I had drug test for a job an I pass the test on m Monday ,then on a Tuesday night at 9:30 I had urine test on that Tuesday an did not pass it.. it came back with a positive of 51 an da cut off is 50,so could you explaine to me if this is possible?

Based on the cutoff I'm guessing the fail was for THC?

When a substance is near cutoff detection is quite literally a flip of the coin. Screening tests have an allowable variance of +/- 20%.

Were both tests urine screens?