Salivary SARS-CoV-2 antigen rapid detection: A prospective cohort study

Abstract

Background and aim: SARS-CoV-2 quick testing is relevant for the containment of new pandemic waves. Antigen testing in self-collected saliva might be useful. We compared salivary and naso-pharyngeal swab (NPS) SARS-CoV-2 antigen detection by a rapid chemiluminescent assay (CLEIA) and two different point-of-care (POC) immunochromatographic assays, with results of molecular testing.

Methods: 234 patients were prospectively enrolled. Paired self-collected saliva (Salivette) and NPS were obtained to perform rRT-PCR, chemiluminescent (Lumipulse G) and POC (NPS: Fujirebio and Abbott; saliva: Fujirebio) for SARS-CoV-2 antigen detection.

Results: The overall agreement between NPS and saliva rRT-PCR was 78.7%, reaching 91.7% at the first week from symptoms. SARS-CoV-2 CLEIA antigen was highly accurate in distinguishing positive and negative NPS (ROC-AUC = 0.939, 95%CI:0.903-0.977), with 81.6% sensitivity and 93.8% specificity. This assay on saliva reached the optimal value within 7 days from symptoms onset (Sensitivity: 72%; Specificity: 97%). Saliva POC antigen was limited in sensitivity (13%), performing better in NPS (Sensitivity: 48% and 66%; Specificity: 100% and 99% for Espline and Abbott respectively), depending on viral loads.

Conclusions: Self-collected saliva is a valid alternative to NPS for SARS-CoV-2 detection by molecular, but also by CLEIA antigen testing, which is therefore potentially useful for large scale screening.

Keywords: COVID-19; Chemiluminescence; Naso-pharyngeal swab; Point-of-care.

Fig. 1

SARS-CoV-2 antigen in NPS and saliva. The antigen was assayed by CLEIA in subjects classified as negative or positive on the basis of NPS rRT-PCR and subdivided on the basis of the time lapse between symptoms onset and testing (Days). The upper graph shows the results obtained in NPS samples. The dotted line (1.34 ng/L) is the cut-off recommended by the manufacturer. The lower graph shows the results obtained in salivary samples. The dotted lines are the cut-off (0.67 ng/L) and the limit of detection (0.2 ng/L) recommended by the manufacturer. In both graphs, patients enrolled within 7 days and classified as negative are all but one (open square) outpatients. Among rRT-PCR positive results, open squares represent the three outpatients who were found to be positive.

Fig. 2

Kinetics of SARS-CoV-2 antigen and Ct values in NPS and saliva. Two consecutive samples (seven days apart) were available in a series of inpatients, who were subdivided on the basis of days from symptoms onset to enrollment in three groups: within 7 days (group A), between 7 and 14 days (group B), after 14 (group C). The p values reported were obtained after Wilcoxon rank test for paired data.

Fig. 3

Comparison of NPS and saliva SARS-CoV-2 rapid antigen testing. Percentages of positive results for SARS-CoV-2 antigen testing by means of rapid immunochromatographic assays (Abbott and Espline) and CLEIA in NPS and saliva after subdividing samples on the basis of viral load (Ct ranges) at molecular analyses.