How accurate is a pregnancy blood test 7 to 8 days after conception?

My friend had a blood pregnancy test done 7 to 8 days after possible conception and the result came back negative. She is still having pregnancy symptoms. She asked the lady who gave her the results and the lady said that the test should not be done until atleast 10 days after conception to be most accurate. How accurate is a pregnancy blood test 7 to 8 days after conception? Thanks in advance!