Comparison of RT-qPCR and Digital PCR Methods for Wastewater-Based Testing of SARS-CoV-2

Wastewater-based epidemiology is an important tool for monitoring SARS-CoV-2 and other molecular targets in populations, using wastewater as a pooled sample. We compared the sensitivity, susceptibility to inhibition, and quantification of reverse transcription quantitative PCR (RT-qPCR), microfluidic well digital RT-PCR (RT-dPCR), and droplet digital RT-PCR (RT-ddPCR) measurements of SARS-CoV-2 (N1 gene target) and Pepper Mild Mottle Virus (PMMoV) RNA in 40 wastewater RNA extracts. All three methods were highly sensitive, but appeared less accurate at very low concentrations. Lower inhibition was observed for RT-ddPCR than RT-qPCR with both SARS-CoV-2 and PMMoV targets, but inhibition appeared to be mitigated by dilution of template RNA. The concentrations of N1 and PMMoV from all three methods were significantly correlated (Pearsons r=0.97-0.98 for N1 and r=0.89-0.93 for PMMoV), although RT-qPCR reported higher concentrations than digital methods. Taken together, this study provides support for the application of all three methods in wastewater-based epidemiology, with additional guidelines for the use of RT-qPCR. Impact StatementPCR-based assays are the current standard for sensitive, specific, rapid pathogen quantification in environmental samples, including wastewater. The increased availability of multiple digital PCR technologies necessitates side-by-side comparison between platforms, including traditional qPCR, to guide the application of these methods. Specifically, this work can inform interpretation of wastewater SARS-CoV-2 PCR data, as reported to public health agencies for pandemic response.