are called by their first names. Carter has stated
before that his friends call him "Carter" anyhow. (This hasn`t
stopped some characters, like Harper and Chloe, from addressing him