I've had call backs too and it was always nothing. They always err on the side of caution, as others have said, want to have a good clear baseline. Don't stress, feel strong that you went it to do this, that you are being proactive about your healthcare. Don't stress about what might be, that is the whole point of a screening. Its not positive until its positive, right now they are just trying to get you the best baseline data possible. By all means call your doc if you are concerned, and ask about getting on a cancellation list. But, really, try not to worry until you know there is really something to worry about.

