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. A grainy appearance of the milk after the addition of the chemical indicates low progesterone for this kit. Some kits require that foremilk strippings be used while a few others utilize postmilk strippings.