The COVID-19 pandemic, caused by the SARS-CoV-2 virus, has had a profound impact on global health. Since the identification of the first cases in Wuhan in December 2019, the scientific community has been working tirelessly to develop accurate and reliable diagnostic tests for the virus. In this context, this article provides a comprehensive review of the historical development, updates, and guidelines regarding diagnostic methods for COVID-19, including antigen, molecular, and serology tests.
Historical Context and Diagnostic Test Development
The journey of SARS-CoV-2 diagnostic test development has been swift and intense. After the first genome sequence release, the U.S declared a Public Health Emergency (PHE) in early 2020, which was subsequently declared a pandemic. By September of the same year, numerous SARS-CoV-2 Nucleic Acid Amplification Tests (NAAT) became available, and over 250 U.S FDA Emergency Use Authorization (EUA) diagnostic tests were in use. As of August 2023, this number has expanded to over 439 FDA EUAs, and two NAATs have received full FDA authorization.
The transition from EUA to 510(k) test approval began in May 2022, when the U.S PHE ended. This period also marks the significant milestone of nearly 6 billion COVID-19 NAAT and antigen tests estimated to have been completed globally by mid-2022.
NAAT & Antigen Testing
NAAT and antigen testing have played crucial roles in diagnosing SARS-CoV-2 infections. A recent study conducted between October 2021 and January 2022 found that the performance of antigen tests was optimized if symptomatic persons were tested twice and asymptomatic persons were tested three times. It was also observed that SARS-CoV-2 viral load peaks around day 4 in a highly immunized population.
Specimen Source and Collection for Molecular Testing
The Infectious Diseases Society of America (IDSA) suggests that for symptomatic individuals suspected of having COVID-19, swab specimens should be collected and tested from either the nasopharynx, anterior nares, oropharynx, or midturbinate regions; saliva, or mouth gargle. The performance characteristics of NAAT differ depending on the sample site, with sensitivity and specificity varying for saliva, oropharyngeal swab, anterior nares swab, combined AN/OP swab, midturbinate swab, and mouth gargle.
For symptomatic individuals suspected of having COVID-19, anterior nares and midturbinate swab specimens may be collected for SARS-CoV-2 NAAT by either patients or healthcare providers. Notably, a high concordance was found between Ct value results of self-collected versus healthcare worker collected nasal swabs.
Antigen Testing and Self-Collection
The IDSA panel suggests either observed or unobserved self-collection of swab specimens for antigen testing if self-collection is performed. However, unsupervised individuals may misinterpret rapid antigen test results, as participants often misread positive antigen results and did not always understand how to interpret or respond to test results.
Use of NAAT to Reduce SARS-CoV-2 Transmission
The IDSA panel advises against routine SARS-CoV-2 NAAT in asymptomatic individuals without a known exposure to COVID-19 who are being hospitalized or undergoing a medical or surgical procedure. The panel also suggests against routinely repeating NAAT in patients with COVID-19 to guide release from isolation.
Serologic Testing
Serologic testing provides informationabout past or current infection with the virus. The global seroprevalence of SARS-CoV-2 antibodies is nearly 100%, while in the US, the infection-induced seroprevalence of SARS-CoV-2 antibodies (anti-nucleocapsid) is nearly 80%. However, the role of serologic testing in COVID-19 diagnosis and management has been limited due to several factors.
Firstly, the IDSA recommends against using IgG antibodies to provide evidence of COVID-19 infection in symptomatic patients with a high clinical suspicion and repeatedly negative NAAT. Secondly, while serologic testing can assist with the diagnosis of multisystem inflammatory syndrome in children (MIS-C), it is recommended that both IgG antibody testing and NAAT be used to provide evidence of current or recent past COVID-19 infection.
When evidence of previous SARS-CoV-2 infection is required, the IDSA suggests testing for SARS-CoV-2 IgG, IgG/IgM, or total antibodies three to five weeks after symptom onset and against testing for SARS-CoV-2 IgM. Furthermore, when evidence of prior SARS-CoV-2 infection is desired, the IDSA suggests using serologic assays that target nucleocapsid protein rather than spike protein.
For individuals with previous SARS-CoV-2 infection or vaccination, the IDSA suggests against routine serologic testing given no demonstrated benefit to patient-important outcomes. Serologic testing may be useful for diagnosing MIS-C in pediatric patients, especially when NAAT or antigen testing is negative, to provide evidence of recent COVID-19 infection.A negative spike antibody test may identify immunocompromised patients who are candidates for immune therapy such as convalescent plasma or monoclonal antibodies, or to prioritize the administration of monoclonal therapies when supplies are limited.
In conclusion, NAATs remain the most sensitive tests for diagnosing COVID-19, and the sensitivity of antigen tests can be improved by repeat testing. For molecular testing, anterior nares and oropharynx alone are acceptable specimens but they have lower sensitivity than other sources. Optimal testing algorithms for diagnosis and identifying resolution of infectivity are likely to continue to evolve as population immunity increases.
