After having one of my test fail, might I suggest what I did. First, do your test first thing in the morning before you eat, drink or brush your teeth. Secondly, use a clean soft bristle toothbrush and work it lightly over the insides of your cheeks, without swallowing the spit. Then spit into the test tube. This should give you a adequate amount of skin cell buildup inside your mouth and render enough DNA, that your test should be fine . If that does not work, I really don't know what will.