Ovulation prediction kit (OPK) measures the Luteinizing Hormone (LH) in your urine. Small amount of LH is present in the urine every day and its level gradually increases as ovulation approaches. The LH surges within 24 hours before ovulation and OPK is positive the day before you ovulate.
But some conditions could give you false positive result. These include
Polycystic ovarian syndrome(PCOS) where there is a raised blood level of the LH
Premature menopause where the ovary ages prematurely in women below the age of 35
Perimenopause or menopausal transition where the level of LH is high