The stick-on, or forehead strip, thermometer is a simple device used for quickly checking for a fever. Its main advantage is convenience, offering a non-invasive way to screen for elevated temperatures, especially in children. These strips are not meant to replace more precise clinical devices but serve as a preliminary indication of body heat. They provide a rapid visual assessment that helps determine whether a more accurate temperature measurement is necessary.
Understanding the Liquid Crystal Mechanism
The science behind these strips relies on thermochromic, encapsulated liquid crystals layered onto a plastic strip. These crystals change color when exposed to specific temperatures. The molecules within the liquid crystals are in a cholesteric phase, forming a helical structure.
As the temperature rises, the pitch of this helix changes, altering how light is reflected from the strip’s surface. Different colors correspond to different temperature thresholds. Colors typically progress from tan or red at lower temperatures to green and then blue as the temperature increases, providing a continuous, visual indication of the skin’s surface temperature.
Proper Placement and Usage
Accurate use begins with ensuring the forehead is clean and completely dry. Any moisture, such as sweat or oils, can interfere with heat transfer and result in a falsely low reading. The strip should be held firmly at both ends and pressed flat against the center of the forehead, maintaining full contact with the skin.
Keep the strip in place for the manufacturer’s recommended time, typically around 15 seconds. This waiting period allows the strip to absorb enough thermal energy from the skin to properly activate the liquid crystals. Reading the strip immediately upon application will often lead to an inaccurate or incomplete reading.
Interpreting the Temperature Display
The most accurate reading occurs while the strip is still on the forehead, immediately after the required waiting time. The temperature is indicated by the illuminated colored square, as each number corresponds to a specific crystal mixture. If only one number displays a bright green color, that number represents the body temperature.
Green is generally the color used to signify the exact temperature reading. If the temperature falls precisely between two marked values, the strip will often display two colors simultaneously (e.g., blue on one side and tan or red on the other). In this case, the actual temperature is considered to be the midpoint between the two numbers indicated. For example, if the strip shows blue at 99.0°F and tan at 99.5°F, the approximate reading is 99.25°F.
Limitations and Verifying Accuracy
A stick-on thermometer measures the skin’s surface temperature, not the body’s core temperature. This surface temperature is highly susceptible to external influences, making the strips less reliable than devices that measure internal heat. Ambient air temperature, drafts, or a sweaty forehead can easily skew the displayed result.
These thermometers are best utilized as a screening tool to monitor temperature trends or quickly check for a significant elevation. If the strip indicates a high temperature or fever is suspected, the result must be verified with a more accurate device. Oral, rectal, or tympanic (ear) thermometers provide a truer measure of core body temperature and should be used for definitive diagnosis.