Jo - HCG levels in pregnancy double every four days - therefore, a test in a week or so should increase significantly - no need to wait longer.
Yes, blood tests are very good, but no test is 100% accurate, and there is always the possibility for human error, mixed up labels and the like - rare, but it does happen.
Treating this at an early stage is easier than at a later stage, so I wouldn't wait several weeks - but I think there's no harm in waiting one week and repeating the HCG just to be sure. You are still so early that even if it was a tubal pregnancy (which seems unlikely) one week would probably not make any difference.
