What is the accuracy in a level 2 sonogram when detecting down syndrome?

Recently I had blood work come back showing a postitive result for Down Syndrome. I have heard numerous times that there can be false positives although the statistical percentage is 5%. I had a level 2 sonogram done to try to see if they could detect any problems and they said the results were great- showing no indication of any abnormalities. They still offered me a amnio. Does anyone know that statistic of the sonogram and detection? I really dont want to do the amnio but want to also be prepared. Thanks in advance. Any others experience appreciated.