Performance characteristics of a quantitative PCR assay on repository stool specimens and smeared filter-paper cards

Stool repositories are a valuable resource for retrospective analyses including quantitative PCR assays to distinguish between asymptomatic shedding and clinical disease. The suitability of archival specimens for this purpose is unclear and requires assessment. We conducted a pilot study to evaluate pathogen detection by TaqMan Array Card (TAC) in travelers' diarrhea (TD) stool specimens stored for 1-13 years, as well as the impact of transporting specimens on Whatman FTA Elute cards (FTA Cards) on detection.

The positive percent agreement (PPA) for TAC on stool vs. microbiologic testing was lower than our a priori PPA estimate of 80% for most pathogens: Shigella spp. (100% [95%CI 69-100%]), enterotoxigenic E coli (ETEC) (63% [95%CI 49-75%]), Campylobacter spp. (66% [95%CI 43-85%]) and Norovirus (37% [95%CI 16-61%]). Use of the FTA card resulted in a further reduction of PPA. Our findings suggest that archival specimens may lead to insensitive detection on quantitative PCR assays due to degradation of nucleic acid with prolonged storage, although our limited sample size precluded us from evaluating the impact of storage duration on nucleic acid yield. Additional studies are needed to understand the impact of storage duration on quantitative PCR data.