A progesterone receptor assay is a laboratory test that is conducted on a biopsy specimen of breast cancer to determine whether the surface of the cancer cells contain receptors (molecules that bind) to the hormone, progesterone. If the surface of the cancer cells contain progesterone receptors, the cancer cells are termed progesterone receptor-positive (PR-positive). If the surface of the cancer cells lack progesterone receptors, the cancer cells are termed progesterone receptor-negative (PR-negative).
When progesterone that circulates in the blood binds to the surface of PR-positive breast cancer cells, the cancer cells are stimulated to multiply.
PR-positive breast cancer may be treated with hormonal approaches that block the production, binding, or action of progesterone.