Analysis of SARS-CoV-2 reverse transcription-quantitative polymerase chain reaction cycle threshold values vis-à-vis anti-SARS-CoV-2 antibodies from a high incidence region

Abstract

Objectives: To examine the relationship between antibody status and cycle threshold (Ct) values, the prognostic value of the latter for COVID-19 patients, and the inter-assay comparability of SARS-CoV-2 Ct values.

Methods: In 347 COVID-19 inpatients, SARS-CoV-2 Ct values (via reverse transcription-quantitative polymerase chain reaction) on admission were compared between 2 assays and correlated with the antibody response (in the course of the disease), the clinical course and the time since onset of symptoms.

Results: Ct values for 2 of 3 target genes showed significant differences between the 2 assays used (P=0.012 and P<0.0001). Ct values were significantly higher for antibody positive patients (P<0.0001) and positively correlated with the amount of time since onset of symptoms (R: 0.332-0.363; P<0.001). Patients with fatal outcomes showed higher viral loads than survivors (P<0.0001).

Conclusions: Ct values depend strongly on assay used and target gene examined and should not be used as quantitative values to guide therapeutic or diagnostic decisions. The inverse association between antibody status and viral load suggests that antibodies contribute to the elimination of the virus, independent of the outcome, which is influenced by the viral load on admission and might depend more strongly on other parts of the immune response.

Keywords: COVID-19; Ct value; RT-qPCR; SARS-CoV-2; anti-SARS-CoV-2 antibodies.