It's an interesting time in America. The majority of states (although not Iowa) have legalized marijuana for medical use. Most states, including Iowa, have legalized low-THC CBD, a cannabis derivative, for medical purposes. And, last year, the federal government legalized hemp, a relative of marijuana that contains almost no THC, the psychoactive compound in marijuana. However, states that have anti-hemp laws have generally not lifted them.
If a drug field test comes back positive, that is generally considered enough to justify an arrest. Field tests are inadmissible in most courts, so further testing must be performed by a lab. Unfortunately, lab results can take months to come back. If a field test is inaccurate, an innocent person could be stuck in jail for a long time before the inaccuracy is discovered.