RESULTS
Rapid antigen test with phlegm specimens detects SARS-CoV-2
more sensitively than with nasal swab specimens
To know the performance of the rapid antigen test using phlegm specimens
in SARS-CoV-2 detection, we compared its sensitivity with that using
popular nasal swab specimens. In this test, the “T” band from the
phlegm specimen was very strong, but invisible from the nasal swab
specimen (Figure 1A, test 1 vs test 2). This result indicated
that the patient was infected by SARS-CoV-2, which was detected by the
rapid antigen test with the phlegm specimen but not with the nasal swab.
The patient continued the antigen test with nasal swab specimens (Figure
1A, test 3), and interestingly found 12 hours later the test with a
nasal swab specimen was positive. This result reminded us that the
sensitivity in detection of SARS-CoV-2 using rapid antigen tests with
phlegm specimens is higher than with nasal swab specimens. Hence, we
serially diluted the phlegm specimen 3-fold for more tests (Figure 1B).
The 2187-fold dilution showed a weak “T” band (Figure 1B, test 9)
compared to the fewer dilutions (Figure 1B, test 2-8), and the 6561-fold
dilution had similar levels of SARS-CoV-2 antigen to those of the
original nasal swab specimen (Figure 1B, test 10 vs test 1), suggesting
the sensitivity in detection of SARS-CoV-2 using an antigen test with
phlegm specimens was about 6000-fold higher than that with nasal swab
specimens in this case. These results confirmed that rapid SARS-CoV-2
antigen tests with phlegm specimens have higher sensitivity in detection
of SARS-CoV-2 than with nasal swab specimens and could be an alternative
evaluation technique for COVID-19 diagnosis.
Rapid antigen test with phlegm specimens accurately detects
SARS-CoV-2
Among the total of 41 volunteers who volunteered for
SARS-CoV-2 tests, 13 patients had
a scratchy sensation in the throat when specimens were collected, and
their phlegm specimens were positive, but the matched nasal swab
specimens were negative; One patient with slight fever had both phlegm
and nasal swab specimens tested positive; Seventeen patients with early
symptoms of COVID-19 had phlegm specimens tested positive with lack of
nasal swab specimens; the rest 10 volunteers without any COVID-19
symptom showed negative phlegm specimens in antigen tests. All 31
patients with detected positive phlegm specimens had typical COVID-19
symptoms in a later stage indicating the accuracy of rapid antigen tests
using phlegm specimens was 100%. Considering the collection of phlegm
specimens is much milder and easier than that of nasal swab specimens,
these results suggested that rapid antigen tests for SARS-CoV-2 with
phlegm specimens had high accuracy and can be a good substitute for
those with nasal swab specimens.
Rapid antigen test with phlegm specimens earlier detects
SARS-CoV-2 infections than with nasal swab specimens
We have demonstrated that the rapid SARS-CoV-2 antigen test with phlegm
specimens is more sensitive than that with nasal swab specimens. Hence
it is possible that patients can use this test to determine whether they
are infected by SARS-CoV-2 earlier than with nasal swab specimens. To
test this hypothesis, we chose 3 patients who had positive phlegm in the
SARS-CoV-2 antigen test and negative nasal swab to continue antigen
tests with nasal swab specimens (Figure 2). Patient 1 had a positive
nasal swab specimen 13 hours after SARS-CoV-2 was first detected in the
phlegm specimen (Figure 2A, test 4 vs test 2), suggesting the SARS-CoV-2
antigen test with the phlegm specimen detected viral antigen earlier
than with the nasal swab specimen for 13 hours. Similarly, Patient 2 and
Patient 3 were determined as SARS-CoV-2 infected cases by phlegm
specimens earlier than by nasal swab specimens for 24 hours (Figure 2B
test 5 vs test 1) and 42 hours (Figure 2C test 5 vs test 1)
respectively. Taken together, rapid antigen tests with phlegm specimens
detect SARS-CoV-2 12-42 hours earlier than those with nasal swab
specimens. Considering the rule of early detection, the better specimen
for rapid SARS-CoV-2 antigen tests is from phlegm, but not from nasal
swabs.
