Microbiology Diagnosis of CI. Difficile infection is made by which of the following methods? A Toxin gene detection by polymerase chain reaction (PCR) B Culture C Enzyme - linked immunosorbent assay (ELISA) D Nagler's reaction 💡 Explanation Diagnosis of CI. Difficile infection is made by which of the following methods? ✓ Correct Answer: A. Toxin gene detection by polymerase chain reaction (PCR)