OOPs...to answer your question realsitically it is best to begin testing from 8dpo...there are very few cases where a faint +ve shows up at 7dpo...it takes the egg to travel down the fellopian tube 6-7 days before it swims around in the uterous that again could take from a couple of hours to 2 days in some...until it begins to completely embedd itself into the uteral lining....it is also documeted that since that takes longer in some pregnancies than others the hcg hromone already released into the blood at the time of the first embedment, takes another 2 days to enter your urine...as to be detected in the hpt's sesitive hpt's which test up to 10mIU of hcg are available at online pharmacies...the store brands start checking for hcg at 20mIU minimum and above. i think in cases where it is already detected at store brands must be becasue embeddment was quicker than usual or many cases the higher the amount of HCG present so early on in some pregnancies can idnicate multiple pregnancy.
however it is important to note that storng +ve of hpt's esp even before a period is due also need to be investigated or taken notice of esp if accompanied with pain as this can indicate molar pregnancy or ectopic pregnancies.
hope this has helped....
ps: i think this time i did feel many twinges and a dull ache in my ovaries and lower abdome..at 8dpo hence i tested..they say you cannot feel the egg embedd but i think in my case i did...8dpo it began embedding and by the evening of 9dpo the first hcg units entered my urine (hence the imaginary line)..and by 10dpo i could detect them....ona normal store brand even at 5 days before my period was due...