Antigen detection assay

Another COVID-19 detection method involves the direct detection of SARS-CoV-2 viral particles using immunoassays (Ji et al., 2020). The SARS-CoV-2 nucleocapsid protein may be detected in nasopharyngeal swabs and urine samples of COVID-19 patients within 3 days of onset of fever (Diao et al., 2020).
A Cochrane systematic review found that sensitivity varied considerably across studies (from 0–94%). Based on eight evaluations in five studies on 943 samples, the average sensitivity was 56.2% (95% CI 29.5–79.8) and average specificity was 99.5% (95% CI 98.1–99.9) (Dinnes et al., 2020). Data for individual antigen tests were limited, with no more than two studies for any test. There were no studies in asymptomatic individuals.
For asymptomatic individuals, a non-peer-reviewed study showed that for a pre-test probability of 5%, the negative predictive value was 99.6% (95% CI 99.5–99.7) and the positive predictive value was 81.5% (95% CI 65.0–93.2) (Alemany et al., 2020). At this pre-test probability, the estimated number of false-negative and false-positive values per thousand tests were 4 (95% CI 3–5) and 12 (95% CI 4–27), respectively. The authors stressed the need for confirmatory testing of positive tests with nucleic acid amplification techniques in these circumstances (Alemany et al., 2020).
In comparison with rRT-PCR, rapid antigen detection tests tend to have a lower sensitivity, and owing to the increased risk of false-negative results, some authors consider such tests only as an adjunct to rRT-PCR tests (Siam et al., 2020). Alternatively, antigen detection tests have the advantage of being simple to perform and can play a role in settings where accessibility to rRT-PCR tests is limited or in symptomatic patients with a high viral load and within the first 5–7 days after symptom onset (Lai and Lam, 2020). The viral load is directly related to the sensitivity of the test (Dinnes et al., 2020).

Antibody assays

Serologic tests are essential because they provide information on patients who have been infected and already recovered, and asymptomatic patients who were never diagnosed (Ravi et al., 2020). In a study that followed the immunological response in COVID-19 patients, three types of seroconversion were observed: synchronous seroconversion of IgG and IgM (nine patients), IgM seroconversion earlier than that of IgG (seven patients), and IgM seroconversion later than that of IgG (ten patients) (Long et al., 2020). A study profiling the early SARS-CoV-2 humoral response found that the median time for IgM detection was five days after symptom onset; IgG was detected at a median of 14 days after symptom onset (Guo et al., 2020).
For SARS-CoV-2, the IgG and IgM produced specific to the S and N proteins are of particular diagnostic interest. A study indicates that the S protein tends to cause a more significant immune response than the N protein, eliciting neutralizing antibodies (Amanat et al., 2020). However, other studies argue that the N protein is more immunogenic, as it is expressed abundantly during active infection (Johns Hopkins Center for Health Security, 2020).
Some examples of serologic tests to measure patient antibodies include rapid diagnostic tests (RDTs), enzyme-linked immunoassays (ELISAs), chemiluminescent immunoassays (CLIAs, not to be confused with the CLIA acronym: Clinical Laboratory Improvement Amendments), and neutralization assays (Ravi et al., 2020), performed only at specialized laboratories. Another review (Deeks et al., 2020) found that some differences were noted by test technology, with CLIA methods appearing more sensitive (97.5%, 95% CI 94.0–99.0) than ELISA (90.7%, 95% CI 83.3–95.0) or colloidal gold immunoassay (CGIA)-based lateral flow assays (90.7%, 95% CI 82.7–95.2) for IgG/IgM (there were also differences in sensitivity for IgG but no differences for IgM). There was little clear evidence of differences in specificity between technology types.
Essential considerations for antibody testing include timing of the test, previous infection, immune status of the individual, and cross-reaction, which can alter the test results (Siam et al., 2020).

CRISPR technology

The CRISPR gene-editing tool has been utilized to construct an accurate, faster, and simple-to-use SARS-CoV-2 detection test. The DNA Endonuclease-Targeted CRISPR Trans Reporter (DETECTR) assay is based on CRISPR–Cas12 and can distinguish SARS-CoV-2 with no cross-reactivity for related coronavirus strains, using N gene genomic RNA, within 40 minutes (Broughton et al., 2020).

Loop-mediated isothermal amplification (LAMP)

Loop-mediated isothermal amplification (LAMP) is a method of isothermal DNA replication. It utilizes six DNA oligonucleotides that hybridize with eight different regions of a target molecule in an accelerated format. Reverse transcriptase can be included to improve sensitivity within the reaction when detecting an RNA target (RT-LAMP), such as SARS-CoV-2 RNA (Rabe and Cepko, 2020).