DISCUSSION
Early and accurate diagnostic tests are essential to curb the spread of SARS-CoV-2 and control the ongoing COVID-19 pandemic. Since the current variant of the SARS-CoV-2 virus has high transmissibility and evades human immunity, it is urgent to determine whether one is infected by SARS-CoV-2 who has some early common COVID-19 symptoms such as cough, low fever, and headache. Early and accurate detection helps patients to adjust their lifestyle by increasing sleep, stopping alcohol and smoking, separating them from young or old family members or members having basic diseases, and storing some required foods and medicines for recovery time. Considering the reduction of available nuclei acid test sites and cross infections by other positive patients, people tend to have a self-test with rapid SARS-CoV-2 antigen test with nasal swab specimens at home to know whether they are infected.
PCR test has been a gold standard to diagnose SARS-CoV-2 infection because of its sensitivity. However, it has shortcomings such as false negative results, crossing infections, long procedure time, and professional personnel and instrument involvement. As a substitutive test, rapid SARS-CoV-2 antigen tests are carried out using a specimen of nasal swabs. The rapid antigen test has several advantages such as high accuracy, portable use, and no requirement for professional background. However, the essential shortcoming is the low sensitivity in detection of SARS-CoV-2 compared to the PCR test. Currently, our research updated the collection method of specimens and used phlegm samples for rapid SARS-CoV-2 antigen tests. Using this new sampling method, we demonstrated the sensitivity in detection of SARS-CoV-2 in rapid antigen tests is about 6000-fold higher than that using nasal swab specimens. Importantly this sensitivity enhancement of rapid antigen tests with phlegm specimens made it comparable to PCR tests for diagnosing SARS-CoV-2 infection. Two reasons possibly contribute to the elevated sensitivity. First, the common symptom caused by the omicron variant of SARS-CoV-2 is sore throat suggesting virus titer is high in throat secretion; Second, more viruses are picked in the tests by dipping phlegm.
Using the rapid antigen test with phlegm specimens, from 41 volunteers we diagnosed 31 positive cases of SARS-CoV-2 infection, and these cases all developed as typical COVID-19 suggesting the method has 100% accuracy. Thirteen cases showed negative rapid antigen tests with nasal swab specimens but positive phlegm specimens, indicating rapid antigen tests with phlegm specimens can earlier detect SARS-CoV-2 infection cases than with nasal swab specimens. From 6 cases, we found the advanced time by rapid antigen test with phlegm specimens is about 12-42 hours compared to the tests with nasal swab specimens (Figure 1-4). The advanced time is a big window for patients to arrange the following plan and prevent their family members from further infection, and for physicians to start specific therapy against severe symptoms at the early stage of COVID-19 development. Even though the antigen test with phlegm specimens and the PCR test detected SARS-CoV-2 at the same time, technically the former test notified of the output result ~10 hours earlier than the latter one (Figure 3, 4). During recovery from COVID-19, antigen tests with phlegm specimens better determine a negative time, which is comparable to PCR tests (Figure 5). It helps patients to wisely make decisions for recovery avoiding the side effects of improper treatments. Taken together, antigen tests with phlegm specimens accurately and early detect SARS-CoV2, and better determine the recovery time suggesting a good aid to monitor the health conditions of COVID-19 patients.