Preliminary tests performed right away on the sample are not as reliable. If you believe you have a false positive then wait for lab results or see if you may test again.
Several years back I ate a poppy seed muffin from Costco for breakfast, about 45 minutes prior to a urine test. To my surprise I tested positive for opiates. The preliminary test only could prove the presence of certain substances but not the amounts. The lab test later showed that the levels of whatever it was that was detected was far below the threshold for a positive. I would have had to eat hundreds of muffins to get the level over that threshold. However, at the time of the test I was pretty much treated like a heroin addict.