This blog post explores the principles and features of LFIA kits utilizing antigen-antibody reactions, examining how they enable fast and accurate diagnostics at low cost.
Test kits are widely used for diagnosing health conditions or examining bloodstains at crime scenes. Various scientific principles are applied in kit production, with a demand for kits that enable easy, fast, and accurate testing at low cost. To meet this need, diverse kits analyzing components present in samples by applying antigen-antibody reactions are being developed.
An antigen-antibody reaction refers to an immune response where an antigen binds to an antibody that reacts specifically only to that antigen. With advances in antibody manufacturing technology, various types of kits utilizing lateral flow immunoassay (LFIA), which offers high portability and short analysis times, are being developed. Notably, these kits can be easily used not only in medical settings but also at home, leading to their increasingly widespread adoption. For example, pregnancy test kits and diagnostic kits for specific infectious diseases are designed based on LFIA technology for easy home use. This represents a significant innovation that provides convenience to users while enabling the efficient utilization of medical resources.
Using an LFIA kit, the presence or absence of the target component to be detected in a liquid sample can be easily confirmed by observing the line that appears on the kit. The LFIA kit has a long, flat, rod-like shape, with the sample pad, binding pad, reaction membrane, and absorbent pad arranged sequentially in that order. The sample absorbed by the sample pad moves through the conjugate pad, forming a complex, then passes through the reaction membrane to the absorbent pad where excess sample is absorbed. The complex on the conjugate pad is formed by a specific substance binding to a marker material, such as gold nanoparticles or fluorescent beads. The marker material produces color through a chromogenic reaction; the specific substance bound to this marker varies depending on the kit design.
Typically, the reaction membrane of a kit detecting a single target component features antibodies immobilized in two parallel bands. The band closer to the sample pad is the test line, while the other is the control line. If the label material lands on either the test line or the control line, a reaction line appears due to the colorimetric reaction. The presence or absence of the target component can be determined by observing whether the test line appears colored. The appearance of a colored control line indicates the test has proceeded correctly. This structural feature enables LFIA kits to provide rapid and accurate testing in diverse environments.
LFIA kits are primarily manufactured using direct or competitive methods, and the significance of the test line’s coloration differs depending on the method. In the direct method, the specific substance within the complex is an antibody capable of binding to the target component. If the sample contains the target component, it first binds to this antibody and subsequently binds to the fixed antibody on the test line. Therefore, if the test line becomes colored, it is determined that the target component was detected in the sample. In contrast, in the competitive method, the specific substance within the complex is the target component itself, not an antibody against it. If the sample contains the target component, the target component in the sample and the target component in the complex compete to bind to the antibody on the test line. If the target component in the sample is present in sufficient quantity, it prevents the target component in the complex from binding to the antibody on the test line, so the test line does not develop color. The direct method is used for detecting bacteria or large-molecular-weight proteins, while the competitive method is used when the target component is small, such as antibiotics.
Meanwhile, for test kits, accuracy is crucial alongside portability and speed. To measure a kit’s accuracy, multiple tests are performed using the kit, and the results are analyzed. If the kit determines the sample contains the target component, this is called a positive result. If the target component is actually present in the sample, it is a true positive; if absent, it is a false positive. Conversely, if the kit determines the target component is absent in the sample, it is called a negative result. If the target component is truly absent, it is a true negative; if the target component is present but the kit indicates absence, it is a false negative. In reality, no kit can completely eliminate false positives or false negatives.
The kit’s accuracy is determined through multiple test results. Accuracy is the probability of obtaining the correct test result when analyzing a sample. Accuracy is divided into sensitivity and specificity. Sensitivity is the proportion of samples containing the target component that the kit correctly identifies as positive. Specificity is the proportion of samples without the target component that the kit correctly identifies as negative. While a kit with high sensitivity and specificity, thus high overall accuracy, is ideal, this is often not the case in practice. Therefore, kits must be selected based on the situation, considering either sensitivity or specificity.
In conclusion, LFIA kits possess many advantages as rapid, convenient, and accurate testing tools. Their scope of application is expanding in both medical and non-medical fields, and with technological advancements, increasingly sophisticated and diverse kits are expected to be developed in the future. These developed kits will play a significant role in facilitating health management in daily life, enabling rapid response in various situations, and improving the quality of life for many people.
