Diagnosis of Clostridioides difficile - Induced Diarrhea Stool assay for glutamate dehydrogenase (GDH) antigen and C difficile toxin and polymerase chain reaction (PCR) test for the toxin gene Sometimes sigmoidoscopy C difficile - induced diarrhea should be suspected in any patient who develops diarrhea within 2 months of antibiotic use or 72 hours of hospital admission.