- An immunochromatographic assay (ICA), also commonly referred to as a lateral flow assay (LFA), is a rapid and user-friendly diagnostic method used to detect the presence or absence of a specific analyte—such as proteins, hormones, or pathogens—in a sample. This technique is widely applied in clinical diagnostics, food safety, environmental monitoring, and point-of-care testing due to its simplicity, portability, and quick results, typically within 5–30 minutes.
- The core principle of immunochromatographic assays is based on the antigen-antibody interaction combined with capillary action. The assay device usually consists of several components: a sample pad, a conjugate pad, a nitrocellulose membrane with test and control lines, and an absorbent pad. When a liquid sample (e.g., blood, urine, saliva) is applied to the sample pad, it migrates by capillary action through the membrane.
- On the conjugate pad, antibodies labeled with a detectable marker—such as colloidal gold nanoparticles, latex beads, or fluorescent dyes—are immobilized. If the target analyte is present in the sample, it binds to these labeled antibodies and continues to migrate. The complex reaches the test line, where another antibody specific to the analyte is immobilized, forming a visible line due to the accumulation of the label. A control line, which contains antibodies that capture the labeled antibody regardless of analyte presence, ensures the test is functioning correctly.
- ICAs are generally qualitative or semi-quantitative, providing a simple yes/no answer for the presence of the target molecule. However, advances in detection methods, such as the use of fluorescence or smartphone-based image analysis, have enabled more quantitative interpretations.
- One of the most well-known examples of an immunochromatographic assay is the home pregnancy test, which detects human chorionic gonadotropin (hCG) in urine. Other common applications include tests for infectious diseases (e.g., COVID-19 antigen tests), drug screening, and food allergen detection.
- In summary, immunochromatographic assays are powerful tools for rapid diagnostics. Their robustness, ease of use, and adaptability to various targets make them especially valuable in resource-limited settings and for on-site testing where laboratory infrastructure is not available.