Rapid antigen test with phlegm specimens earlier notifies
patients of SARS-CoV-2 infections than PCR test
Since PCR test is a gold standard in SARS-CoV-2 detection and phlegm
specimens in rapid antigen test enhance sensitivity in detection, next
we would like to know which of the two methods is better for detection
of SARS-CoV-2. We picked a volunteer who was exposed to a positive case
in the early stages of COVID-19 for further study. On Day 0, the
volunteer had a negative phlegm specimen by the rapid SARS-CoV-2 antigen
test (Figure 3, test 1), and the PCR test was notified of a negative
result on the next day (Figure 3, 0 h). On Day 1, the volunteer
collected phlegm specimens for antigen tests and determined that the
sample was positive (Figure 3, 0 h). The SARS-CoV-2 antigen levels
increased in a later stage shown by the stronger “T” bands with time
increment (Figure 3, test 2, 4, 5, 7). On Day 2, 35 hours later, his
antigen test with a nasal swab showed a positive result (Figure 3, test
12), as confirmed our conclusion that phlegm specimens in rapid antigen
tests determine SARS-CoV-2 infection earlier than nasal swab specimens
for about 12-42 hours. More interestingly, the specimen was collected
from the patient for a PCR test on Day 1 when the phlegm specimen was
first positive (Figure 3, 0 h), and was notified of a positive result 11
hours (Figure 3, 11 h). Hence, technically rapid antigen tests with
phlegm specimens earlier notify patients of a positive result than PCR
tests, though the two tests have a similar sensitivity in detection of
SARS-CoV-2.
Rapid antigen test with phlegm specimens facilitates
monitoring health of COVID-19 patients
Considering rapid antigen tests with phlegm specimens can identify the
early SARS-CoV2 infection, it is possible to monitor the health
conditions of COVID-19 patients using this test. On Day 1, one volunteer
had a positive phlegm specimen in the rapid antigen test (Figure 4, test
1). Due to the weak “T” band in the antigen test with the phlegm
specimen and negative nucleic acid sample two days ago, we deduced Day 1
was the earliest time in detection of SARS-CoV-2 in the patient.
Simultaneously, the patient had a PCR test. Since the report of PCR
tests took time, but the patient was notified of a positive nucleic acid
RNA of SARS-CoV-2 on Day 2, 10 hours after the first phlegm specimen was
positive (Figure 4, 10 h vs 0 h), as is consistent with the
previous result (Figure 3, 11 h vs 0 h). On Day 2, the phlegm
specimens remained positive, but the nasal swab specimen tested positive
21 hours after the first phlegm specimen was positive (Figure 4, test 9vs test 1). The patient was then thirsty and had a slight cough
and an increased body temperature (37.9 °C) 29 hours after the first
phlegm specimen was positive (Figure 4, 21 h and 29 h). Hence, rapid
antigen test with phlegm specimens facilitates monitoring health of
COVID-19 patients.
Rapid antigen test with phlegm specimens better determines
patient recovery from SARS-CoV-2 infection
The sensitivity in detection of SARS-CoV-2 using phlegm specimens in
rapid antigen tests is higher than using nasal swab specimens, and
comparable to that using oropharyngeal swab specimens in PCR tests.
Hence, it is possible that a rapid antigen test with a phlegm specimen
could be used to determine whether a patient is recovered from
SARS-CoV-2 infection. To this end, we investigated the virus antigen
levels using rapid antigen tests with phlegm specimens in 3 volunteers,
and the tests with nasal swab specimens were used as a contrast (Figure
5). These volunteers were all infected by SARS-CoV-2 and had common
symptoms of COVID-19 by omicron, such as a sore throat, runny nose, and
headache. Till Day 9 – Day 11, SARS-CoV-2 was still detectable, but the
titer was very low, as shown by the weak “T” band in the rapid antigen
test with phlegm specimens (Figure 5A test 1 & 3, 5B test 1, 5C test
3), but not with nasal swab specimens (Figure 5A test 2 & 4, 5B test 2,
5C test 4). Depending on the variation of immunity background, each
patient may take a different time to recover. As expected, we found
while the phlegm specimen was negative in the rapid antigen test, the
nuclei acid sample was simultaneously negative in the PCR test on the
same day for both patients (Figure 5B, test 3 and green▽; 5C test 5 and
green▽). Taken together, rapid SARS-CoV-2 antigen tests with phlegm
specimens better direct patients’ recovery from COVID-19 infection.