From early 2020, a high demand for SARS-CoV-2 tests was driven by several testing indications, including asymptomatic cases, resulting in the massive roll-out of PCR assays to combat the pandemic. Considering the dynamic of viral shedding during the course of infection, the demand to report cycle threshold (Ct) values rapidly emerged. As Ct values can be affected by a number of factors, we considered that harmonization of semi-quantitative PCR results across laboratories would avoid potential divergent interpretations, particularly in the absence of clinical or serological information. A proposal to harmonize reporting of test results was drafted by the National Reference Centre (NRC) UZ/KU Leuven, distinguishing four categories of positivity based on RNA copies/mL. Pre-quantified control material was shipped to 124 laboratories with instructions to setup a standard curve to define thresholds per assay. For each assay, the mean Ct value and corresponding standard deviation was calculated per target gene, for the three concentrations (10(7), 10(5) and 10(3) copies/mL) that determine the classification. The results of 17 assays are summarized. This harmonization effort allowed to ensure that all Belgian laboratories would report positive PCR results in the same semi-quantitative manner to clinicians and to the national database which feeds contact tracing interventions. ; UZ/KU Leuven: as National Reference Center for Respiratory Pathogens, is supported by Sciensano, which is gratefully acknowledged. We would like to acknowledge additional staff members of the participating laboratories that contributed to this study: Jean-Luc Gala, Benoit Kabamba, Elsa Wiam, Valentin Coste, Paul Blanpain, Jean Ruelle, Ari Serbetciyan, Nicolas Pinte, Ophélie Simon and the entire UCLouvain federal testing platform COVID-19 team (all affiliation 4), Aurore Demars (affiliation 7), Laura Vanden Daele (affiliation 9), Hanne Valgaeren (affiliation 22), Guillaume Bayon-Vicente (affiliation 23), Fabrice Bureau, Claire Gourzonès and Joey ...