This is a summary, written by members of the CITF Secretariat, of:

Wilson M, Qiu Y, Yu J, Lee BE, McCarthy DT, Pang X. Comparison of Auto Sampling and Passive Sampling Methods for SARS-CoV-2 Detection in Wastewater. Pathogens. 2022 Mar 16;11(3):359. doi:

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-funded study, published in Pathogens, found that overall, the passive sampler is a rapid, reliable, and cost-effective device for use in wastewater surveillance. The passive sampler containing absorbent materials or membranes is placed in a targeted sewage catchment to capture viruses for a defined period of time. This method could be used as an alternative sampling method for the detection of SARS-CoV-2 in wastewater in small sewage systems with low water flow. This study was led by Dr. Xiaoli Pang in collaboration with Dr. Bonita Lee (University of Alberta).

In this study, the authors looked at the detection of SARS-CoV-2 from sewage samples collected by a passive sampler and an automatic sampler installed in a wastewater draining system at a local hospital. An automatic sampler is a common wastewater sampling method that is installed to collect 24-hour composite samples. The study also looked at grab sampling, which is a convenient and easy method because installation of special equipment is not required. It may miss viral shedding discharges to sewers depending on the time of day and associated human activities, providing less representative surveillance.

Key findings:

  • Cotton swabs and electronegative membranes of the passive sampler showed similar detection rates and Ct values using RT-qPCR targeted on SARS-CoV-2 RNA for the nucleocapsid protein (N1 and N2) gene.
  • The passive method performed as well as a standard auto sampling methods, with no significant differences between N1 and N2 Ct values.
  • There were two days where the grab/composite samples failed to detect SARS-CoV-2, while the passive samples were positive for SARS-CoV-2. The explanation for this could be that the passive samplers were placed in the sewage outflow for more than 24 hours and hence allowed detection of viral shedding events prior to the start of the automatic sampling period.
  • The passive sampler is more cost-effective, less labor-intensive, and has a shorter sample processing time before testing compared to the traditional grab or automatic sampling methods.

The findings suggest that passive sampling could be a valuable tool for ongoing monitoring efforts, especially in resource-poor settings and smaller communities. It is more suitable for small sewage systems with low water flow, such as schools, hospitals, university campuses, and long-term care facilities. Implementing passive sampling in wastewater surveillance may enhance early detection of SARS-CoV-2 spread; testing of students, staff, or residents of a facility; and disease prevention measures.

Between April and June 2021, 15 passive samplers were placed at a local hospital’s wastewater outflow alongside an autosampler. RT-qPCR was used to detect SARS-CoV-2 in the samples after processing and RNA extraction.