Identification of a pseudo-outbreak of Clostridium difficile infection (CDI) and the effect of repeated testing, sensitivity, and specificity on perceived prevalence of CDI.
OBJECTIVE
To describe a pseudo-outbreak of Clostridium difficile infection (CDI) caused by a faulty toxin assay lot and to determine the effect of sensitivity, specificity, and repeated testing for C. difficile on perceived CDI burden, positive predictive value, and false-positive results.


METHODS
Outbreak investigation and criterion standard.


METHODS
Patients hospitalized at a tertiary care hospital who had at least 1 toxin assay for detection of C. difficile performed during the period from July 1, 2004, through June 30, 2006.


METHODS
The run control chart method and the chi(2) test were used to compare CDI rates and the proportion of positive test results before, during, and after the pseudo-outbreak. The effect of repeated testing was evaluated by using 3 hypothetical models with a sample of 10,000 patients and various assay sensitivity and specificity estimates.


RESULTS
In November of 2005, the CDI rate at the hospital increased from 1.5 to 2.6 cases per 1,000 patient-days (P < .01), and the proportion of positive test results increased from 13.6% to 22.1% (P < .01). An investigation revealed a pseudo-outbreak caused by a faulty toxin assay lot. A decrease of only 1.2% in the specificity of the toxin assay would result in a 32% increase in perceived incidence of CDI at this institution. When calculated by use of the manufacturer's stated specificity and sensitivity and this institution's testing practices, the positive predictive value of the test decreased from 80.6% to 4.1% for patients who received 3 tests.


CONCLUSIONS
Specificity is as important as sensitivity when testing for CDI. False-positive CDI cases can drain hospital resources and adversely affect patients. Repeated testing for C. difficile should be performed with caution.
Department of Medicine, Washington University School of Medicine, Barnes-Jewish Hospital, St Louis, Missouri 63110, USA.
Marina
M
Litvin
Kimberly A
KA
Reske
Jennie
J
Mayfield
Kathleen M
KM
McMullen
Peter
P
Georgantopoulos
Susan
S
Copper
Joan E
JE
Hoppe-Bauer
Victoria J
VJ
Fraser
David K
DK
Warren
Erik R
ER
Dubberke
D000046:Academic Medical Centers; D016360:Clostridioides difficile; D003428:Cross Infection / Q000175:diagnosis / Q000453:epidemiology; D004196:Disease Outbreaks; D004761:Enterocolitis, Pseudomembranous / Q000175:diagnosis / Q000453:epidemiology*; D004768:Enterotoxins / Q000032:analysis; D004797:Enzyme-Linked Immunosorbent Assay / Q000592:standards; D005189:False Positive Reactions; D006801:Humans; D008923:Missouri / Q000453:epidemiology; D015995:Prevalence; D011933:Reagent Kits, Diagnostic; D012680:Sensitivity and Specificity
D004768:Enterotoxins; D011933:Reagent Kits, Diagnostic