Clostridium difficile
Clostridium difficile is a Gram-positive, spore-forming, anaerobic bacillus that is associated with pseudomembranous colitis. The disease formerly known as C. difficile-associated disease (CDAD) is now called C. difficile infection (CDI). CDI ranges in severity from mild diarrhea to fulminant colitis. Risk factors for CDI include antibiotic use within three months prior to symptom onset and exposure to a health-care setting. Colonization with C. difficile is common in hospitalized patients (20-40%), while only 3% of healthy adults are colonized. CDC data indicates that the incidence and severity of CDI has been increasing since the year 2000. The recently described hypervirulent C. difficile strain has now been reported in 38 states, & presents a new challenge for infection control. Community-acquired infections, many without identifiable risk factors, are increasing as well.
Alterations of normal gut flora, resulting in overgrowth of C. difficile, are believed to initiate CDI. Production of exotoxins A & B by the organism subsequently results in colonic mucosal damage, and detection of these toxins is the basis for diagnostic laboratory tests. The majority of U.S. laboratories utilize enzyme immunoassay (EIA) for this purpose, due to rapid turn-around time & ease of use. However, the sensitivity & specificity of EIA is widely variable, as low as 50% and 70%, respectively. More accurate toxin detection is achieved by cell culture cytoxicity assays, however this technique requires 48 to 72 hours to obtain results. Conversely, there are recent favorable reports in the literature regarding the utility of real-time PCR for CDAD diagnosis (Ann Intern Med. 2009;151;176-179, JCM 2009; 47; 3211-3217).
CDC data indicates that the incidence and severity of CDAD has increased since the year 2000. Two recent publications (NEJM 2005; 353:2433-2449) describe an apparently new, more virulent strain of C. difficile that has been responsible for hospital outbreaks in the U.S. and Quebec, Canada. This epidemic strain differs from common strains in that it produces 16 times more toxin A and 23 times more toxin B, which may result from a deletion in the negative regulator gene, tdC. The epidemic strain was also resistant to fluoroquinolones, and prior use of fluoroquinolones was identified in 52% of cases. More severe CDAD is associated with the epidemic strain, including more frequent toxic megacolon, leukemoid reaction, shock, and death, particularly in the elderly.
Equally alarming, community-acquired cases of severe CDAD in individuals with minimal risk factors have been reported recently (MMWR 2005;54:1201-1205). Analysis of the organism responsible for two of these infections showed they were not caused by the epidemic strain. Another report suggests that use of proton pump inhibitors increases the risk of community-acquired CDAD by 2-3 times (JAMA 2005; 294:2989-2995).
Hand hygiene is of particular importance in reducing the incidence of CDAD. Of note is that hand-washing with soap and water is necessary for C. difficile eradication, as its spores are resistant to alcohol-gel based preparations. In light of the apparent changing epidemiology of CDAD, the CDC has stressed the importance of judicious antibiotic use, and the need for a high index of suspicion for community-acquired CDAD in patient with severe diarrhea.
Common nonspecific laboratory abnormalities in patients with Clostridium difficile-associated disease (CDAD) include leukocytosis and hypoalbuminemia. Fecal leukocytes are detected in 50-60% of cases. Gram stains of fecal specimens are of no value, since C. difficile is only a small part of the fecal flora, even among patients with severe colitis. Likewise, anaerobic stool cultures are of little use in the diagnosis, due to the inability to distinguish between toxigenic and nontoxigenic strains.
The most commonly used diagnostic tests for CDAD are enzyme immunoassays (EIA) that detects both cytotoxin A & B. A positive test indicates the presence of Clostridium difficile. Normal stools are negative for toxins.
The following criteria should be followed before requesting this test;
- the patient must have diarrhea,
- the history should suggest antibiotic-associated disease
- babies less than 2 years old should not be tested routinely, since they have such high asymptomatic carrier rates (15-70%).
False negative EIA results may occur because, unlike blood, stool is not a homogenous sample. Toxin can occasionally be detected in one sample and not another.
A PCR test that targets the tcdA & tcdB genes that encode for toxin A & B has been demonstrated to have better sensitivity and specificity than EIA. PCR testing has replaced C. difficile toxin A/B EIA in many hospital laboratories.
Due to the complexity of PCR tests for C. difficile toxin testing, guidelines for frequency of testing have recently been published (Ann Intern Med. 2009;151:176-179). These guidelines were designed to limit the false diagnosis of C. difficile infection in patients that are colonized with the organism. The guidelines suggested that only patients with 3 or more loose stools per day for at least 1 to 2 days be tested. Testing criteria include:
- Testing only one stool specimen per patient per day
- In the event of a positive test result, not testing any further specimens for 10 days
- Only testing loose or soft stool specimens
In addition, the guidelines stated that “test of cure” is imprudent because PCR remains positive for several days to weeks following treatment.
Specimens should be submitted fresh (refrigerated unless transported to the laboratory immediately) or in Cary-Blair transport media.
