Author: Zhu Yuqing
Unit: Shanghai Clinical Laboratory Center
Recently, a video was circulating on the Internet: someone cut the orange, squeezed the juice on the antigen reagent test board, and not long after, a purple-red horizontal line appeared in the T area. As a result, some people got the conclusion that "you can't eat oranges before doing antigen testing, otherwise you will be positive". Is this true?
You may see this conclusion that everyone wants to try it, so Xiaobian specially used orange juice to carry out the same experiment, using three different brands of new coronavirus (2019-nCoV) antigen detection kits, the experimental results are as follows:
Brand 1
After the orange juice was dripped, there was no purple-red horizontal line in the C area of the reagent plate, and only a purple-red horizontal line appeared in the T area, indicating that the experiment was invalid.
Brand 2
The dripping orange juice did not immediately appear purple horizontal line in the T area of the reagent board, and the purple horizontal line appeared immediately in the C area, but when the results were read within the specified time after the sample was applied (it must be noted here that the result reading time specified by each brand is different, and the T-zone strip that appears after the reading time cannot be used as a positive result), a very, very faint purple horizontal line appeared in the T area, and the result was shown as a weak positive.
Brand 3
The brand 3 result is consistent with brand 2, and the result is shown as weak positive.
Do oranges affect antigen test results?
Can the above experiments lead to the results that orange juice causes false positives? We have even seen the results of tests with various drinks and liquids, so what is the truth?
None of these results indicate that an orange or other liquid antigen test is "positive" or "weakly positive" because the antigen test kit is tested against a human sample as described in the instructions, such as a nasal swab, a nasopharyngeal swab, and not other liquids such as orange juice. Antigen detection should be carried out in strict accordance with the instructions, so that the results obtained have reference significance, such as the operation does not meet the standard, the results obtained are actually meaningless.
Eating oranges will not affect the antigen test results in principle. At present, the antigen test in Shanghai uses a nasal swab, and the normal diet does not affect the sample composition of the nasal swab - even if part of the orange juice enters the nasal cavity, the concentration is usually not very high, which is completely different from the orange juice directly into the kit. So there's no need to worry about eating oranges or eating other foods and beverages that will affect the antigen test results.
Why does orange juice have false positives?
The antigen detection rapid test strip uses colloidal gold immunochromatography or latex method.
When performing antigen detection, after sampling, the sampling rod will be placed in the sample extraction solution matching the kit, and then the liquid in the tube will be dripped into the reagent strip after extrusion. The main components of this sample extract will vary from brand to brand, and if the extract is used interchangeably, or without specialized extracts and human samples, the optimal performance of the reagent cannot be guaranteed, in short, it is a disorder of the test system.
What is the specific testing principle?
The antigen detected is on the membrane protein outside the virus, and we perform the detection by using antigen antibodies to bind to each other, and then use colloidal gold as a marker to develop color. Since the mutual binding of proteins mainly relies on hydrogen bond binding force, electrostatic gravity, van der Waals force, and hydrophobic effect, when the salt concentration in the reaction system changes, non-specific binding between antibodies is easy to occur, resulting in false positive results. In addition, when the pH of the reaction environment is close to the antibody isoelectric point, the condensation reaction will occur, resulting in false yang or false yin results. Not to mention, directly dripping orange juice into the reagent slats, so that the pH of the solution is too low, and there is vitamin C in the orange juice, a large number of reducing agents can easily lead to abnormal aggregation of chlorelloic acid colloidal gold, so that part of the colloidal gold precipitate. In the T-line position, because the film is sometimes shallow, the colloidal gold analyzed by the part is directly precipitated in it, and it does not run forward with the liquid chromatography, resulting in a shallow imprint on the T-line position, resulting in false positive results.
In addition, recently some citizens have shown that when using antigen detection, the unsampled or directly dipped swab was added to the extraction solution and dripped, and the result was that there was no band in the T zone, and there was a band in the C zone, which was believed to be a problem with the reagent slats. In fact, the emergence of the quality control line in the C area just shows that this reagent slat is good, and we use this daily to judge whether this experiment is successful or whether the reagent is well preserved. As long as we drop the sample extraction solution into the slat, even if it is not sampled, the liquid will flow along the direction of chromatography shown in the figure above, so that the quality control line of the C region appears, and whether the line appears in the T region depends on whether the sample extract contains the new crown virus antigen.
The principle of this reaction is very similar to that of the pregnancy test stick, one bar - squad leader, is a negative result; two bars - squadron leader, is positive.
Therefore, when we conduct antigen self-testing, we must strictly follow the requirements of the instructions for standardized operation, so that we can better avoid false positive and false negative results.
Source: Shanghai Clinical Laboratory Center
Editor: Ren Mileage Reviewer: Xiao Ran