When can i test to see if im pregnant

Patient: I ovulate 8 days before my period and I am now 5 days late on my period I bought a fertility monitor and I ovulated on the 16-18 of sept I was wondering when I will get a deffinate positive test I have had for 5 days now a very very very faint line but its not getting any darker when can I test

Symptoms: Missed period

Doctor: