Can having sex with a person that isnt clean cause another person to fail a UA. For instance being the female and having sex the night before with your partner in bed and then getting up the next am and taking a UA. Is it possible for the semen to leak down and mix in with the urine causing the sample to become tainted? After sex if you arent up and walking around sometimes it just kind if sits up there until you are up and about. So its odd that it would come down and out and mix with urine the next am. All i know is I was clean and passed my home tests, the clinic test but failed the lab test. I am the one who asked for the tests. I would not do that if i weren't ! I just dont understand. How could I pass my test and the clinics test but fail the lab test? The only thing i can think of is trace amounts from my partner the night before that was so light it wasnt detectable on OTC tests.