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.