During the coronavirus disease 2019 (COVID-19) pandemic, COVID-19
testing is crucial, as it enables early detection and halting the spread
of infection throughout the community. Real-time reverse-transcriptase
polymerase chain reaction (Real-time RT-PCR) testing is the predominant
method for COVID-19 testing, and the cycle threshold value (Ct value) is
used to determine COVID-19 positivity. There are many ongoing studies
using Ct value, and the present study aims to examine time series
distribution during the pandemic using Ct values at the national level
and analyze the association with time-varying reproduction number (Rt)
to discuss the utilization of laboratory-based COVID-19 test results. We
used Real-time RT-PC results collected by Seegene Medical Foundation
from the index case in Korea in February 2020 to January 2022 in Korea.
The distribution of daily Ct value ( RdRp/S target) was examined,
and it was compared with the daily count of newly diagnosed cases and Rt
to determine the usability of Ct values. Moreover, time lag was applied
to the daily count of newly diagnosed cases to analyze the association
between Ct values and Rt. During the COVID-19 pandemic, Ct values
declined in general, while viral load increased progressively. After Ct
values dropped markedly, the number of newly diagnosed cases rose
substantially, and the association analysis also confirmed that the
daily count of newly diagnosed cases declined with increasing Ct values.
The time series trend of the Ct values was also similar to that of Rt, a
classic marker used as a predictor of the trends of the pandemic, and
when compared to the actual count of newly diagnosed cases, Ct values
can be used to predict new diagnoses earlier than Rt. The fact that the
Ct values were more sensitive to a substantial rise of new COVID-19
cases than Rt was in the early days of the pandemic also support this.
We examined the potential of Ct values as a predictor of new COVID-19
cases in real-time using nationally collected Ct value data. Further, we
proposed the use of Ct values as an index reflecting the degree of viral
load, so the findings of this study can be used as valuable evidence to
support public health decisions for response and resource distribution.