Abstract
Diagnostic methods based on SARS-CoV-2 antigens detection are a promising alternative to SARS-CoV-2 RNA amplification. We evaluated the automated chemiluminescence-based Lumipulse® G SARS-CoV-2 Ag assay on saliva samples, using Simplexa™ COVID-19 Direct assay as a reference test. Analytical performance was established on a pool of healthy donors’ saliva samples spiked with the 2019-nCoV/Italy-INMI1 isolate, whereas clinical performance was assessed on fresh saliva specimens collected from hospitalized patients with suspect or confirmed COVID-19 diagnosis. The limit of detection (LOD) was 0.65 Log TCID50/mL, corresponding to 18,197 copies/mL of SARS-CoV-2 RNA. Antigen concentrations and SARS-CoV-2 RNA were highly correlated (r = 0.99; p < 0.0001). Substantial agreement (80.3%) and significant correlation (r = −0.675; p = 0.0006) were observed between Lumipulse® G assay results and Ct values on clinical samples, with 52.4% sensitivity and specificity 94.1%. Sensitivity exceeded 90.0% when calculated on samples with Ct < 25, and specificity was 100% when excluding samples from recovered patients with previous COVID-19 diagnosis. Overall, chemiluminescence-based antigen assay may be reliably applied to saliva samples to identify individuals with high viral loads, more likely to transmit the virus. However, the low positive predictive value in a context of low SARS-CoV-2 prevalence underscores the need for confirmatory testing in SARS-CoV-2 antigen-positive cases.
Highlights
As the ongoing COVID-19 pandemic is growing fast, accounting for more than 121 million laboratory-confirmed cases and more than 2.69 million deaths reported around the world, reliable and rapid detection methods are increasingly necessary to diagnose and track patients with COVID-19 worldwide
In Italy, following health authorities mandate, we evaluated the performance of a novel antigenic test using saliva samples, namely the Lumipulse® G SARS-CoV-2 Ag assay (Fujirebio, Tokyo, Japan), which received the CE marking for qualitative and quantitative detection of the SARS-CoV-2 nucleoprotein (N) antigen on both saliva and nasopharyngeal swab samples, based on the chemiluminescent enzyme immunoassay technology
The low limit of detection (LOD) of the assay, established using a pool of fresh saliva samples from healthy donors spiked with the INMI SARS-CoV-2 isolate, was 0.65 log10 TCID50/mL corresponding to 18,197 copies/mL
Summary
As the ongoing COVID-19 pandemic is growing fast, accounting for more than 121 million laboratory-confirmed cases and more than 2.69 million deaths reported around the world (https://www.worldometers.info/coronavirus, accessed on 18 March 2021), reliable and rapid detection methods are increasingly necessary to diagnose and track patients with COVID-19 worldwide. Rapid antigen detection tests currently deserve great attention because they are intrinsically less laborious, require a few minutes to results and have the potential to satisfy the pressing demand for early SARS-CoV-2 infection diagnosis [3,4,5]. It is worthy to note that, in some countries, rapid antigen detection tests are suggested as first-line diagnostic testing [6], and recently ECDC recommended clinical validations of rapid antigen tests (https://www.ecdc.europa.eu/en/publications-data/options-use-rapid-antigentests-covid-19-eueea-and-uk, accessed on 19 November 2020). In SARS-CoV-2 diagnosis, saliva has entered the shortlist of clinical samples to which apply the current laboratory tests since recent studies have shown that molecular tests performed on saliva had sensitivity and specificity comparable to those observed with nasopharyngeal swab samples [7,8,9,10,11,12]. On 8 May 2020, the U.S Food and Drug
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