There are 2 types of Down syndrome tests.
1. Screening test which will not give you a yes or no but provide you with the probability. Higher or lower.
2. Invasive diagnostic test which has an accuracy of higher than 90%. However, it also has a 0.3% chance of leading to a miscarriage.
If you have only done the screening test and your odd is high, you may proceed with a diagnostic testing.
I have 2 friends who during the screening tested to have high probability of having a down syndrome baby but further diagnosis shows that babies are healthy with no Down syndrome.