The drs usually get the tests that are more sensitive. most HPT's detect 20 or 25 MIU. the drs can get some that detect 10MIU and a blood test is positive if your levels are anything above 5. It is different for every person and every pregnancy. The HCG levels are only and estimate and they range very widely during the first weeks especially. here is a list of what the levels are.
# 3 weeks LMP: 5 - 50 mIU/ml
# 4 weeks LMP: 5 - 426 mIU/ml
# 5 weeks LMP: 18 - 7,340 mIU/ml
# 6 weeks LMP: 1,080 - 56,500 mIU/ml
# 7 - 8 weeks LMP: 7, 650 - 229,000 mIU/ml
# 9 - 12 weeks LMP: 25,700 - 288,000 mIU/ml
# 13 - 16 weeks LMP: 13,300 - 254,000 mIU/ml
# 17 - 24 weeks LMP: 4,060 - 165,400 mIU/ml
# 25 - 40 weeks LMP: 3,640 - 117,000 mIU/ml
# Non-pregnant females: <5.0 mIU/ml
So up until you reach 6 weeks since your LMP, your levels can be low. Once they hit six weeks they should be up over 1,000 and really show up on a test.You must also remember that these levels are based on O day as two weeks after LMP so if you O later the levels will be different too.