A survey last year found CIDT methods were used most often for Campylobacter (10%) and Shiga Toxin producing E.coli (STEC) (19%).
Lab practices to detect Campylobacter infections have changed at Foodborne Diseases Active Surveillance Network (FoodNet) sites with the proportion of clinical labs using a CIDT increased from fewer than 3% in 2004 to 15% in 2014.
This could change as more labs adopt commercial DNA-based syndrome panels, said the Morbidity and Mortality Weekly Report.
CIDTs are also becoming more widely used for the diagnosis of STEC infections.
Comparing clinical lab practices in 2007 and 2014, the use of antigen-based and DNA-based methods to detect Shiga toxin or the genes encoding the toxins increased from 11% to 60%.
A positive CIDT report was associated with almost all STEC reports, and most of these (90.4%) were confirmed by culture.
Pros and cons of CIDTs
CIDTs detect the presence of a specific antigen or genetic sequence of a germ and do not require isolation and identification of living organisms so are quicker to do and give results sooner than traditional culturing methods.
However, they do not give the information needed to characterize organisms that cause infections.
Because they do not provide isolates (the organism that caused the illness), it is not possible to run tests that determine an organism’s DNA fingerprint, strain or subtype or resistance pattern.
The delay to the outbreak investigation means contaminated products may remain on shelves and more people may become sick.
Currently, isolates from culture are forwarded from clinical labs to public health labs for additional testing, including antimicrobial susceptibility, serotyping, pulsed-field gel electrophoresis (PFGE) and whole genome sequencing (WGS).
The US Centers for Disease Control and Prevention (CDC) is encouraging labs to perform reflex culturing (i.e., culturing specimens with positive CIDT results for intestinal bacteria).
It is working with companies that make CIDTs so that specimens are collected to keep the organism alive for culturing; developing new CIDTs to supply information about subtype, antibiotic resistance, and virulence; and adapting surveillance systems to meet the public health challenges.
FoodNet identified 38,666 culture-confirmed cases and positive CIDT reports of Campylobacter, Salmonella, Shigella, STEC, Vibrio, and Yersinia during 2012–13.
Among the 5,614 positive CIDT reports, 2,595 (46%) were not confirmed by culture - because the specimen was not cultured or a culture did not yield the pathogen.
In the 2,497 positive CIDT reports of Campylobacter, 539 (22%) were confirmed by culture, 1,099 were culture-negative, and 859 had no culture.
A total of 2,409 positive CIDT reports of STEC, 2,205 (92%) were confirmed by culture, 110 were culture-negative, and 94 had no culture.
For Salmonella, 115 (37%) were confirmed by culture, eight were culture-negative, and 185 had no culture.
The report said as more clinical labs adopt CIDTs, collection and detailed characterization of bacterial isolates to support public health activities will fall more heavily on public health labs.
“The increased reliance on CIDTs will create a burden for public health laboratories and will have a significant impact on clinical practice, outbreak detection, and the ability to monitor disease burden and trends.
“Public health surveillance programs rely on the ability to distinguish among strains and serotypes of pathogens to detect foodborne outbreaks and monitor the effectiveness of specific public health and food safety interventions by regulatory agencies and the food industry.”
Currently, the extent of culture-independent diagnostic practices by clinical laboratories and the future impact on public health surveillance are unknown, it added.