Utility of Stool PCR for the Diagnosis of COVID-19: Comparison of Two Commercial Platforms

Abstract

The ability to detect SARS-CoV-2 in the upper respiratory tract ceases after 2 to 3 weeks post-symptom-onset in most patients. In contrast, SARS-CoV-2 can be detected in the stool of some patients for greater than 4 weeks, suggesting that stool may hold utility as an additional source for diagnosis. We validated the Cepheid Xpert Xpress SARS-CoV-2 and Hologic Panther Fusion real-time RT-PCR assays for detection of viral RNA in stool specimens and compared performance. We utilized remnant stool specimens (n = 79) from 77 patients with gastrointestinal symptoms. Forty-eight patients had PCR-confirmed COVID-19, and 29 either were nasopharyngeal/oropharyngeal PCR negative or presented for reasons unrelated to COVID-19 and were not tested. Positive percent agreement between the Cepheid and Hologic assays was 93% (95% confidence interval [CI]: 81.1% to 98.2%), and negative percent agreement was 96% (95% CI: 89% to 0.99%). Four discrepant specimens (Cepheid positive only, n = 2; Hologic positive only, n = 2) exhibited average cycle threshold (C_T ) values of >37 for the targets detected. Of the 48 patients with PCR-confirmed COVID-19, 23 were positive by both assays (47.9%). For the negative patient group, 2/29 were positive by both assays (6.9%). The two stool PCR-positive, nasopharyngeal/oropharyngeal PCR-negative patients were SARS-CoV-2 IgG positive. Our results demonstrate acceptable agreement between two commercially available molecular assays and support the use of stool PCR to confirm diagnosis when SARS-CoV-2 is undetectable in the upper respiratory tract.

Keywords: COVID-19; SARS-CoV-2; diagnostics; stool PCR.

FIG 1

Correlation of C_T values for Cepheid and Hologic Panther Fusion SARS-CoV-2 assays. Hologic ORF1a C_T values are plotted verse Cepheid E and N2 C_T values for stool specimens testing positive by both assays (n = 27).

FIG 2

Detection of SARS-CoV-2 assay amplification targets in positive stool specimens up to 33 days after initial upper respiratory PCR result. Cepheid SARS-CoV-2 gene targets for individual patient stool specimens are plotted versus the number of days from the first positive nasopharyngeal/oropharyngeal PCR result (n = 25) or first initial negative nasopharyngeal/oropharyngeal PCR for patients with only negative upper respiratory PCR findings (n = 2).