▎ WuXi AppTec content team editor
Recently, China's fight against the new crown epidemic in many places has ushered in another round of challenges.
In order to further optimize the detection strategy of the new crown virus, according to the needs of epidemic prevention and control, the comprehensive group of the joint prevention and control mechanism of the State Council of The People's Republic of China decided to start promoting the monitoring mode of "antigen screening and nucleic acid diagnosis", and add antigen detection as a supplement to nucleic acid detection.
So, what exactly is antigen detection and how is it done? Let's take a brief look at science today.
What is the difference between COVID-19 antigen testing and nucleic acid testing?
To illustrate this issue, let's first look at the structural diagram of the new crown virus.
Image source: References[2] (translated by WuXi AppTec content team)
In simple terms, antigens are substances that can induce the body to produce an immune response, and the antigens of the new crown virus include 4 specific proteins.
If you think of the new coronavirus as a lychee, the antigen is its "peel" (S protein, M protein, E protein) and "membrane" (N protein), while nucleic acid (RNA) is "pulp".
Therefore, antigen detection is the use of antibodies and antigen binding principle, to see if there is a virus "peel" and / or "membrane" presence, the current approval of the new crown antigen test, generally to detect N protein mainly [3].
Nucleic acid testing, on the other hand, depends on whether the virus has left the pulp fragments (nucleic acid gene fragments) behind.
The China Medical Administration pointed out in the Policy Interpretation of the Application Plan for The Detection of New Coronavirus Antigens (Trial) that:
Nucleic acid testing is still the basis for confirming the diagnosis of new crown virus infection, and antigen testing can be used as a supplementary means for screening specific populations, which is conducive to improving the ability of "early detection".
Where primary level medical and health establishments have nucleic acid testing capabilities, nucleic acid testing shall be preferred; those that do not have nucleic acid testing capabilities may conduct antigen testing, and do a good job of medical personnel training and patient communication and guidance.
Isolation observers and community residents conducting antigen testing shall carefully read the instructions and standardize operations, and once the antigen test is positive, they should immediately report to the relevant departments; when necessary, nucleic acid testing should be carried out to confirm.
In other words, although antigen detection cannot replace nucleic acid detection as a basis for confirming the diagnosis, its main advantages are:
Fast speed (usually within 30 minutes of the results);
Saves material, time and labor costs, antigen testing in suspected populations, and if positive, early shunting and rapid management;
Home self-testing is convenient and can also reduce the risk of infection caused by queuing up for nucleic acid personnel to gather.
Who needs antigen testing and who doesn't?
The "Application Plan for Antigen Detection of The New Coronavirus (Trial)" points out [4] that there are three types of people who are suitable for antigen testing:
1． A person who goes to a primary medical and health institution and has symptoms such as respiratory tract and fever and has symptoms within 5 days
Among them, institutions with nucleic acid detection capabilities prefer nucleic acid detection; if they do not have nucleic acid testing capabilities, antigen testing is carried out.
Image credit: 123RF
2. Isolate observers
This includes home isolation observation, close and sub-close contact, entry isolation observation, sealing area and personnel in the control area.
During the isolation and observation period, nucleic acid testing is carried out in accordance with the requirements of the current relevant prevention and control programs, and antigen self-testing is carried out once a day for the first 5 days.
3. Residents of communities with antigen self-testing needs
Community residents who have self-testing needs can purchase antigen detection reagents for self-testing through retail pharmacies, online sales platforms and other channels.
So, which community residents need to do it, and which residents don't need to do it?
The Basic Requirements and Procedures for Self-Testing for Antigens of the New Coronavirus states that antigen testing is generally used during the acute infection period, that is, sample testing of suspected populations within 7 days of the onset of symptoms [5].
On March 15, Li Jinming, deputy director of the Clinical Laboratory Center of the National Health Commission of China, said at a press conference that in areas with low prevalence of the new crown virus, the antigen test of the new crown virus will detect more false positives [6], so it is recommended:
Antigen testing should be used in high-risk, high-prevalence clusters of infected populations [7].
If the general population does not have symptoms and does not have a relevant epidemiological travel history, do not do the new crown virus antigen test.
Antigen testing may be more accurate if measured several times
Since rapid antigen testing has limited evidence of screening effects on asymptomatic populations, is there any way to improve the accuracy of results?
Recently, a study published in JAMA Network Open, a sub-journal of the Journal of the American Medical Association[8], compared the rapid antigen screening data and nucleic acid test results of nearly 180,000 asymptomatic people and found that:
In screening for asymptomatic populations, two consecutive positive antigen tests (1 hour apart) can greatly improve the accuracy of positive results compared with 1 positive antigen.
Put simply, of the nearly 180,000 asymptomatic people who participated in the local workplace screening program, 623 (0.35% of the total population) tested positive for the first time, and only 38% of nucleic acid retests were true positive.
