Table 3.
Comparison of studies that examine cycle threshold (CT) values of SARS-CoV-2–positive samples at the population level
| Study characteristics | This study | Hay et al [24] | Walker et al [25] | El Zein et al [31] |
| Source population | El Paso, Texas metropolitan statistical area | Nearly all hospital admissions into Brigham & Women’s Hospital in Boston, Massachusetts | United Kingdom’s national COVID-19 Infection Survey, which provided a representative sample of the United Kingdom | All patients who tested positive for SARS-CoV-2 at the Detroit Medical Center |
| Type of testing | Testing based on symptoms or testing for nonsymptomatic purposes (eg, travel and work) | 2 weeks of symptomatic testing and 4.5 months of surveillance testing | Surveillance testing | —a |
| Sample size | 148,410 samples and 36,306 positive tests | — | 843,85 samples and 1892 positive tests | 708 hospitalized patients and 282 ambulatory patients |
| Biological sample types included | Mostly nasopharyngeal swabs and small numbers of anterior nares, salivary, or unknown sample types. | Nasopharyngeal swabs | Nose and throat swabs | Nasopharyngeal swabs |
| Timeframe (duration) | September 15, 2020, to January 11, 2021 (around 4 months) |
April 3, 2020, to November 10, 2020 (around 7 months) | April 26, 2020, to October 11, 2020 (around 5.5 months) |
April 4, 2020, to June 5, 2020 (around 2 months) |
| Gene target(s) of RT-PCRb | Nucleocapsid, spike, and ORF1abc genes | — | Nucleocapsid, spike, and ORF1ab genes | Nucleocapsid gene |
| CT-based value(s) | Daily median CT value | Distribution, median, and skew of CT values | Mean and median CT values | High, medium, and low viral load (CT values of ≤25, 26-36, and ≥37, respectively) |
| Outbreak measure | Rtd, count of individuals hospitalized with COVID-19, and change in percent positivity | Rt | Positivity rate | Mortality |
| Outcome(s) of study | Negative correlation between median CT and Rt, negative correlation between median CT and hospitalization count (with time delay), and negative correlation between median CT and percent positivity | Correlation between Rt and median and skewness of CT values among positive surveillance specimens | Declines in mean and median CT values preceded increases in positivity rates. | Downward trend in viral load coincided with a decrease in the number of deaths |
aNot available.
bRT-PCR: reverse transcription polymerase chain reaction.
cORF1ab: open reading frames 1ab.
dRt: transmission rate.