against since they can vary broadly from year to year. For human urinary LH, one IU is most recently defined as 1/189th of an ampule denoted 96/602 and distributed by the NIBSC, corresponding to approximately 0.04656 µg of LH protein for a single IU, but older standard versions are still widely in use. Predicting ovulation The detection of a surge in release of luteinizing hormone indicates impending ovulation. LH can be detected by urinary ovulation predictor kits (OPK, also LH-kit) that are performed, typically daily, around the time ovulation may be expected. A conversion from a negative to a positive reading would suggest that ovulation is about to occur within 24–48 hours, giving women two days to engage in sexual intercourse or artificial insemination with the intention of conceiving. The recommended testing frequency differs between manufacturers. For example, the Clearblue test is taken daily, and an increased frequency does not decrease the risk of missing an LH surge. On the other hand, the Chinese company Nantong Egens Biotechnology recommends using their test twice per day. If testing once per day, no significant difference has been found between testing LH in the morning versus in the evening, in relation to conception rates, and recommendations of what time in the day to take the test varies between manufacturers and healthcare workers. Tests may be read manually using a color-change paper strip, or digitally with the assistance of reading electronics. Tests for luteinizing hormone may be combined with testing for estradiol in tests such as the Clearblue fertility monitor. The sensitivity of LH tests are measured in milli international unit, with tests commonly available in the range 10–40 m.i.u. (the lower the number, the higher the sensitivity). As sperm can stay viable in the woman for several days, LH tests are not recommended for contraceptive practices, as the LH surge typically occurs after the beginning of the fertile window. Disease states Excess In children with precocious puberty of pituitary or central origin, LH and FSH levels may be in the reproductive range instead of the low levels typical for their age. During the reproductive years, relatively elevated LH is frequently seen in patients with polycystic ovary syndrome; however, it would be unusual for them to have LH levels outside of the normal reproductive range. Persistently high LH levels are indicative of situations where the normal restricting feedback from the gonad is absent, leading