Currently, in the United Kingdom, the recommended method for diagnostic testing and screening for COVID-19 is quantitative (real-time) reverse transcription polymerase chain reaction (PCR) analysis of viral RNA extracted from upper respiratory tract samples. A positive result indicates that SARS-CoV-2 RNA has been detected with a cycle threshold (Ct) value of less than 38 cycles of amplification (n.b. this is a locally agreed value, which is based on analysis of proficiency testing performance and other local testing data. Cycle threshold (Ct) values inversely correlate to the amount of target nucleic acid in the sample. Thus, a lower cycle threshold (Ct) value indicates a greater amount of target nucleic acid in a sample i.e. a higher viral load.
A recent scoping literature review has revealed only a handful of published studies to date that explore an association between cycle threshold (Ct) values and disease severity or clinical outcome in patients infected with the SARS-CoV-2 virus. The majority of these studies have been conducted in China with small patient sample sizes. Thus, a larger-scale study which explores the relationship between viral load and disease severity is required; indeed, the World Health Organization (WHO) has recently highlighted this as an area which needs further research.
