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).