Will eating oranges turn you positive? Is antigen testing accurate? Here's everything you need to know!

Recently, a video on the Internet has attracted a lot of attention:

Someone dropped a drop of orange juice squeezed from a fresh orange onto the new Guan antigen test plate. Soon, a purple-red horizontal line appeared in the T area. The orange juice was also "positive"! Many people exclaimed, is there a fake antigen test? Will it be possible to do antigen tests in the future?

A netizen used orange juice to do an antigen test at home, and the result was positive. Source: Screenshot from the Internet

First, the conclusion:

Orange juice, tangerine juice, or even some other fruit juices may all test positive. This is not because the antigen test is fake, but because the test method is problematic - the antigen test kit tests nasal swab samples, not orange juice. If you insist on using orange juice for the test, it is normal for it to show a positive result. This is a " false positive ".

Why does this happen? Let’s start with what antigen testing is.

01

Why are antigen tests effective?

There are currently three main methods for testing whether there is a virus in a sample: nucleic acid testing, antigen testing , and antibody testing . The first two are both direct methods for detecting the virus.

The novel coronavirus is an RNA virus whose genes encode multiple structural protein shells, mainly four types: surface spike protein (S), membrane protein (M), envelope protein (E) and nucleocapsid protein (N). Technically, by quickly detecting the presence of structural protein antigens (detecting N protein antigen or detecting N and S protein antigens), it can be directly proved that the sample contains the novel coronavirus.

Schematic diagram of the novel coronavirus structure. Image source: J. Med. Chem. 2020, 63, 21, 12359-12386

Antigen detection reagents are generally suitable for samples from infected areas, such as nasal swabs . The S protein is on the surface of the virus, and it determines whether the coronavirus can infect humans or even further spread from person to person. It is easier to capture in the test, but the S protein is prone to mutation; while the N protein is on the surface of the virus nucleocapsid, and its gene sequence is relatively conservative and stable, and is not prone to mutation. Double antigen detection of N protein and S protein can effectively ensure the detection of the new crown virus.

02

Key features of antigen tests

At a press conference on March 15, 2022, Li Jinming, deputy director of the Clinical Laboratory Center of the National Health Commission, introduced that the sensitivity of the antigen detection reagents approved by the country is between 75% and 98%, and the specificity is between 95% and 99%.

In other words, the answer to the question in the title about whether the antigen test is accurate is that if used correctly, it is quite accurate .

Antigen testing has four characteristics:

The first is convenience . The rapid antigen test kit can be used for self-testing at home according to the instructions and operating procedures.

The second is speed . Antigen testing takes 10-20 minutes to produce results.

The third is economic . Antigen testing does not require any instruments or equipment, does not require professional personnel to collect samples, and is currently cheap, with low overall social costs.

Fourth, individuals can undergo testing independently , avoiding the risk of cross-infection when people queue up for screening.

The "New Coronavirus Antigen Self-Test Basic Requirements and Process" states that the new coronavirus antigen test is generally used for sample testing during the acute infection period, that is, within 7 days of the onset of symptoms in suspected people . Both positive and negative results of the suspected people should be further tested for nucleic acid. The positive result can be used for early diversion and rapid management of the suspected people, but it cannot be used as a basis for confirming new coronavirus infection.

As a supplementary means of screening specific populations, antigen testing can be used for screening in multiple scenarios, which is conducive to greatly improving the ability of "early detection", screening out possible infected people at the first time, and helping to control the source of infection in a timely manner.

Because of its strong stability and simple operation, antigen testing can detect infected people faster, take isolation measures earlier, and demarcate control areas when facing multiple scattered or clustered epidemics.

03

Why does dropping orange juice cause a "false positive"?

Antigen detection rapid test strips use the latex method or colloidal gold immunochromatography method. When the sample extract is dropped onto the reagent strip, it will penetrate forward, passing through the T area, which is the test area, and the C area, which is the quality control area.

If the extract contains the novel coronavirus, the protein on its surface will combine with the corresponding protein antibody in the test area due to some complex physical processes, showing a red mark. Therefore, when the T area and C area show red at the same time, it means the result is positive.

So, what happens when orange juice is used as the sample extract?

