Test still positive

So I had a miscarriage. I've bled for 4 1/2 days. What is strange to me is that my pregnancy tests are still positive. They are not as dark as they were of course but they still have lines. I know it takes some time for hcg to leave your system. But this is why I'm confused... last Monday the 12 my dr informed me that my quant had dropped from 50 to 38 and expect to miscarry. So my thing is if my number was 38 last Monday how in the world am I still getting positive pregnancy tests?