1-2 ounces
A sample of less than one cup (eight ounces) is required for the test.
For a DOT test 45ml. For a drug free workplace test 30ml. If you are doing an immediate test enough so it can be read on the temperature gauge and then the test is to be put inside cup, then urine...
Caffeine will not cause a false positive in drug tests.
You can get home screening test kits for as little as $2-3. A full laboratory confirmation test will obviously be more expensive.