Clinical Microbiology Newsletter Vol. 11, No. 4 February 15, 1989 Diagnosis of Antibiotic-Associated Disease Caused by Clostridium difficile Hannah Wexler, Ph.D. Microbial Diseases Research Laboratory VA Wadsworth Medical Center Los Angeles, California 90073 Clostridium difficile is considered to be the leading cause of antibiotic-asso- ciated pseudomembranous colitis (AAPMC) and may also cause antibi- otic-associated diarrhea (AAD). Rec- ommended diagnostic laboratory routines usually include culture for the organism and/or testing for cytotoxin. A major disadvantage of the tissue- culture technique for diagnosing C. d/f- ficile-associated disease is its lack of availability in many hospitals. For hospitals that have only sporadic cases of C. difficile-associated disease, it is costly and time-consuming to maintain viable cell lines, although the recent commercial availability of convenient cytotoxicity testing materials has im- proved this situation. Another problem is that there is no commonly used or standardized methodology for cytotoxin testing. Various alternatives to the cy- totoxin testing technique have been proposed and evaluated during the past few years (1-28). Specificity and Sensitivity When reviewing these reports, the laboratorian should consider the preva- lence of a disease in the sample popu- lation tested, as it will profoundly affect the positive and negative predic- tive values of the method (29). Whether an investigator includes all specimens received during a specified time period (i.e., a relatively low- prevalence population) or only spec- imens that were from patients who had been positively identified as having AAPMC (i.e., a high-prevalence popu- lation) will influence the predictive values reported. For example, a diag- nostic test with a specificity and sensi- tivity, each, of 99% would have a positive predictive value (PPV) of 16% in a population where the prevalence is 1%, a PPV of 50% where the preva- lence is 5%, and a PPV of 95% where the prevalence is 50% (29). Results reported by several investigators using various testing procedures are summa- rized in Table 1. For consistency, all values of sensitivity, specificity, pre- dictive value, and accuracy were calcu- lated using the formulas described by Galen and Gambino (29). Except when otherwise noted, cytotoxin testing served as the "gold standard" because it was the test most commonly used by others in evaluations and comparisons. The use of cytotoxin testing as the standard may be misleading because as noted previously, some investigators have found documented cases of C. difficile-associated disease that had negative cytotoxin results and positive results on some other assay (which in these calculations would be considered "false positives") (2, 20, 30). For ex- ample, Brown et al (5), found latex to have a lower sensitivity compared to cytotoxin, but these authors found that 5 of 6 latex negative, cytotoxin positive patients had an unlikely diagnosis of C. difficile-associated disease. Indeed, they found that the latex test correlated better with the presence of disease than did the cytotoxin assay. Similar results were found in some cases with both ELISA (2, 30) and CIE (21) assays. In spite of the cautions necessary in inter- pretation, this was the most reasonable way to compare the results found by various investigators. Non-Tissue-Culture Methods Several laboratories have developed ELISA assays for Toxins A and/or B In This Issue Diagnosis of Antibiotic-Associated Disease Caused by Clostridium difficile ...................... 25 A critique of the laboratory methods used for the diagnosis of pseudomembranous colitis and an- tibiotic-associated diarrhea Bacteremia due to Leuconostoc species ....................... 29 A case report Erysipelothrix rhusiopathiae septicemia .................... 30 A case report Letter to the Editors ........... 32 CMNEEJ 11(4)25-32,1989 Elsevier 0196-4399/89/$0.00 + 02.20