Depends on methodology used and the methodology used will vary
depending on why it was ordered.

Some urine tox screens are exactly that being only a screen with
positive or negative much like a urine pregnancy test kit. These are
generally used in hospital settings and no confirmations are done and
are reported out as presumptive positive. The next level are the
semiquantitative assay in which a number is generated and depending on
the number, concentration units, then it is reported out as negative or
presumptive positive after it supersides a cut-off level. The lower
concentrations where false positives are common with the first
screening kit are thus avoided. The next level up involves drug abuse
clinic and medical legal cases such as perinatal testing where
confirmatory testing is mandatory on any and all positive results.

Ampicillin would react in some immunoassays such as EMIT but the higher
level GCMS is not immunological and is very chemically specific.
Ampicillin does not cross-react in our test system. That's where
competition comes in and it makes systems better by such competition.
Gas Chromatography/Mass Spectroscopy doesn't really have any false
positives and also uses cut-off concentrations for positive. Not only
does it give a positive but the actual metabolite or metabolites will
be identified. Within the cocaine group are three metabolites:
Benzoylecgoine, Cocaethylene, and Ecgoine. Most of the lower testing
involves a group catagory without distinction within the group.

The answer to the question depends on the methodology used and if
confirmatory testing was done with GC/MS.