What's the earliest you can get a positive result?

Rebecca
Hi all just wondering how soon a pregnancy test would show a positive result. I had a scan last Saturday and was told I was only 6 weeks. I got a positive pregnancy test on the 18th of March but that would of meant I would of conceived a week before. Is it possible for a test to show that quick? I'm being rescanned next Tuesday but I'm convinced I'm further than 6 weeks. My lmp was 13th Feb. 
Thanks xx