Lateral flow tests are simple devices designed to detect the presence of a specific substance, known as an analyte, in a liquid sample. These rapid diagnostic tools have become common in various settings, from homes to clinics, due to their speed and ease of use. They deliver results within minutes, often between five and thirty, without requiring specialized laboratory equipment. This accessibility has made them a popular choice for quick assessments.
Understanding Lateral Flow Technology
Lateral flow technology relies on immunochromatography, where a liquid sample moves across a porous material by capillary action. A typical lateral flow test strip is composed of several distinct pads on a backing card. The process begins on the sample pad, which receives the liquid and prepares it for analysis, sometimes filtering out unwanted components.
The sample then flows into the conjugate pad, which contains dried, reactive molecules known as conjugates. These conjugates are typically antibodies attached to colored particles, such as gold nanoparticles, which appear ruby red. Following the conjugate pad, the liquid moves onto a nitrocellulose membrane, which serves as the central reaction platform. Finally, an absorbent pad at the end of the strip draws the fluid through the system, maintaining consistent flow and preventing backflow.
How Lateral Flow Tests Function
When a liquid sample is applied, it rehydrates the labeled antibodies in the conjugate pad. If the target substance (analyte) is present, it binds to these mobile labeled antibodies, forming an analyte-antibody complex. This complex then travels along the nitrocellulose membrane.
The membrane contains specific lines of immobilized antibodies. As the complex reaches the “test line,” the immobilized antibodies capture the analyte-antibody complex, concentrating the colored particles at this location. This accumulation creates a visible line, indicating a positive result.
Further along the strip, a “control line” appears regardless of whether the target analyte is present. This line contains molecules that bind to any unbound labeled antibodies, confirming that the sample flowed correctly through the strip and the test components are active. The color intensity of the test line can sometimes relate to the amount of analyte present, and some tests can be read by a digital reader for quantitative results.
Everyday Uses of Lateral Flow Tests
Lateral flow tests are widely applied due to their rapid results and user-friendly nature. One of the most recognized applications is the home pregnancy test, which detects the hormone human chorionic gonadotropin (hCG) in urine. These tests offer a convenient and private method for early pregnancy detection.
During the COVID-19 pandemic, rapid antigen lateral flow tests detected SARS-CoV-2 proteins from nasal or throat swabs. These tests provided results within 15 to 30 minutes, aiding in prompt identification and isolation of cases. Lateral flow tests are also commonly used in drug screening to detect illicit drugs or their metabolites in biological samples like urine. These tests offer a cost-effective and portable solution for on-site screening in workplace or law enforcement settings.
Reading and Trusting Test Results
Interpreting lateral flow test results involves observing the presence or absence of lines on the strip. A positive result shows two colored lines: one at the test line and one at the control line. A negative result displays only the control line, indicating the absence of the target analyte. If no control line appears, the test is invalid, suggesting an issue with the test itself or the sample application, and a new test should be performed.
The accuracy of lateral flow tests is described by two metrics: sensitivity and specificity. Sensitivity refers to the test’s ability to correctly identify positive cases, meaning the proportion of people with the condition who test positive. Specificity measures the test’s ability to correctly identify negative cases, or the proportion of people without the condition who test negative.
Many lateral flow tests demonstrate high specificity, often above 99%, reducing the likelihood of false positive results. Factors such as the timing of the test relative to infection, the concentration of the target substance in the sample, and proper sample collection can influence the accuracy of the results. If there are concerns about a result, or symptoms persist, seeking professional medical advice is recommended.