Is The Cipla Pregnancy Test Reliable?

Dear doctor, my girlfriend and i had intercourse some time last month before her period which was expected on the 16 of july,but she still has not got her period till today which makes it 18 days with no sign of period.. We are confused we had unprotected sex but i am sure i did not allow any semen to enter her.. So today i purchased a cipla pregnancy test. She will do the test in the morning. Can we rely on the test? If it shows negative?