Abstract: An apparatus and method to deter circumvention of sobriety breath tests. As a precondition to passing the test, the invention requires an operator/test subject to apply a suction at the sample delivery port within a predetermined time of delivering a breath sample thereinto. The delay, if any, between the required sucking and the delivery of the breath sample is selected to be short thereby deterring a substitution of gas sources. In this way, the invention effectively deters circumvention of sobriety breath tests due to attempts to supply bogus gas samples from a positive pressure gas source incapable of applying a reverse pressure gradient.