ABSTRACT
The relationship between N-antigen concentration and viral load within a specimen and across different specimens is essential for interpretation of rapid diagnostic tests (RDT) clinical performance in different use cases. A prospective study was conducted in Porto Velho, Brazil, to investigate RDT performance in different specimen types as a function of the correlation between antigen concentration and viral load. The study included 214 close contacts with recent exposures to confirmed cases, aged 12 years and older and with various levels of vaccination. Antigen concentration was measured in nasopharyngeal swab (NPS), anterior nares swab (ANS), and saliva specimens. Reverse transcriptase (RT)PCR was conducted on the NPS and saliva specimens, and two RDTs were conducted on ANS and one on saliva. Antigen concentration correlated with viral load when measured in the same specimen type but not across specimen types. Antigen levels were higher in symptomatic cases compared to asymptomatic/oligosymptomatic cases and lower in saliva compared to NPS and ANS samples. Discordant results between the RDTs conducted on ANS and the RT-PCR on NPS were resolved by antigen concentration values. The analytical limit-of-detection of RDTs can be used to predict the performance of the tests in populations for which the antigen concentration is known. The antigen dynamics across different sample types observed in SARS-CoV-2 disease progression support use of RDTs in nasal samples. Given lower antigen concentrations in saliva, tests using saliva is expected to require improved analytical sensitivity to achieve clinical sensitivity similar to testing of nasal samples.
Subject(s)
Nasopharyngitis , Severe Acute Respiratory SyndromeABSTRACT
Rapid diagnostic tests (RDTs) that detect antigen indicative of SARS-CoV-2 infection can help in making quick health care decisions and regularly monitoring groups at risk of infection. With many RDT products entering the market, it is important to rapidly evaluate their relative performance. Comparison of clinical evaluation study results is challenged by protocol design variations and study populations. Laboratory assays were developed to quantify nucleocapsid (N) and spike (S) SARS-CoV-2 antigens. Quantification of the two antigens in nasal eluates confirmed higher abundance of N than S antigen. The median concentration of N antigen was 10 times greater than S per genome equivalent. The N antigen assay was used in combination with quantitative RT-PCR to qualify a panel composed of recombinant antigens, inactivated virus, and clinical specimen pools. This benchmarking panel was applied to evaluate the analytical performance of the SD Biosensor STANDARD Q COVID-19 Ag test, Abbott Panbio COVID-19 Ag Rapid Test, Abbott BinaxNOW COVID-19 Ag test, and the LumiraDx SARS-CoV-2 Ag Test. The four tests displayed different sensitivities toward the different panel members, but all performed best with the clinical specimen pool. The concentration for a 90% probability of detection across the four tests ranged from 21 pg/mL to 102 pg/mL of N antigen in the extracted sample. Benchmarking panels provide a quick way to verify the baseline performance of a diagnostic and enable direct comparison between diagnostic tests.
Subject(s)
COVID-19 , Virus DiseasesABSTRACT
The global COVID-19 pandemic has created an urgent demand for large numbers of inexpensive, accurate, rapid, point-of-care diagnostic tests. Analyte-based assays are suitably inexpensive and can be rapidly mass-produced, but for sufficiently accurate performance they require highly optimized antibodies and assay conditions. We used an automated liquid handling system, customized to handle arrays of lateral flow immunoassay (LFA) tests in a high-throughput screen, to identify anti-nucleocapsid antibodies that will perform optimally in an LFA. We tested 1021 anti-nucleocapsid antibody pairs as LFA capture and detection reagents with the goal of highlighting pairs that have the greatest affinity for unique epitopes of the nucleocapsid protein of SARS-CoV-2 within the LFA format. In contrast to traditional antibody screening methods (e.g., ELISA, bio-layer interferometry), the method described here integrates real-time reaction kinetics with transport in, and immobilization directly onto, nitrocellulose. We have identified several candidate antibody pairs that are suitable for further development of an LFA for SARS-CoV-2.
Subject(s)
COVID-19ABSTRACT
Inexpensive, simple, rapid diagnostics are necessary for efficient detection, treatment and mitigation of COVID‑19. Currently, the primary diagnostic tool being utilized is reverse transcription polymerase chain reaction (RT-PCR). RT-PCR delivers results with good sensitivity and excellent specificity, but is expensive, prone to access challenges and is often slowed by transport to centralized testing laboratories. Antigen-based assays are inexpensive and can be rapidly mass-produced and deployed, with lateral flow assays (LFAs) being the most common inexpensive antigen test. To date, few antigen-detecting LFAs for COVID-19 have been commercialized. Herein, we present an open source LFA using commercially available antibodies and materials for the detection of SARS-CoV-2. Using an optical reader with comparable sensitivity to a visual read, the LFA yielded a Limit of Detection (LOD) of 23 TCID50/mL (95% CI of 9.1 to 37 TCID50/mL), equivalent to 1.4x105 copies/mL (95% CI of 5.5x104 to 2.3x105 copies/mL) irradiated virus in pooled nasal matrix. This LOD meets the criteria suggested by WHO for diagnosis of acute SARS-CoV-2 infection in a point of care format. A clinical evaluation and further testing is ongoing.
Subject(s)
COVID-19ABSTRACT
The SARS-CoV-2 pandemic has created an unprecedented need for rapid diagnostic testing to enable the efficient treatment and mitigation of COVID-19. The primary diagnostic tool currently employed is reverse transcription polymerase chain reaction (RT-PCR), which can have good sensitivity and excellent specificity. Unfortunately, implementation costs and logistical problems with reagents during the global SARS-CoV-2 pandemic have hindered its universal on demand adoption. Lateral flow assays (LFAs) represent a class of diagnostic that, if sufficiently clinically sensitive, may fill many of the gaps in the current RT-PCR testing regime, especially in low- and middle-income countries (LMICs). To date, many serology LFAs have been developed, though none meet the performance requirements necessary for diagnostic use cases, primarily due to the relatively long delay between infection and seroconversion. However, based on previously reported results from SARS-CoV-1, antigen-based SARS-CoV-2 assays may have significantly better clinical sensitivity than serology assays. To date, only a very small number of antigen-detecting LFAs have been developed. Development of a half-strip LFA is a useful first step in the development of any LFA format. In this paper we present a half-strip LFA using commercially available antibodies for the detection of SARS-CoV-2. We have tested this LFA in buffer and measured an LOD of 0.62 ng/mL using an optical reader with sensitivity equivalent to a visual read. Further development, including evaluating the appropriate sample matrix, will be required for this assay approach to be made useful in a point of care setting, though this half-strip LFA may serve as a useful starting point for others developing similar tests.