Will a hair strand test show all drug abuse?

Detection of abuse. Diagnosis of factitious diarrhea can be made by colonic barium x-ray, stool & urine testing for phenolphthalein, emetine, bisacodyl, anthraquinolones, & fecal fluid can be analyzed for osmolality & electrolytes. See also my earlier answers on healthtap regarding colon lining changes seen on colonoscopy, as well as the physical manifestions that may present in suspected laxative abusers.