Why do doctors draw blood?

a blood check doesn't always have to do with your particular illness at the time you have it taken. the doctor looks at your charts and may see that you haven't had a blood work up in a while and draws blood to check your colesteral-sugar levels-infections-hiv -anemia and many other things as well as drugs and booze. it's just a way of giving your body a small tune-up and to head off any problems before they become big problems