Answer given by Ms Kyriakides on behalf of the European Commission
For a positive Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) reverse transcription polymerase chain reaction ( RT-PCR) test with a high Cycle threshold (Ct) value, e.g. > 35, repeated testing should be considered.
An exact value needs to be defined locally in a laboratory and may be specific to the assay used. Each SARS-CoV-2 RT-PCR assay will have a slightly different limit of detection. A typical RT-PCR assay will have a maximum of 40 thermal cycles.
High Ct values (low viral load) can be attributed to several clinical scenarios whereby the risk of infectivity may be reduced but interpretation requires clinical context.
Live and potentially infectious viruses have been isolated in laboratory cell cultures from samples exhibiting high Ct values (>36) — to what extent this indicates a potential transmission risk from person-to-person is not fully understood.
The European Centre for Disease Prevention and Control does not recommend a specific maximum amplification cycle threshold for RT-PCR tests. However, it is recommended that if the Ct values are high, e.g. > 35, repeated testing should be considered.
-  Singanayagam A, Patel M, Charlett A, Lopez Bernal J, Saliba V, Ellis J, et al. Duration of infectiousness and correlation with RT-PCR cycle threshold values in cases of COVID-19, England, January to May 2020. Euro Surveill. 2020 Aug;25(32) Available at: https://www.ncbi.nlm.nih.gov/pubmed/32794447
-  https://www.ecdc.europa.eu/en/covid-19/latest-evidence/diagnostic-testing ; https://www.who.int/publications/i/item/laboratory-testing-strategy-recommendations-for-covid-19-interim-guidance