A bag specimen is useful *only* if it's negative.  A positive specimen from
a bag specimen is meaningless, in my opinion (and the studies back me up).
A catheter specimen is better but not foolproof either, since bacteria can
be pushed up into the bladder, and if a small specimen obtained, may have
significant growth.

A suprapubic puncture is about the only *virtually* foolproof way of getting
a specimen which means something (but even that can be messed up, because
people are infinitely resourceful at finding ways of messing things up).

Jack Newman, MD, FRCPC