I will try my best to answer your query.
test kit are 99.9% accurate.
A late period and recurring negative pregnancy test results can lead to a great deal of stress
, worry, confusion. Common reasons that might be responsible for late periods are stress, illness, problems with the ovaries etc. USG pelvis will helpful to rule out the problem with ovaries.
If your LMP is with in 48 days pregnancy can be terminated by using drugs.
So i advice you to meet the OBG specialist get your self examined and rule out the cause.
Hope my answer will be helpful for you.