How long before HPT comes back with BFN

Ok I had I miscarriage that started on May 31.  They checked my levels which was 2500 to 2600. Went into the doctor on Monday the first retested and was down to 316. They told me to recheck with home test in a week. I on Monday I took another still positive so they retested my levels they was 40. Dr told me they should be negative in a week. We are sitting at a week and still positive. Not only that but the test are getting darker lines at least to me