With my current pregnancy and my last one I was given two different HCG levels as being considered positive as they used different labs. One was over 5 then current one was over 20 or 25 (sorry can't remember).

When I had my early m/c in March I'd tested positive 3 times with 3 different brands over a week but then started spotting, then extremely light bleeding. When the BT came back it had a level of 3 which the doctor told me meant I might be pregnant but it was too early to tell, or I'd just had a very early miscarriage (which is what it turned out to be) as two days later it was back to zero.

Leave it a few more days and test again with a HPT, if it comes up positive see another GP and as for another BT - I think some labs called it a HCG quantative test.