I took a COVID-19 antigen test after drinking Coke, and it turned out positive? It's probably because you didn't do it right

With the recent recurrence of the epidemic, many places have gradually used the new crown antigen rapid test kit, and the frequency of testing has gradually increased. Many people lamented that they are almost experiencing a stress reaction after wiping with cotton swabs every day. When they see the cotton swabs, their noses are sore and they want to cry. Some people have also discovered a problem with the antigen test kit, that is, if you drop acidic drinks such as cola, lemonade, and orange juice on the test paper, a positive result will appear.

Some people say that the coronavirus is hidden in cola, oranges and lemons (obviously very unreliable, our readers should not be fooled), and some people say not to drink these drinks before taking an antigen test, otherwise it will be a false positive. What is going on, and can these drinks still be safe to drink?

Too long to read version:

Drinks such as cola and orange/lemon juice may affect the pH of the sample and produce false positive results.

The buffer solution in the antigen rapid self-test kit can provide a stable pH environment and improve detection accuracy.

Therefore, be sure to use the antigen self-test kit according to the instructions.

Drinking Coke can give you a false positive? It's not impossible

In fact, in 2021, the BBC reported that some British students dropped cola and orange juice on the antigen test kit and then skipped classes with a false positive result. Why drinks can make the test paper ineffective has to start with the working principle of the test paper.

At present, the method for rapid detection of new coronavirus antigens is to take samples from the upper respiratory tract (whether from the nose or mouth, the swab collects samples from the same part, that is, the posterior pharyngeal wall), mix them with buffer, and then pass the sample through the test paper for testing.

Generally speaking, there are two areas on the test paper, namely the C area and the T area. C is the reference area and T is the test area. If a colored line appears in the C area, it means that the test process is successful. If a line also appears in the T area at the same time, then the result is positive. If there is no line in the T area, it is negative. However, if there is no color in the C area, it means that the test is invalid, and no matter what appears in the T area, you can stop looking.

A common antigen test kit on the market |

Judging from the testing process alone, the antigen test kit is very similar to the pregnancy test stick that everyone is familiar with. In fact, their working principles are also very similar. They both allow antigens to combine with labeled antibodies. Currently, the most common method on the market is colloidal gold immunochromatography (GICA).

The so-called colloidal gold method, as the name suggests, uses gold in a colloidal state as a detection reagent.

In chemistry, colloid is a very special state. For example, rust that has just formed in water is neither dissolved in water nor precipitated, forming a very unstable system. If it is affected by some external factors at this time, it is easy to form precipitation. In the early years, there was also a method of treating water with alum. The components of alum are water and potassium aluminum sulfate. The aluminum in it will form a colloid in water, combine with impurities in the water, and finally precipitate together.

When light passes through a colloid, a bright path is formed, which is the result of light scattering by the suspended particles.

Colloidal gold is gold in a colloidal state. It is also often called gold nanoparticles and has adsorption properties. Colloidal gold is often no longer the common golden yellow, but can present different colors according to different parameters. The color change on the test paper is actually the result of colloidal gold combining with certain specific substances.

From this picture, we can clearly see the basic principle of the new coronavirus antigen test paper test | Ruijin Hospital

When a person is infected with the new coronavirus (there is a window period after infection, when the amount of virus in the upper respiratory tract is small and the antigen may not be detected. Antigen test strips are mainly suitable for the acute infection period of the new coronavirus), the protein on the surface of the new coronavirus (ie "antigen") in the samples taken from the mouth and nose during the test will combine with the colloidal gold-labeled antibodies and move forward on the test strip in a chromatographic manner (from left to right in the picture above).

In the above picture, purple represents the T zone and blue represents the C zone. The T zone contains the components of the new coronavirus antibodies, which will bind to the antigen protein. Therefore, if the subject is in the acute infection period, colloidal gold will inevitably aggregate in the T zone, and eventually a clear line will appear.

At the same time, the test solution also contains another auxiliary marker. Under normal circumstances, it will also combine with colloidal gold and eventually aggregate in area C. If the test paper fails or there is a serious problem with the detection method, it may not show color.

Regardless of the T-zone result, as long as the C-zone does not show color, the test is invalid | China Medical Network

Why can Coke and orange juice fool the test strips?

The combination of protein and test paper material is essentially a chemical reaction and will be affected by pH. During the normal test process, the sample needs to be added with a buffer solution, usually PBS phosphate buffer, to ensure that the pH does not change significantly.

On the contrary, if there is no buffer solution, the pH of the system changes, and some proteins that originally have a very low degree of binding with T-zone substances may become more easily retained and enriched by them, leaving a line in the T-zone.

If drinks such as cola and orange juice are directly used on the test strip without buffer treatment, it will obviously give wrong results. And if oral sampling is performed just after drinking cola, false positives may indeed occur if the test is not carried out according to the correct process.

How to use the antigen box?

If the test is not conducted correctly according to the process, it may result in a false positive, which requires us to be more cautious during special periods.

An antigen test kit mainly contains three parts: a sterile swab (cotton swab), a test box, and an extraction tube containing a buffer. Some test kits also include an operating table (for placing the extraction tube) and a recovery bag (for storing the test box).

A common antigen test kit | Oriental Gene

According to the operating specifications released by China News Network, antigen self-testing can be divided into five steps:

The first step is sampling (stirring): the self-examination person inserts the nasal swab paper into the nasal cavity about 1cm-1.5cm, and stirs the left and right nostrils 5 times respectively, for no less than 15 seconds each. People aged 2-14 years old will have someone else take the sample on their behalf.

Step 2 and step 3: elution (stirring and squeezing): place the swab immediately after collecting the sample into the sampling tube, and rotate or squeeze the outer wall of the sampling tube to fully elute it into the preservation solution.

Step 4, sample loading (one drop): squeeze out the liquid on the swab head and discard the swab, cover the sampling tube cap, and drop the liquid vertically into the sample hole of the test card.

Step 5, Interpretation (look): According to the reagent instructions, determine the test results. The appearance of only C line indicates negative, the appearance of both C line and T line indicates positive, and the appearance of only T line indicates that the test result is invalid.

Usually, as long as you strictly follow these five steps, the antigen test kit can work normally. However, in actual operation, there are many factors that affect the detection process. The first is that you have just drunk an acidic beverage, and then the test process is not standardized, or the test system is not ideal, and the pH is prone to fluctuations. However, after this statement was mentioned, some people did experiments, and the final results showed that even if color appears in the T zone, it is only a lighter red, which is still quite different from the color of a positive reaction.

Therefore, according to the principle of colloidal gold test paper, we can understand: before the antigen test, it is best to avoid drinking some acidic beverages. If you happen to drink them, just rinse your mouth, which will help the accuracy of the test; even if the acidity and alkalinity of the mouth are different from usual, if you strictly follow the specifications, under normal circumstances, it will not affect the test results.

In fact, no matter what type of testing method, whether medical or chemical, it is very necessary to establish the testing conditions and strictly implement them, otherwise some strange results will be obtained. There are many similar things in life, such as not testing fasting blood sugar after eating, and not using tea to impersonate urine for testing. The results obtained by ignoring the operating requirements are meaningless. Only by strictly following the conditions can reliable conclusions be drawn.

Author: Sun Yafei, flip through

Edited by: Odette

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