Viral culture and immunofluorescence for the detection of SARS-CoV-2 infectivity in RT-PCR positive respiratory samples.

BACKGROUND: Knowing how long SARS-CoV-2-positive individuals can remain infective is crucial for the design of infection prevention and control strategies. Viral culture is the gold standard for detecting an active-replicative virus and evaluating its infectious potential. OBJECTIVE: To assess the correlation of SARS-CoV-2 infectivity with the number of days from symptom onset and the Ct value, using culture as a reference method. Also, to describe a detailed protocol for SARS-CoV-2 culture and immunofluorescence confirmation based on our experience with other respiratory viruses. STUDY DESIGN: 100 consecutive respiratory samples positive for SARS-CoV-2 by RT-PCR from different subjects were inoculated into VERO E6 cells. RESULTS: Viral isolation was successful in 58% of samples. The median number of days from symptom onset for culture-positive samples was 2, and 15 for culture-negative samples. Six positive cultures were obtained in patients ≥14 days after symptom onset, all of whom were immunocompromised or with severe COVID-19. The mean Ct value was 12.64 units higher in culture-negative than in culture-positive samples. The probability of successfully isolating SARS-CoV-2 in samples with a Ct value <22 was 100%, decreasing to 3.1% when >27. CONCLUSIONS: Our findings show a significant positive correlation between the probability of isolating SARS-CoV-2 in culture, fewer days of symptoms and a lower RT-PCR Ct value. SARS-CoV-2 infectivity lasts no more than 14 days from symptom onset in immunocompetent individuals. In contrast, in immunocompromised patients or those with severe COVID-19 infectivity may remain after 14 days. Ct value <22 always indicates infectivity.

Viral Culture Confirmed SARS-CoV-2 Subgenomic RNA Value as a Good Surrogate Marker of Infectivity.

Determining SARS-CoV-2 viral infectivity is crucial for patient clinical assessment and isolation decisions. We assessed subgenomic RNA (sgRNA) as a surrogate marker of SARS-CoV-2 infectivity in SARS-CoV-2-positive reverse transcription PCR (RT-PCR) respiratory samples (n = 105) in comparison with viral culture as the reference standard for virus replication. sgRNA and viral isolation results were concordant in 99/105 cases (94%), indicating highly significant agreement between the two techniques (Cohen's kappa coefficient 0.88, 95% confidence interval [CI] 0.78 to 0.97, P < 0.001). sgRNA RT-PCR showed a sensitivity of 97% and a positive predictive value of 94% to detect replication-competent virus, further supporting sgRNA as a surrogate marker of SARS-CoV-2 infectivity. sgRNA RT-PCR is an accurate, rapid, and affordable technique that can overcome culture and cycle threshold (CT) value limitations and be routinely implemented in hospital laboratories to detect viral infectivity, which is essential for optimizing patient monitoring, the efficacy of treatments/vaccines, and work reincorporation policies, as well as for safely shortening isolation precautions.