Statistics stops you. Blood testing is all about statistics. You measure a certain number of things in the blood test, which gives you an idea of how much of th

Statistics stops you. Blood testing is all about statistics. You measure a certain number of things in the blood test, which gives you an idea of how much of the thing you're looking for is in the blood. Some of these things (bacteria, virus, etc) only needs to be in the blood in very low concentrations to be harmful, and to get a good reading you need a larger sample.