Figure 2. Reported SARS-CoV-2 concentrations in wastewater before and after Wisconsin Wastewater Monitoring Program proficiency testing (PT) intervention and laboratory resubmission of corrected concentrations in study of implementation and early outcomes of laboratory proficiency testing program for National Wastewater Surveillance System, United States, 2024. Each point represents the reported concentration of SARS-CoV-2 from a given laboratory workflow, averaged across gene targets and replicates (n = 3). Total number per PT round is larger than the number of participating laboratories because of multiple workflows tested by some participants. Lines connect results from the same laboratory before and after correction; dotted lines indicate results from new participants in round 2. Red lines highlight results that required substantial correction reflecting issues such as unit conversion errors, misapplied processing factors, or other calculation discrepancies in the initially reported data identified during PT data review; no re-analysis of samples was performed. Blue shaded area denotes the interquartile region across laboratories before and after correction; line denotes median. gc/L, gene copies per liter.