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.
The concentration of progesterone may be twice as much in postmilk compared to foremilk. It is essential to use the type of milk that is specified for the kit you are using. Low progesterone does not indicate the exact time to inseminate the cow. Low progesterone 19-23 days after insemination is a very accurate indication the cow is open. However, high progesterone obtained during this time is not an absolute sign the cow is pregnant.