Passive breathalysers only deliver what is called “qualitative” result. They provide a written qualification as to whether alcohol is present of not. They do not measure accuracy. For an accurate result, you need a “quantitative” breathalyser (operating in Standard mode using a mouthpiece). A breathalyser operating in “Standard mode” with a mouthpiece analyses the subject’s deep lung air sample, and quantifies the result as a numeric blood alcohol concentration (BAC) reading.