Baby soap residue causing false positives in marijuana tests A shocking number of babies are testing positive for marijuana use. Baby wash, baby shampoo, bedtime bath soap -- all seemingly innocent products used in nurseries every day. But scientists found them to be the culprit behind a spike in false positive marijuana results in baby's urine, according to the journal clinical biochemistry. The study found it had to do with how the baby's urine is captured. Those tests usually get more than just urine; they were capturing the soap residue on the baby's skin. Scientists say none of these soaps actually contain any marijuana, just that something in the soap is triggering the false positive. Scientists ultimately decided this was an important study to let hospitals know that when a test comes back positive for marijuana they should do a blood test, too. Click for more from My Fox New York.