I use similar tubes on a daily basis at my work.
You must drink water prior to taking the test, any alcohol present in the mouth will give a false positive
You cannot smoke within 5min of taking this test, again a possible false positive.
The tube only gives you a positive or negative result based on the % listed on the side of the tube. These tubes are %.08 which is the legal driving limit in most states.
If you are interrested in actually purchasing these in quantity so you can be a safe driver, I would recommend Redwood Bio Tech.