Simply put, orange juice is rich in vitamin C. Vitamin C has a strong reducing effect. It will react with the chloroauric acid on the antigen test rapid test strip, causing abnormal aggregation of colloidal gold and precipitation of some colloidal gold. Because the surface of the T zone is relatively rough and bumpy, these precipitated colloidal gold may settle in it, showing a light red color . This is how the "false positive" that shocked everyone in the online video appeared.

The main components of sample extracts may vary from brand to brand. If the extracts are used interchangeably, or if the extracts and human samples specified in the instructions are not used, the optimal performance of the reagents cannot be guaranteed and will cause a disorder in the test system.

04

How to use antigen testing accurately?

At this point, some people may be worried again: So before doing the antigen test, do we not eat oranges, drink orange juice, or eat other foods rich in vitamin C?

In fact, this worry is unnecessary because current antigen tests mostly use nasal swabs when taking samples.

Even if a person eats a lot of oranges, it is unlikely that the orange juice will enter the nasal cavity - even if a little bit of it goes in, the concentration will not be too high. Therefore, there is no need to worry about affecting the results of the antigen test because of eating oranges or drinking orange juice.

It is not difficult for individuals to use antigen testing accurately. Detailed instructions for operation are available in the kit instructions. You can also save the following picture for reference:

Schematic diagram of the novel coronavirus antigen test operation. Photo provided by the author

In order to better assist in epidemic prevention and control, after the test is completed, the user can upload the results on the relevant platform through the unique identification code on the antigen test reagent plate.

Schematic diagram of uploading the results of the new coronavirus antigen test. Photo provided by the author

05

10 points to note when using the test kit

When using the new coronavirus antigen test kit, you don't need to pay attention to your diet. The following 10 points are what you need to pay attention to:

1. Check whether the outer packaging is intact and whether it is damaged or has been opened.

2. Check the production date and expiration date. Do not use after the expiration date.

3. Observe whether there is a medical device registration certificate number approved by the State Food and Drug Administration.

4. Check whether it contains the complete UDI code.

5. After unpacking, check whether the product components are complete, including: reagent plate (the smallest usage unit must have a digital code), lysis solution or sample extraction solution (different test kits have different names), disposable sampling swab, and instruction manual.

Product composition of the novel coronavirus antigen detection kit. Image provided by the author

6. To avoid missing variant strains, test kits with the intended purpose of detecting N and S dual antigens and a minimum detection limit of S1-S7 can be used if conditions permit. The sensitivity and specificity values ​​of the reagents are marked in the instructions of the test kit. Reagents with a detection threshold of less than 80 TCID50 can be accurately detected.

7. The operation must be carried out strictly in accordance with the corresponding instructions. When using, place the reagent plate flat on a horizontal surface.

8. The new coronavirus antigen detection product is only suitable for human samples specified in the instructions, and cannot be used for environmental testing, food testing, etc.; directly adding samples with lysis solution or sample extraction solution or directly inserting the cotton swab into the sample collection tube without taking samples are both incorrect operations.

9. When taking samples, you need to use a disposable sampling swab that comes with the reagent. Either a nasal swab or a throat swab will do. Do not use an ordinary cotton swab as a substitute.

10. After the test, the swab, lysate sample extraction tube, reagent plate, etc. must be placed in a sealed bag, and the results must be uploaded to the local health code system or epidemic prevention staff according to the requirements of each jurisdiction. If the person is under quarantine, the management personnel should refer to medical waste or handle it according to procedures; community personnel who test negative can be treated as general garbage, and if they test positive, they will be handed over to medical institutions for treatment as medical waste when they are finally transferred.

In epidemic prevention and control, antigen testing can be used as a simpler and more convenient means of auxiliary testing. Antigens can further shorten the testing time and avoid the risk of cross-infection during the process of queuing for nucleic acid testing. It is especially suitable for large-scale on-site emergency testing, which is conducive to timely taking necessary isolation measures for highly contagious patients.

It is important to remind the general public not to do antigen testing at will, as antigen testing cannot replace nucleic acid testing. When the result of an antigen test is positive, it must be confirmed by nucleic acid testing .

Author: Public Health Physicians Branch of Chinese Medical Doctor Association

Review | Jin Dong, Associate Researcher, Institute of Infectious Diseases, Chinese Center for Disease Control and Prevention