The SELFCheck menopause home test kit is an immunological test for detection of FSH (Follicle-stimulation-hormone) in urine.
This is because the number of ovarian follicles (that contain your eggs) falls as you begin to enter menopause. Because there are fewer follicles, your body produces less of a hormone called Inhibin B that brings FSH levels down. Therefore, your FSH rises and this is a marker for being in the menopause.
When a woman's FSH blood level is consistently elevated to 30 mIU/mL or higher, and she has not had a menstrual period for a year, it is generally accepted that she has reached menopause.