@fify-- That's not true. The serum pregnancy test will not be able to detect HCG levels until after seven days of pregnancy.
So you may definitely get a false negative if you get the test too soon. You have to wait some more and get the test again.
Normally the HCG levels in a woman who is not pregnant is zero. If the serum test shows HCG levels above 5 units, she is pregnant. But labs don't always take the same reference levels for pregnancy. So one lab might require HCG levels above 5 to consider the woman pregnant, while others will look for 10 units or more.
So it's a good idea to talk to the lab about what their reference values are for the HCG pregnancy test and when the best time is to get the test.