Evaluation of the relationship between quantitative PCR results and cell culturing of SARS2-CoV with respect to symptoms onset and Viral load - a systematic review

BackgroundViral culture is currently the most accurate method to demonstrate viability and infectivity of Severe acute respiratory syndrome Coronavirus (SARS-2 CoV). Routine clinical diagnosis, however, is mostly performed by PCR - based assays that do not discriminate between infectious and non-virus. Herein, we aimed to determine the correlation between positive viral cultures and either PCR positivity, the Cycle Threshold (Ct) or the number of viral copies. MethodsA systematic electronic literature search was performed and studies that reported both viral SARS-CoV-2 culture and PCR-based assays were included. A separate search for samples from blood, urine, stool, breast milk and tears were performed. To convert Ct values reported in the reviewed studies were to viral genomic copies, calibration experiments with four different reaction performed, using quantified RNA molecules. ResultsA total 540 articles were reviewed, and 38 studies were included in this review. Out of 276 positive-culture of non-severe patients, 272 (98.55%) were negative ten days after symptoms onset, while PCR assays remained positive for up to 67 days. In severely ill or immunocompromised patients positive-culture was obtained up to 32 days and out of 168 cultures, 31 (18.45%) stayed positive after day 10. In non-severe patients, in Ct value greater than 30 only 10.8% were still culture-positive while in Ct >35 it was nearly universally negative. The minimal calculated number of viral genome copies in culture-positive sample was 2.5 x 103 copies / mL. These findings were similar in immunocompromised patients. Recovering positive culture from non-respiratory samples was sporadically obtained in stool or urine samples. Conversion of Ct values to viral genome copies showed variability between different PCR assays and highlighted the need to standardize reports to correctly compare results obtained in different laboratories. ConclusionDuring the pandemic phase, non-severe COVID-19 patients who are recovering and are not immuno-suppressed, can be regarded as non-infectious, within 10 days from symptom onset, or with Ct value greater than 35 (or a calculated viral load lower than 1.2x103 copies / mL). These findings have important implications for recovering patients and asymptomatic patients, with respect to isolation criteria. The conversion of Cq values to viral genome copies described herein may be useful in future work, enabling a more standardized comparison between results reported in different studies from different laboratories.