INTRODUCTION
The coronavirus disease 2019 (COVID-19) pandemic continues worldwide,
which is caused by severe acute respiratory syndrome coronavirus 2
(SARS-CoV-2) [1,
2]. The current variant of SARS-CoV-2 is
omicron (B.1.1.529), having high
transmissibility and evading human immunity
[3]. SARS‑CoV‑2 is a positive-sense
single-stranded RNA virus [4]. The
gold-standard method to detect SARS-CoV-2 is
nucleic acid amplification tests
(NAAT, here named PCR test) for the reverse transcribed cDNA from viral
RNA [5]. Considering the increased
number of SARS-CoV-2 infected people, the need for testing for potential
infection increased as well. In addition, it is possible to be infected
when a nucleic acid specimen is collected by other COVID-19 patients,
hence most people choose to use a rapid SARS-CoV-2 antigen self-test for
early diagnosis. Omicron appears to be more like a cold for some people,
with commonly reported symptoms including a sore throat, runny nose, and
headache [6]. The symptoms in each
individual are different due to various immunity backgrounds, hence
there is a war to fight against SARS-CoV-2 around the world. Early
diagnosis, early quarantine, and early therapy are gold rules to conquer
SARS-CoV-2 and COVID-19. Among them, early diagnosis is most important,
since it helps patients to adjust their lifestyle, protect some special
family members with basic diseases, and determine whether they need
therapies.
In vitro, early diagnosis depends on molecular tests, which
consists of PCR test, antibody assays, and antigen testing
[7, 8].
Here we focus on PCR tests and antigen testing. PCR test in principle is
reverse transcription quantitative PCR (RT-qPCR), which is the most
sensitive method to detect SARS-CoV-2. Regarding conventional PCR tests,
RNA including viral RNA and host RNA is prepared from respiratory
specimens, and then reverse transcribed to cDNA. qPCR is performed to
determine the Ct value of the viral RNA in specimens. Generally, the
whole procedure takes 3.5-4.0 hours not considering other affected
factors, and required professional technicians and instruments. For
large-scale PCR tests, normally specimen is collected on the first day,
and the output returns on the second day. Nasal and oral swabs are
commonly used for PCR tests. Due to discomfort associated with nasal
swab collection and the shortage of professional healthcare personnel,
an alternative specimen is from saliva (referred to as spit) which is an
extracellular fluid produced and secreted by salivary glands in the
mouth. Wyllie et al. reported that PCR tests with saliva
specimens are comparable to those with nasal swab specimens
[8], and it is reported that the
SARS-CoV-2 virus was detected in the first week of symptom onset.
Antigen test detects viral proteins, rather than viral nucleic acid
[8, 9].
Viral proteins from swabs are extracted with sample extraction buffer
and applied for nitrocellulose membrane to flow. The viral proteins will
be captured by antigen-specific antibodies coupled with colloidal gold
in the filter and detected by antigen-specific antibodies at the “T”
line. As a technique control, antibodies by flowing will be detected in
the “C” line suggesting the quality control of a correct flow. Rapid
antigen tests with colloidal gold can detect active infections within 15
min, quite faster than PCR tests. Another advantage of rapid antigen
test is that it does not require professional personnel. Currently, a
nasal swab is recommended for rapid SARS-CoV-2 antigen tests. Collection
of nasal specimens using swabs has discomfort and cannot be applied for
all age patients. Alternate specimens for rapid antigen tests are saliva
and phlegm. It was reported that saliva specimens were used for rapid
SARS-CoV-2 antigen test, but the sensitivity was not concluded against
nasal swab specimens. Different from saliva, phlegm is mucus produced by
the respiratory system searched to Wikipedia. It often refers to
respiratory mucus expelled by coughing. No conclusive study reported the
sensitivity of phlegm specimens against nasal swab specimens in rapid
antigen tests for SARS-CoV-2.
This study aims to compare the sensitivities of the methods: rapid
SARS-CoV-2 antigen test with nasal swab specimens and with phlegm
specimens. We demonstrate rapid antigen tests with phlegm specimens have
much higher sensitivity than with nasal swab specimens. The detection
time of active SARS-CoV-2 with phlegm specimens is advanced 12-42 hours
than with nasal swab specimens. Considering processing time in PCR
tests, phlegm antigen tests notify patients of the output report 10
hours earlier than PCR tests.