A case of diarrhoea

This is the sigmoid mucosa of an elderly inpatient who has developed diarrhoea.  
WHAT SINGLE TEST WOULD YOU RECOMMEND NEXT?

■ FBC

Of course it's not sensitive nor specific and does not exclude diagnosis if normal. Nevertheless, a very high WBC level is often found in fulminant pseudomembranous colitis.

■ Obtain a set of stool cultures

Probably not a priority and they should have done it already!

■ ELISA for C.diff toxin in the stool

Yes! Results available within a few hours with sensitivity of 65%-85% and specificity 95%-100%

■ Stool PCR for C.diff

Seems like a sensible investigation in patients with unexplained, new-onset diarrhoea but perhaps not the single most important test...

■ Abdominal X-ray

Important IF there is significant abdominal distension

explanation

Of course this is a case of Clostridium difficule associated colitis (pseudomembranous colitis). Patients usually present with diarrhoea, abdominal pain, and leukocytosis, and a history of recent antibiotic use. Other common symptoms include fever, abdominal tenderness, and distension.

In a symptomatic patient with typical colonic pseudomembranes such as in this case, arguably it would be sensible to recommend that treatment is immediately started.  

Stool testing should be considered in any patient with unexplained, new-onset diarrhoea (defined as 3 or more unformed stools in 24 hours in a patient not taking laxatives).  Your local institution will probably have protocols for how patients should be investigated and all of the above test may well be part of the algorithm.  When I was on the wards, toxin tests were favored over culture for diagnosis of C.diff because it was the toxins which mediate disease. detection of toxins was faster and correlated better with symptoms.  However, there was a move towards 'molecular tests' (PCR for the bacterium) from 2009 because of concern that patients with C.diff could be missed by toxin tests. Of course, this raised the question; do toxin-negative patients with a positive C difficile PCR test result require treatment? Several studies have now indicated that about half of the patients with positive PCR test for C. diff do not experience adverse events without treatment and do not need treatment. For this reason, PCR testing for C.diff should not be used as the stand-alone diagnostic test. Instead it's patients with clinical disease (diarrhoea) AND a positive toxin assay who should be treated!  There are lots of references for this statement and here is an open access article in JAMA: 

Treatment is to discontinue the responsible antibiotic and start therapy with oral vancomycin or fidaxomicin.  Up to 50% of patients have a relapse after discontinuation of antibiotics, but most respond to a second course of therapy. In those who relapse again, faecal microbiota transplantation is very effective.