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. 2021 Mar 6;6(10):6528–6536. doi: 10.1021/acsomega.1c00166

Figure 3.

Figure 3

Clinical significance of SARS-CoV-2 RT-PCR results. (a) Timeline of SARS-CoV-2 infectivity taking into account our own findings and those of others.39,40 (b) Ct values (target IP2) as a function of time after the symptom onset in nasopharyngeal swab specimens of COVID-19 patients. Ct of specimens with undetectable SARS-CoV-2 RNA were set to 50. (c) Correlation of Ct values with SARS-CoV-2 infectivity. Vero E6 cells were infected with 10-fold dilutions of a SARS-CoV-2 isolate. The plates were incubated for 6 days in 5% CO2 at 37 °C and examined daily using an inverted microscope (ZEISS Primovert) to evaluate the extent of the virus-induced cytopathic effect in cell culture. The calculation of the estimated virus concentration was carried out by the Spearman and Karber method48,49 and expressed as TCID50/mL (50% tissue culture infectious dose). TCID50/mL values were transformed to PFU mL–1 using the formula PFU mL–1 = TCID50/mL × 0.7.50 RNA extraction and RT-PCR (target IP4 and target IP2) were performed in duplicate for each dilution. Ct of dilutions with undetectable SARS-CoV-2 RNA were set to 50.