What is the key diagnostic method for identifying C. difficile infection?

The key diagnostic method for identifying Clostridioides difficile infection is the cytotoxin assay of stool. This test detects the presence of toxins produced by C. difficile that are responsible for the clinical manifestations of the infection, such as diarrhea and colitis. The toxins primarily involved are toxin A and toxin B, with toxin B being particularly pathogenic.

While stool cultures can also be used, they may not be the most effective or timely method for diagnosis since C. difficile can be part of the normal intestinal flora in some individuals, leading to potential false positives. Endoscopy, although useful in examining the colon for pseudomembranous colitis associated with severe C. difficile infections, is not used primarily as a diagnostic tool due to its invasive nature and the availability of less invasive and effective tests. Blood cultures are not useful for diagnosing this infection since C. difficile typically presents with gastrointestinal symptoms without systemic spread to the bloodstream.

Thus, the cytotoxin assay is favored due to its specificity for the pathogenic effects of the organism, making it the key diagnostic method for C. difficile infection.