Viral cultures, Polymerase Chain Reaction Cycle Threshold Values and Viral Load Estimation for SARS-CoV-2 Infectious Potential Assessment in Hematopoietic Stem Cell and Solid Organ Transplant Patients: A Systematic Review

BackgroundOrgan transplant recipients are at increased vulnerability to SARS-CoV-2 due to immunosuppression and may pose a continued transmission risk especially within hospital settings. Detailed case reports including symptoms, viral load and infectiousness, defined by the presence of replication-competent viruses in culture, provide an opportunity to examine the relationship between clinical course, burden and contagiousness, and provide guidance on release from isolation. ObjectivesWe performed a systematic review to investigate the relationship in transplant recipients between serial SARS-CoV-2 RT-PCR cycle threshold (Ct) value or cycle of quantification value (Cq), or other measures of viral burden and the likelihood and duration of the presence of infectious virus based on viral culture including the influence of age, sex, underlying pathologies, degree of immunosuppression, and/or vaccination on this relationship. MethodsWe searched LitCovid, medRxiv, Google Scholar and WHO Covid-19 databases, from 1 November 2019 until 31 December 2021. We included studies reporting relevant data for transplantees with SARS-CoV-2 infection: results from serial RT-PCR testing and viral culture data from the same respiratory samples. We assessed methodological quality using five criteria, and synthesised the data narratively and graphically. ResultsWe included 10 case reports and case series reporting on 38 transplantees. We observed a relationship between proxies of viral burden and likelihood of shedding replication-competent SARS-CoV-2. Two individuals shed replication-competent viruses over 100 days after infection onset. Lack of standardisation of testing and reporting platforms precludes establishing a definitive viral burden cut-off. However, most transplantees stopped shedding competent viruses when the RT-PCR cycle threshold was above 30 despite differences across platforms. ConclusionsViral burden is a reasonable proxy for infectivity when considered within the context of the clinical status of each patient. Standardised study design and reporting are essential to standardise guidance based on an increasing evidence base.