◎ Science and Technology Daily reporter Zhang Jiaxing
Recently, a wave of brain-hole operations have triggered onlookers - orange juice or cola is dripped into the small holes of the new crown antigen test plate, and the test results can even show "two lines"... There are rumors that eating oranges and drinking Coke will affect the test results, and some netizens directly call out that false positives are not too easy to come. However, from the perspective of developers, these so-called "false positives" are completely out of thin air.
"At the end of the day, it's basic common sense that any test product that wants to get exact results should follow the instructions." Xu Lei, director of R&D at BGI Inyuan Flux, told Science and Technology Daily that although some antigen detection products have gone out of the laboratory, the process of testing and verification is ultimately an experiment, and users should follow the basic test rules.
Citizens wait in line for nucleic acid sampling at a nucleic acid sampling site in Balizhuang Street, Chaoyang District, Beijing, on April 25. Image source: Xinhua News Agency
Orange juice and cola can show the sun
Zhu Yuqing, a professor in the Clinical Immunology Department of Shanghai Clinical Laboratory Center, conducted a confirmatory test on the above operations, and found that some test kits can identify "bad tests" and show that the tests are invalid; some kits will be "blinded", the quality control line is normal, and show weak positives.
The reason for this is that different substrate components are used in the approved antigen kits.
So why do orange juice and cola make some antigen tests falsely positive?
True positive color development is due to the antigen detection captures the protein of the virus, the reaction of the antibody binding to the viral protein occurs at the detection line, and the color development reaction is stimulated, and both the quality control line and the detection line are colored.
Orange juice and cola can also make the detection line color, because the two acidic solutions directly stimulate the color development reaction, so that the presence of viral proteins can be colored. In chemical reactions, the pH value within the solution system is crucial, determining the dissociation and binding of ions.
Imagine that adding white vinegar to hot milk in daily life will lead to flocculent precipitation, it is not difficult to understand the protein denaturation caused by the PH value (the antibody bound to the detection board is also a protein), due to the change in pH changes the chemical structure, which in turn leads to the further occurrence of its color development reaction, resulting in false positives. Zhu Yuqing explained in his popular science article: directly drop orange juice into the reagent slats, the PH value 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 chloroharic acid colloidal gold, part of the colloidal gold precipitation, report false positive.
In the design of the kit, there is a special design to avoid changes in the PH value, that is, the buffer solution. This solution acts as a "buffer" for the small amount of pH fluctuations caused by sweat, air, etc., avoiding the entire reaction system from causing drastic changes due to small changes, which in turn affect the results. Even so, the buffer solution cannot "buffer" the destructive effects of artificial addition of a large number of acidic substances on the entire experimental results.
False positives are rejected, and antigen testing is seen in the trick
For ease of detection, antigen and antibody reactions and color development reactions are aggregated onto a thin test plate. However, due to the physical adsorption method of colloidal gold immunochromatography technology, antigens/antibodies are easy to detach from the surface of gold particles, and the markers are unstable, so that orange juice and cola can change the reaction environment and bring false positives.
Image source: Visual China
As of April 27, the NmPA has approved 30 new coronavirus antigen detection reagent products. The detection principle of all approved products can be divided into three latex methods, colloidal gold methods, and fluorescent immunochromatography methods.
The principle of the three methods is the double anti-sandwich method, but the color signal is different.
The signal of the colloidal gold method is nano gold particles, which is a visible light color signal, usually observable by the naked eye; the latex method is coated with a color source, which is also visible light, and the results are observed by the naked eye; the signal of the fluorescent immunochromatography method is a fluorescent substance, which needs to be given a suitable excitation light to emit the corresponding fluorescence, so it is usually necessary to observe the detection results with a specific instrument, but the fluorescent substance is more stable than the colloidal gold, suitable for semi-quantitative detection, so the sensitivity of fluorescence immunochromatography is higher than the previous two.
"The color chromogenic signal is more stable and will avoid the occurrence of false positives caused by some violations of the norm." Xu Lei said that in order to increase the stability of the chromogenic signal, BGI's antigen detection products use nano-microspheres as carrier technology, combined with fluorescent marker probes, which greatly improve the stability of color development in terms of carrier and luminescence. In addition, the detection signal does not rely on naked eye observation, but is quantified by fluorescence recognition devices, so the information provided is more diagnostic and about 10 times more sensitive than traditional colloidal gold chromatography detection techniques.
It can be seen that it is not easy for antigen detection to be "false positive", obviously it is out of line with the operation, do not let oranges and cola back the pot again.