How accurate is the Downs syndrome blood test in pregnancy?

Ive been told to decide if i want the blood test for downs at 16 weeks. Ive heard its not very accurate and gives false high risk results quite often (my friend had one, she had another blood test to be sure and it turns out she wasn't high risk).
I still don't know if to have it or not as im young (20) and i don't want the unnecessary worry of a false high risk result, we worry enough in pregnancy!
Does anybody know how accurate it is??
I have googled it but it seems to be different numbers on different sites.
Thanks!

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