They'd only carry it insofar as they'd be bringing them to the drug recognition expert, the same as they would for a breath test. They get them roadside. They make the determination that a person is impaired by alcohol. They bring them back and put them before an expert. The expert is the evidentiary breath technician.
For the drugs, it's really no different; they observe something that's not right and they take them to an expert to be evaluated.