What do you do when the quad screen comes out postive but the sonogram shows nothing?

I had the quad screen done and I tested 1 out of 166 for my baby to have downs syndrom. I went and got the sonogram done and that showed everything looks perfect. (size,weight,bones, heart chambers etc.) The doctor said this test was 80% accurate. The dr. said its up to me if I wanted an amnio. Does this mean my baby has it ? The % of miscarrage is high. I am confused. Please someone give me some advice.