How do ovulation test strips work?

The detection principle of ovulation test strips is based on the changes of luteinizing hormone (LH) in women’s bodies during the menstrual cycle, as follows:

I. Hormonal changes during the female menstrual cycle

1. Hormonal changes during the follicular phase
   • During the follicular phase of the menstrual cycle, the estrogen level in a woman’s body gradually increases. This increase promotes the gradual maturation of follicles in the ovaries. As the follicles continue to mature, the estrogen level reaches a peak.
   • When the estrogen reaches its peak, it has a positive feedback effect on the hypothalamus – pituitary, stimulating the hypothalamus to secrete gonadotropin – releasing hormone (GnRH).
2. The appearance of the LH surge
   • The GnRH secreted by the hypothalamus acts on the pituitary, prompting the pituitary to secrete a large amount of luteinizing hormone (LH). The LH level rises sharply, forming a peak, which usually occurs about 24 – 48 hours before ovulation.
   • It is due to the appearance of the LH surge that causes the mature follicle to rupture and release an egg, and this process is ovulation.

II. The detection principle of the test strip

1. Antibody – antigen reaction
   • Ovulation test strips contain monoclonal antibodies that specifically bind to LH. When a woman drips her urine onto the test strip, if there is a sufficient amount of LH in the urine, the LH will specifically bind to the monoclonal antibodies on the test strip.
2. Color – developing reaction
   • In addition to LH antibodies, the test strip also contains color – developing agents. When LH binds to the antibodies, it triggers a series of chemical reactions, causing the color – developing agents to develop color.
   • The concentration of LH in the urine can be judged according to the degree of color development. Generally speaking, if the test strip shows a strong positive (darker color), it indicates that the concentration of LH in the urine is high, predicting that ovulation may occur within the next 24 – 48 hours; if it shows a negative (only the control line develops color), it indicates that the concentration of LH in the urine is low and the ovulation peak has not been reached.