Many people think that DUI testing goes something like this:
Step 1: Put blood, urine or other substance in system;
Step 2: Press start button;
Step 3: Computer prints out exactly what substances were found in blood, urine or other substance.
Not only is that not the case, but there is room for human error and dishonesty. Here is a video that shows how a test can be manipulated to give a desired result.