Page 28 - SARS-COV-2-laboratory-viral-outbreak-perspective-investigation-by-ICR-NIH-MY

Laboratory Procedures for Testing COVID-19

At the same time, WHO shared a protocol based on Corman/Drosten 2020 (Corman/Drosten
earlier developed a protocol for SARS-CoV-1) on the 17th January 2020 which utilized real
time RT-PCR platform to detect COVID-19. The protocol included two gene targets:

● “E gene” = screening target (First line screening assay: E gene assay)

● “RdRp Gene” = confirmatory target (Confirmatory assay: RdRp gene assay)

The interpretation of the result depends on the test kit that was used. Some test kits have Ct
values of 38 as cut off point and some kits have Ct values at 40. Any Ct values below 38 - 40
can be considered as positive whereas Ct values above 40 are considered as negative. We also
have to look at the curve whether it is a sigmoid curve. However, if the initial sample (first
time) has Ct values between 38-40, we will suggest repeating the test with fresh NPS. The Ct
values between 38 and 40 indicate low viral count (<10) and we were not able to confirm
whether it is truly infective or not. Therefore, we will request for a second sample.
Moving forward, WHO also shared that in places with high prevalence of COVID-19 virus,
we do not need to do two target genes. If there is a need to use a single target gene (with only
a confirmatory target), then the laboratory has to perform “correlation study” to compare
those samples with presence of two target genes and re-test with the single target gene.