The nucleic acid test is "Is it infected now"; the antibody test is "Is it infected with J in the past."

On July 7, the Civil Aviation Administration, the General Administration of Customs and the Ministry of Foreign Affairs jointly issued an announcement requiring passengers flying to China (including foreigners and Chinese citizens) to provide a "new coronavirus nucleic acid test negative certificate" before boarding; specifically, Passengers are required to "complete the nucleic acid test within 20 days before boarding", "The test should be carried out at an institution designated or approved by the Chinese embassy or consulate abroad", "Chinese passengers take pictures and upload a negative nucleic acid test certificate through the international version of the epidemic prevention health code" .

1. What is nucleic acid testing?

Nucleic acid testing is currently the "gold standard" for new coronavirus testing, and it is also a direct evidence for judging whether a subject has a new coronavirus.

At this stage, the most commonly used method in clinical practice is fluorescent quantitative PCR. In simple terms, this method is to use probes containing specific gene information of the new coronavirus to find the corresponding sequence in the nasopharyngeal swab specimen collected by the patient. If it can be found, the test is positive; if it is not found, the test is negative.

The English word for nucleic acid test is "Nucleic Acid Test", abbreviated as "NAT; it is usually called "Viral Test" in the United States.

Nucleic acid testing usually uses nasopharyngeal swabs and oropharyngeal swabs to collect specimens. According to the information released by the Shanghai Municipal Health Commission【link】, “Although the viral load in the nasal cavity is higher than that of the oropharynx, the oropharyngeal swab sampling has the characteristics of convenient sampling and less pain for the examinee. Therefore, the specimen is usually obtained by the oropharyngeal swab in daily life. You can use it according to your own tolerance. Choose nasopharyngeal swab or oropharyngeal swab for the degree of acceptance. Because the viral load of nasopharyngeal swab is higher than that of oropharyngeal swab, you must cover your nose when you wear a mask.

2. What is antibody testing?

Antibody testing, as the name suggests, is used to detect new coronavirus-specific IgM/IgG antibodies in human serum, plasma, and venous whole blood samples. It is an indirect evidence to determine whether the subject has been infected with the new coronavirus.

Antibody detection has attracted attention due to its convenient operation and rapid detection, and is usually used as a supplementary method for nucleic acid diagnosis. 3-5 days after the onset of new type of pneumonia, serum specific antibodies are gradually produced. The first appearance is the immunoglobulin IgM antibody, which is produced in about 5-7 days; then the IgG antibody appears, which is produced in about 10-15 days.

Under normal circumstances, IgM antibodies are produced early. Once infected, they are produced quickly, maintained for a short time, and disappear quickly. A positive blood test can be used as an indicator of early infection. IgG antibodies are produced late, last for a long time, and disappear slowly. A positive blood test can be used as an indicator of infection and past infections.

The English word for antibody test is "Antibody Test", also called "Antibody Blood Test".

3. Nucleic acid detection VS antibody detection

The most intuitive difference: the former is a nasopharyngeal swab or an oropharyngeal swab, and the latter is a blood draw.

What are the advantages and disadvantages of the two detection methods?

Nucleic acid detection has the characteristics of early diagnosis, high sensitivity and specificity, but has its limitations. First, improper sampling, improper preservation of specimens, the use of different types of specimens, and the use of reagents from different manufacturers may result in "false negative" nucleic acid test results and missed diagnosis. Second, the detection takes a long time.

Antibody detection sampling is relatively more convenient and samples are more stable. The operation is simple and quick, especially the colloidal gold method. In layman's terms, a drop of blood can be observed for 15 minutes with the naked eye.

But antibody testing also has its limitations. One is that in the period of new coronavirus infection, the human body may not have produced antibodies yet, and the antibodies have a window of fusion. The second is that the sensitivity of the test kit is different, and false negative results will occur. Third, the antibody test may give false positive results due to the presence of interfering substances in the specimen, such as rheumatoid factor and other factors. This is also the "old difficulty" problem that has always existed in serological methods.

4. How to interpret the two test results?

According to the guidelines issued by the White House"Guidance on interpreting Covid-19 Test Results"As follows.

New crown test results	How to interpret?
Virus test positive	You are most likely infected with Covid-19 and can infect people
Virus test negative	You are most likely not infected with Covid-19 now
Positive antibody test	You may have been infected with Covid-19 before
Negative antibody test	You may be uninfected or just infected and have not developed antibodies
Virus test positive + antibody test positive	You are most likely infected with Covid-19 and can infect people
Virus test positive + antibody test negative	You are most likely infected with Covid-19 and can infect people
Virus test negative + antibody test positive	You may be infected with covid-19 for the first time, but you will recover
Virus test negative + antibody test negative	You may never have been infected with Covid-19

5. Where can virus (nucleic acid) testing be done in the United States?

Covid-19 testing is free, but due to limited resources, testing can only be done if it meets the CDC guidelines. When you make an appointment, you will be asked if you have had symptoms such as fever, cough, asthma, diabetes, pregnancy and other health conditions, and if you have a doctor’s advice You do Covid-19 testing and so on. The following places provide Covid-19 virus nucleic acid testing.

In addition to the above national chain stores, state governments and local pharmacy may also provide Covid-19 testing. See the Ministry of HealthThis page. If you are not sure whether they provide Viral Test or Antibody Test, it is recommended to call and ask before making an appointment; the website of the test site may also have instructions, such asCVS made it clear :

What type of test is performed?

Minute Clinic performs a PCR test_; which tests for active infections only. We do not administer antibody tests; which indicate whether the patient has been infected in the past.

6.The Chinese government requires returning passengers to provide a negative nucleic acid test certificate before boarding

China Civil Aviation Network News on July 7: In order to ensure the health and safety of international travel and reduce the risk of cross-border transmission of the epidemic, the Civil Aviation Administration, the General Administration of Customs, and the Ministry of Foreign Affairs jointly issued an announcement requiring passengers on flights to China to be tested negative for the new coronavirus nucleic acid Proof of boarding.

7. Can the U.S. nucleic acid testing meet the requirements of the Chinese government?

According to the announcement of the Chinese government, "testing should be carried out at an institution designated or recognized by the Chinese embassy or consulate abroad." It is understood that no institution has been "designated or accredited" by the embassy or consulate in the United States; some readers reported that they called Consulted with the embassy and consulate, the other party said that the policy has not been implemented yet; in addition, according to the CVS instructions, "rapid-result test" results within 30 minutes, while ordinary lab tests take 6-10 days, so if you want to return home Before doing so, remember to select "rapid-result test", because China's announcement requires nucleic acid testing to be completed within 5 days before boarding.

Compatriots who need to return home must not only prepare an international health code (check in every day for at least 14 days), but also perform nucleic acid testing.

Teach you to punch in the international health code