Each kit will have an expiration date printed on the package or the chemicals within the package. Be sure there is enough time to use the kit before the chemicals expire. Keep kits refrigerated between use. However, do not let the chemicals freeze. Always allow the kits to warm up to 60-70 degrees Fahrenheit before testing the milk samples.
Temperature is critical.
Follow directions carefully and do not take shortcuts. Most kits utilize a change in color to determine the level of progesterone. Occasionally the intensity of the color will not be noticeably different from the control. This may indicate the level of progesterone is borderline between high and low. One kit does not involve a color change.