The primary method to diagnose severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is molecular testing of respiratory samples by reverse transcriptase-polymerase chain reaction (RT-PCR). The results of this SARS-CoV-2 RT-PCR are generally reported qualitatively (positive or negative). However, the quantitative test result (cycle threshold (Ct) or crossing point (Cp)) is directly related to the viral load. This viral load may be related to disease phenotype, morbidity, and mortality of COVID-19 patients.
Study: Initial SARS-CoV-2 viral load is associated with disease severity: a retrospective cohort study. Image Credit: Kirill Kamionskiy/ Shutterstock
There is contrasting evidence to suggest the association between viral load with different clinical parameters and outcome measurements in coronavirus disease 2019 (COVID-19) patients. Since the viral load is highest in the earliest (mostly pre-symptomatic) stages of the infection, followed by a gradual decline and increase in host-immune reactions, the timing of testing becomes crucial in determining the viral load found in respiratory samples.
In a recent study reported in the preprint server medRxiv*, German researchers investigated the utility of initial SARS-CoV-2 viral loads of tests performed at a Public Health testing facility regarding hospital and intensive care units (ICUs) admissions and overall survival.
A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.
The researchers performed a retrospective cohort study on data obtained from 20,207 patients. The analysis was based on a positive SARS-CoV-2 RT-PCR result from combined nasopharyngeal (NP) and oropharyngeal (OP) swabs analyzed between March 17, 2020, and March 31, 2021. They used RT-PCR in the Regional Public Health Laboratory Kennemerland, Haarlem, the Netherlands. They collected data on hospital (and ICU) admissions from the two large teaching hospitals in the Kennemerland region.
On analyzing the samples, the Cp-values at baseline measurement were considerably lower (indicating higher viral loads) in those patients who were eventually hospitalized than those who were not.
On categorizing patients in three SARS-CoV-2 viral load groups, the high viral load group (Cp < 25) was associated with an increased risk of hospitalization than the low viral load group, irrespective of age.
The high viral load group (Cp < 25) was also associated with an increased risk of ICU admission compared to the low viral load group (Cp > 30)
This study was a large-scale one that considered most patients who tested themselves in their early stage of the infection (median two days since onset of symptoms). Hence, the initial SARS-CoV-2 viral load assessed in these patients was more likely to represent a value close to the individual peak viral load than samples taken at a later stage of the disease when hospitalized. This can be one of the reasons behind contrasting study results as well.
In conclusion, the study findings emphasized the added value of reporting SARS-CoV-2 viral load based on Cp-values. These values could be crucial in identifying patients at the highest risk of adverse outcomes such as hospital or ICU admission and who may benefit from more intensive monitoring.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behaviour, or treated as established information
- Souverein, D. et al. (2021) “Initial SARS-CoV-2 viral load is associated with disease severity: a retrospective cohort study”. medRxiv. doi: 10.1101/2021.10.01.21264412.
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