Why use effective concentration?
The concentration of SARS-CoV-2 in a wastewater sample is the starting point for many applications of wastewater-based epidemiology. However, this “raw” concentration is subject to factors that are not relevant to Covid-19 disease activity, such as dilution, varying wastewater flow rates, varying population size, and differences in laboratory methodologies used to measure the virus concentration.
Biobot’s effective concentration is a transparent and future-proof approach to account for these other factors.
The effective concentration is designed to be comparable across all our samples, including across sampling locations and through time.
How do you derive the effective concentration?
First, we use a fecal marker virus, PMMoV, to normalize the SARS-CoV-2 concentration. Normalization accounts for factors like dilution, inflow and infiltration, varying population size, and varying wastewater flow.
Second, we apply an adjustment factor that corrects for different lab methodologies’ different recovery and extraction efficiencies
Why do the raw concentration and effective concentration seem so different?
The raw concentration tries to exactly measure the genome copies per liter in a given sample. Effective concentration instead provides a reliable sense of how Covid-19 activity is changing over time across multiple samples.
How will I know when Biobot changes its lab protocol in the future?
Biobot will note any changes to the methodology used to compute the effective concentration in our website’s public release notes. Our goal is to allow technical experts to scrutinize the raw data while providing a seamless data stream to users focused on public health implications of the data.
A Helpful Analogy: “Feels Like” Temperatures
We use the term “effective concentration” because it is not specific to any methodology. This approach is inspired by the “feels like” temperatures reported by consumer weather services.
The raw temperature is the most important single number for understanding how it will feel when you go outside. But other factors, like wind speed and humidity, are important too. The wind chill temperature is computed using the raw temperature and wind speed, while the heat index is computed using the raw temperature and humidity. The combined indicator, that accounts for both wind speed and humidity, is called just “feels like” temperature.
Just as “feels like” avoids clunky phrasing like “wind- and humidity-adjusted temperature”, we hope that effective concentration can avoid imprecise and confusing language while also allowing for continual improvement in our laboratory methods.