In this blog post, we will look at the principles and features of how LFIA kits using antigen-antibody reactions enable fast and accurate diagnosis at low cost.
Testing kits are widely used to diagnose health conditions or to investigate bloodstains at crime scenes. Various scientific principles are applied to the production of the kits, and it is required to produce kits that can be tested easily, quickly, and accurately at a low cost. In response to this need, various types of kits are being developed that analyze the components present in the sample by applying the antigen-antibody reaction.
An antigen-antibody reaction is an immune response in which an antigen and an antibody that specifically reacts with the antigen bind together. As antibody manufacturing technology has advanced, various types of kits using the lateral flow immunoassay (LFIA), which is highly portable and has a short analysis time, have been developed. In particular, these kits are becoming increasingly widely used because they can be easily used not only in medical settings but also at home. For example, pregnancy test kits and kits for diagnosing certain infectious diseases are designed to be easily used at home based on LFIA technology. This is an important innovation that provides convenience to users as well as efficient use of medical resources.
With LFIA kits, it is easy to check the presence or absence of the target component to be detected in a liquid sample by the lines that appear on the kit. The LFIA kit is a long, flat bar-shaped device with a sample pad, conjugate pad, reaction membrane, and absorbent pad arranged in that order. The sample absorbed by the sample pad moves through the reaction membrane with the conjugate on the conjugate pad to the absorbent pad, where any excess sample is absorbed. The complex in the bonding pad is made by attaching a specific substance to a label material such as gold nanoparticles or fluorescent beads. The label material produces a color through a chromogenic reaction, and the specific substance attached to this label material varies depending on the kit type.
Generally, antibodies are fixed in a strip shape on the reaction membrane of a kit that detects a target component. Among them, the strand closest to the sample pad is the test line, and the other strand is the standard line. When a labeled substance is placed on the test line or the standard line, a reaction line appears due to a color reaction. The presence or absence of the target component can be determined by the reaction line that appears when the test line is colored. When a reaction line with a standard color appears, it indicates that the test has been performed normally. Thanks to these structural characteristics, LFIA kits enable rapid and accurate testing in a variety of environments.
LFIA kits are mainly produced using direct or competitive methods, and the meaning of the color development of the test line differs depending on the method. In the direct method, the specific substance contained in the complex is an antibody that can bind to the target component. If the sample contains the target component, the target component binds primarily to this antibody, and then to the fixed antibody on the test line. Therefore, if the test line develops color, it is determined that the target component has been detected in the sample. On the other hand, in the competition method, the specific substance contained in the complex is not an antibody to the target component, but the target component itself. If the sample contains the target component, the target component of the sample and the target component of the complex compete with each other to bind to the antibody of the test line. If the target component is sufficiently abundant in the sample, the target component of the sample will not be detected because it interferes with the binding of the target component of the complex to the antibody on the test line. The direct method is used to detect bacteria or proteins with a large molecular weight, while the competitive method is used to detect target components with a small size, such as antibiotics.
On the other hand, in addition to portability and speed, the accuracy of the test kit is also important. To measure the accuracy of the kit, multiple tests are performed using the kit and the results are analyzed. If the kit determines that the sample contains the target component, it is called a positive result. If the target component is actually present in the sample, it is called a true positive, and if the target component is not present in the sample, it is called a false positive. On the contrary, if the kit determines that the sample does not contain the target component, it is said to be negative. In this case, if there is no target component, it is called true negative, and if there is a target component, it is called false negative. In reality, there is no kit that can exclude false positives or false negatives.
The accuracy of the kit is determined by the results of multiple tests, and accuracy is the probability of obtaining the correct test result when analyzing a sample. Accuracy is divided into sensitivity and specificity. Sensitivity is the percentage of samples that the kit tests as positive when the target component is present. Specificity is the percentage of samples that the kit tests as negative when the target component is not present. The ideal situation is when a kit has high sensitivity and specificity, but in reality, this is not the case in many cases, so you should choose a kit based on sensitivity or specificity depending on the situation.
In conclusion, LFIA kits have many advantages as a fast, easy, and accurate testing tool. Their range of applications is expanding in both medical and non-medical fields, and it is expected that more sophisticated and diverse kits will be developed in the future as technology advances. These newly developed kits will play a major role in improving the quality of life of many people by making it easier to manage health in everyday life and enabling quick response in various situations.
