When can take beta hcg. blood test?

Mam i want to kwn that when can we take beta hcg blood test after .sex ..???. i want kwn when beta hcg can give most accurate result after sex...because i had sex on 2 oct my g.f got her periods on 9 (that was her due date only ) still i was in doubt so i took beta hcg test on 23 oct... it was negative... now (i read on practo  a girl took test after her missed periods and result was negative... and again after 5 days she again took then she got positive... ) so now i am worried for my case because i took beta hcg test for one time only... can it be wrong... still is there any chance of pregnancy in my case....the case i explained above..  of 2 time test which forced me to ask this question its screen short i have attached...plzz first have a look on that too then reply... sir...i m very much horriefied...