0. Phenobarbital is a barbiturate and will cause a positive test result for barbiturates. Phenytoin (Dilantin) is not a barbiturate; however a metabolite, p-hydroxyphenytoin has minimal cross reactivity on the initial screening test. There is no interfe.... Phenobarbital is a barbiturate and will cause a positive test result for barbiturates. Phenytoin (Dilantin) is not a barbiturate; however a metabolite, p-hydroxyphenytoin has minimal cross reactivity on the initial screening test. There is no interfe...