Final answer:
A confirmatory test for a positive alcohol screening result is often done using gas chromatography or gas chromatography-mass spectrometry, which provides precise measurement of ethanol in the bloodstream.
Step-by-step explanation:
When a screening step of an alcohol test, such as one done with a Breathalyzer, indicates a positive result, a confirmatory test is typically done using a more specific and sensitive method. This confirmatory test often involves gas chromatography (GC) or gas chromatography-mass spectrometry (GC-MS), which can accurately measure the presence and concentration of ethanol (alcohol) in blood samples. The use of GC or GC-MS eliminates or substantially reduces the chances of false positive results by providing a detailed analysis of the chemical composition of the sample.