Ovulation test strips mainly work based on the principle of detecting luteinizing hormone (LH).

During a woman’s menstrual cycle, hormonal levels change cyclically. The hypothalamic – pituitary – ovarian axis (HPO axis) plays a crucial regulatory role in the female reproductive cycle. In a normal menstrual cycle, the anterior pituitary gland secretes follicle – stimulating hormone (FSH), which stimulates the growth and development of follicles. As the follicles mature, they secrete increasing amounts of estrogen.

When the follicles are fully mature and about to ovulate, the estrogen level reaches a peak. This peak exerts a positive feedback effect on the hypothalamus and pituitary gland, prompting the pituitary gland to secrete a large amount of luteinizing hormone (LH). The appearance of the LH peak usually indicates that ovulation is imminent. Generally, the egg is released from the follicle about 24 – 48 hours after the LH peak.

The test part of the ovulation test strip contains monoclonal or polyclonal antibodies specific to LH. When a woman’s urine wets the test strip, the LH in the urine binds specifically to the antibodies on the test strip.

This binding reaction triggers a series of chemical signal changes inside the test strip. Usually, there is a control line (C – line) on the test strip. It is a fixed – color mark used to indicate that the test strip is valid and as a reference for comparison. There is also a test line (T – line).

If the LH concentration in the urine is low, the amount of LH bound to the antibody corresponding to the test line (T – line) is small. After the color – developing reaction inside the test strip, the color of the T – line is lighter than that of the C – line. This is a negative result, indicating that the ovulation peak has not been reached yet.

When the LH concentration in the urine gradually increases, more LH binds to the antibody corresponding to the T – line, and the color of the T – line gradually darkens. When the color of the T – line is similar to or darker than that of the C – line, it indicates that the LH has reached its peak, showing a positive result, suggesting that ovulation is about to occur or is occurring. In this way, ovulation test strips can help women roughly understand their ovulation status.