Reproducibility of cycle threshold values from severe acute respiratory coronavirus virus 2 (SARS-CoV-2) reverse-transcription polymerase chain reaction (RT-PCR) assays

To account for the right censoring of Ct values at 40 cycles, we used a mixed-effects Tobit model that included a random intercept to account for the correlation due to repeated measurement of the same patient as well as fixed effects for the researcher collecting the specimen (A vs B), naris sampled (left vs right), time (0 vs 10), and assay (Abbott m2000 vs Simplexa S vs Simplexa ORF1ab). A more nuanced approach, such as that described by Mowrer et al6 in which Ct might play a role in conjunction with expert consultation and evaluation of clinical status, might be better if the Ct is to be used clinically. [...]to use Ct in such a fashion, the Ct would need to be correlated with culturable virus for each platform in use at an institution because the absolute value of Ct is not reproducible between assays or even with the same assay between institutions.