If the subject has anything with alcohol in their mouth just prior to testing, the alcohol in their mouth will contaminate the test, giving an artificially high reading. However, 15 minutes is sufficient time for all mouth alcohol to dissipate.
Alcohol test protocols are designed to rule out the effects of mouth alcohol. One protocol requires that the subject have nothing in their mouth for 10 to 15 minutes prior to testing. Alternatively, the 15 minute waiting period is observed after an initial positive test, and a second breath test is administered to rule out that the first test was positive due to mouth alcohol.