The principle of color change in glucose test strip is based on enzyme reaction.  In addition, there is a correlation between the volume of the urine sample and the result of the test because the color change is depending on the quantity of substrate on the reagent strip.  Therefore, removing the excess urine from the strip is very important to avoid the difference in results and window frame phenomenon.  Window frame phenomenon means when the color of the glucose portion shows darker around the edges than the center.  If this phenomenon is found, read the color of the center of the test portion.