a, A schematic of how to design, produce, and apply synthetic RNA spike-ins (SRSIs). b, A limited titration experiment where SRSIs of varying concentrations were added to two clinical samples with low and intermediate SARS-CoV-2 CTs. SRSIs were added to the sample at the RNA stage; the sample with a low CT (20) was then normalized to CT 25 at the cDNA stage, whereas the sample with mid CT (26) was not normalized.