After 1 hour, the vast majority of people (569 people) who were positive for the first antigen did the second antigen test, 40% of them were still positive, but 92% of them were true positive for nucleic acid re-examination; the remaining 60% became negative for the second time, of which 95% were true negative after nucleic acid re-examination.
Combining true positive and true negative data, the overall estimated accuracy of the second antigen test was 94%.
Of course, the paper also objectively mentions the limitations of the study. For example, antigen test results are usually more accurate when community infection rates are higher, and the populations targeted in this study have no symptoms.
"But regardless of the accuracy of a single test, the diagnostic value of the second antigen test is still high."
In addition, the specific values in this study can only represent the situation of these populations. In other populations, false positives and true positives are affected by two factors, one is the actual prevalence of the population, and the other is the antigen detection products used (the sensitivity and specificity data of different products are usually slightly different).
How do I perform an antigen self-test?
1. Prepare before self-testing
Mainly wash your hands with hand sanitizer, read the instructions to clarify the process, check whether the reagents are not within the shelf life, and whether the reagent supplies are missing or damaged.
Colloidal gold test strip detection is generally required to be at 14 °C-30 °C at room temperature, so it is necessary to avoid too cold, too hot or too humid environment, otherwise it may lead to abnormal test results.
Image source: References[5] (refer to the specific kit manual)
2． sampling
In primary medical institutions, antigen testing has three kinds of nasopharyngeal swabs, oropharyngeal swabs and nasal swabs; while home self-testing is only in the form of nasal swabs (be prepared to poke the nostrils).
(1) People over 14 years old: take samples by yourself
Blow off the snot with toilet paper first. Carefully unpack the nasal swab and avoid hand contact with the swab head.
Then tilt the head slightly, squeeze the tail of the swab with one hand, enter against one side of the nostril, slowly go backwards along the bottom of the lower nasal passage for 1-1.5 cm, rotate at least 4 times against the nasal cavity (stay time is not less than 15 seconds), and then repeat the same operation on the other nasal cavity with the same swab.
(2) 2-14 years old people: other adults are required to take samples on their behalf
When sampling, the nose is also blown off with toilet paper, followed by a slight tilt of the head.
The person taking the sample carefully unwrapped the outer packaging of the nasal swab, avoiding contact with the swab head with the hand and gently holding the head of the person being collected with one hand. The remaining steps are the same as before, and the depth is recommended to be controlled at 1 cm.
Image source: References[5] (refer to specific kit instructions)
3. Antigen detection
Immediately after the sample is collected, place the nasal swab into the sampling tube.
The swab head should be mixed with rotation in the stock solution for at least 30 sec, while squeezing the swab head at least 5 times by hand across the outer wall of the sampling tube to ensure that the sample is sufficiently elute in the sample tube.
After squeezing the swab head liquid dry by hand across the outer wall of the sampling tube, discard the swab. After the sampling tube is covered, the liquid inside is dripped vertically into the test card sample well.
Then follow the waiting time on the instructions and wait to see the results.
4. Interpretation of results
The positive is "two bars", the negative is "one bar" at "C", and there is no bar at "T".
If there is no bar at "C", whether "T" has a bar or not, it is an invalid result and needs to be re-tested.
Antigen positive or negative, how to deal with it?
1. Self-testing found positive antigen
Regardless of whether there are respiratory, fever and other symptoms, it must be reported to the community (village or town), and the community (village) contact the emergency center to transfer residents to a medical institution that has set up a fever clinic for nucleic acid testing in accordance with the guidelines for the transfer of personnel related to the new crown pneumonia epidemic.
2. Self-testing detected negative antigen
If asymptomatic: close observation, antigen testing or nucleic acid testing if necessary;
If there are symptoms (suspected population): it is recommended to go to a medical institution with a fever clinic as soon as possible for nucleic acid testing; if it is inconvenient to seek medical treatment, you should self-isolate at home, avoid going out, and conduct antigen self-testing once a day for 5 consecutive days.
Note here that quarantined people (whether positive/negative) and community residents self-test positive personnel, used sampling swabs, sampling tubes, test cards, etc. should pay attention to retention, and then the staff will be packed into sealed bags and disposed of as medical waste.
The test supplies used by the self-test negative personnel can be packed into a sealed bag and disposed of as general garbage.
3. The nucleic acid test results of the suspected population are confirmed
Positive nucleic acid: Regardless of whether the antigen test is positive or negative, appropriate measures should be taken according to the person infected with the new crown virus or the patient diagnosed with new crown pneumonia.
Negative nucleic acid and positive antigen test: Treat people infected with the new crown virus, take measures such as centralized isolation, close observation, and continuous nucleic acid testing.