If nothing shows up on a test it doesn't get a red flag. I have used quick fix quite a few times and was also on prescription, but no red flag.

One thing that I have done is, instead of heating the entire bottle of urine and hoping that its the right temp, heat up the temp strip on the UA cup with the hand warmer. Just set the cup on the tray, pour urine into cup, place heating pad on temp strip, check frequently and remove heat pad when its at the right temp... It seemed to work fine for me...