A blood sample is taken before the test and then at 30 minutes, 45 minutes, 60 minutes, 90 minutes, and 120 minutes after administration of GnRH. The first four samples are used to establish a baseline for LH levels in your child’s body. As soon as the fifth sample has been collected, the doctor will interpret its results (if necessary), depending on what time it takes for your child's LH level to rise above 20 IU/L. A final blood sample is usually drawn at six hours after GnRH injection as well. In some children with suspected central precocious puberty who have not achieved an adequate LH response within three hours, additional testing may be required in order to measure their serum testosterone or estradiol concentrations and to exclude other causes of early puberty.