Is a blood hcg test a definite negative at 6 dpo?

my doctor said I was definitely not pregnant, but she tested my hcg with a beta blood test at 6 dpo. I asked her like 3 times if this was too early to tell, but she said the test should pick it up by now. What she says doesn't match what I am seeing online. It seems like 8-9 dpo is the earliest you can test positive with the blood test. Does anyone know if my doctor is wrong?