Alliance for Pandemic Preparedness

February 17, 2021

Misinterpretation of Viral Load in COVID-19 Clinical Outcomes



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Raw RT-PCR cycle threshold (Ct) values may lead to misinterpretation of the total viral burden, according to a study comparing tracheal aspirate (TA) with nasopharyngeal swab (NPS) samples in critically ill COVID-19 patients (n=138). The authors found that in general, TA samples had more total viral RNA than NPS samples, even though there was no difference in Ct value. Samples initially considered to have different viral loads by raw Ct comparison actually had the same viral load, which the authors argue highlights the importance of normalizing results using a reference gene approach in order to draw conclusions related to COVID-19 viral load. The authors also proposed a formula to corrected raw Ct values.

Miranda et al. (Feb 2021). Misinterpretation of Viral Load in COVID-19 Clinical Outcomes. Virus Research.