With the recent recurrence of the epidemic, many places have successively used the new crown antigen rapid detection kit, the detection frequency is gradually encrypted, many people are sighing, every day with a cotton swab to wipe, are about to appear stress reaction, a cotton swab on the nose sour want to shed tears. Some people have also found a problem with antigen test boxes, that is, dripping acidic beverages such as cola or lemonade and orange juice on the test strip will produce a positive result.
Some people say that cola and orange lemons have the new crown virus hidden in them (obviously very unreliable, and our readers should not be deceived), and some people say that do not drink these drinks before doing antigen testing, otherwise they will be false positive. What the hell is going on, can this drink still be drunk with confidence?
Too long not to look at the version:
Beverages such as cola and orange juice lemon juice may affect the pH of the sample, producing false positive results.
The buffer solution in the antigen rapid self-test box provides a stable pH environment and improves detection accuracy.
So be sure to follow the instructions to use the antigen self-test box.
Will drinking Coke fake positives? Not impossible
In fact, in 2021, the BBC reported that some British students dropped Coke and orange juice on antigen kits, and then skipped class with false positive results. Why drinks can make the test strip ineffective, we have to start from the working principle of the test strip.
At present, the method of rapid detection of covid-19 antigen is to take a sample from the upper respiratory tract (whether from the nose or the mouth, the swab is taken from the same part, that is, the posterior wall of the pharynx), mix with the buffer, and then flow the sample through the test strip for testing.
Generally speaking, there will be two areas on the test strip, which are C zone and T zone. C is the reference area and T is the test area. If there are colored lines in the C region, it means that the test process is successful, and if the lines in the T region also appear at the same time, then the result is positive, and the T region without lines is negative. However, if the C region does not show color, then the test is invalid, and the T region can no longer be seen no matter what appears.
A common antigen kit on the market | flipped over
From the perspective of the test process alone, the antigen kit is very similar to the familiar pregnancy test stick, in fact, their working principle is also very similar, are to make the antigen and the antibody with the marker combined, the most common method on the market is colloidal gold immunochromatography (GICA).
The so-called colloidal gold method, as the name suggests, is to use gold in the colloidal state as a detection reagent.
Chemically, colloid is a very special state, such as rust just generated in water, neither dissolved in water, but also not precipitated, forming a very unstable system, if at this time by certain influences from the outside world, it is easy to form a precipitate. In the early years, there was also a method of treating water with alum, and the composition of alum was water and potassium aluminum sulfate, in which aluminum would form a colloid in the water, which would combine with impurities in the water and eventually precipitate together.
Light passes through the colloid, forming a bright pathway, which is the result of the scattered light by the particles suspended in it
Colloidal gold, that is, colloidal gold, it is also often referred to as gold nanoparticles, with adsorption. Gold in the colloidal state is often no longer the usual golden yellow, but can present different colors according to different parameters. The color change on the test strip is actually the result of colloidal gold combined with some specific substances.
From this picture, it is clear that the basic principle of the new crown virus antigen test strip test | Ruijin Hospital
When a person is infected with the new crown virus (there is a period of window after infection, when the amount of upper respiratory tract virus is small, antigens may not be detected, and the antigen test strip is mainly suitable for the acute infection period of the new crown), in the sample obtained from the mouth and nasal cavity when testing, the protein (that is, "antigen") on the surface of the new crown virus will combine with the colloidal gold labeled antibody to advance forward on the test paper in the form of chromatography (from left to right in the above figure).
In the image above, purple represents the T region, blue represents the C region, and the T region contains the components of the new crown antibody, which binds to the antigen protein. Therefore, if the subject is in the acute infection phase, then there will inevitably be colloidal gold aggregation in the T-zone, and eventually a clear line will appear.
At the same time, the assay also contains another auxiliary marker. Normally, it also binds to colloidal gold and eventually aggregates in area C. If the test strip fails, or if there is a serious problem with the detection method, it may not show color.
Regardless of the results of the T zone, as long as the C zone does not show color, it means that the test is invalid | China Medical Network
Why can Coke and orange juice fool the test strips?
The binding of proteins to dipstick substances is essentially a chemical reaction and is affected by pH. During the normal test, the sample needs to be added to the buffer solution, generally PBS phosphoric acid buffer, the role of the buffer solution is to ensure that the pH will not change significantly.
Conversely, if there is no buffer solution, the pH of the system changes, and some proteins that originally have a low degree of binding to the substances in the T region may become more easily intercepted and enriched, and leave a line in the T region.
If beverages such as cola and orange juice are used directly on test strips without buffer treatment, it is obvious that the wrong results will be obtained. If oral sampling is performed just after drinking Coke, if it is not tested according to the correct process, false positives may indeed occur.
How to use the antigen box?
If it is not tested correctly according to the process, it may cause false positive oolong, which requires us to operate carefully during special periods.
An antigen assay kit consists of three main pieces: a sterile swab (swab), a test kit, and an extraction tube containing buffer. Some kits also contain an operator station (for placing the extraction tube) and a recovery bag (for storing the test kit).
A common antigen test kit | Oriental Gene
According to the operating specifications published by China News Network, antigen self-testing can be divided into five steps:
The first step, sampling (stirring): the self-test will put the nasal swab paper into the nasal cavity about 1cm-1.5cm, the left and right nasal cavity are stirred 5 times each, respectively, not less than 15 seconds, 2-14 years old people by others to sample.
The second and third steps, elution (stirring, squeezing and squeezing): the swab after the sample is collected is immediately placed in the sampling tube, and the outer wall of the sampling tube is rotated or squeezed to fully elute into the preservation solution.
Step 4, Loading (drop one drop): Squeeze out the liquid on the head of the swab, discard the swab, cover the sample tube lid, and drop the liquid vertically into the sample hole of the test card.
The fifth step, interpretation (take a look): according to the reagent instructions, determine the test results. Only the C line is negative, the C line and the T line are both positive, and only the T line indicates that the test results are invalid.
Usually as long as you strictly follow these five steps, the antigen detection box can function normally. However, in actual operation, there are many factors that will affect the detection process, first of all, just drank an acidic beverage, and then the test process is not standardized, or the test system is not ideal, and the pH is easy to fluctuate. However, after this statement was mentioned, some people also did experiments, and the final results showed that even if the color of the T zone appeared, it was only a relatively light red, which was still quite different from the color of the positive reaction.
Therefore, according to the principle of colloidal gold test strips, we can understand: before the antigen test, it is best to avoid drinking some acidic drinks, and gargling if you drink it unfortunately, which is helpful for the accuracy of the test; even if the acidity and alkalinity of the mouth are different from usual, if you strictly follow the norms, under normal circumstances, it will not affect the test results.
In fact, no matter what type of test method, medical or chemical, it is very necessary to establish testing conditions and strictly implement them, otherwise some strange results will be obtained. There are many similar things in life, such as not being able to test fasting blood glucose after eating, and not being able to use tea to impersonate urine for testing. The results obtained by the tests carried out in disregard of operational requirements are meaningless, and reliable conclusions can only be drawn if they are operated in strict accordance with the conditions.
Author: Sun Yafei, flip over
EDIT: Flip, odette
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