If you don't wash your hands before testing, the cost of the test is wasted because you can't trust the result and if you can't trust the result, why test at all?
Of all your body parts which pick up foreign bodies, your fingers are tops. Food residues on the table top will leave traces. If you put your hand on the table top, you'll get traces on your hands. Same with door handles, fridge handles, jars, oven door etc etc. Many of these food residues will distort readings but, unless you are a really very bad cook, they won't poison you.