The NTx Urine Test System is an immunoassay that provides a quantitative measurement of the excretion of cross-linked Ntelopeptides of type I collagen (NTx) in human urine samples and can be used as an indicator of human bone resorption. Elevated levels of urinary NTx indicate elevated human bone resorption. Measurement of NTx is intended for use in predicting skeletal response (bone mineral density) to hormonal antiresorptive therapy in postmenopausal women. Measurement of NTx is also intended for the use in therapeutic monitoring of antiresorptive therapy in postmenopausal women, antiresorptive therapy in individuals diagnosed with osteoporosis, antiresorptive therapy in individuals diagnosed with Paget's disease of bone, estrogen-suppressing therapies and identifying the probability for a decrease in bone mineral density after one year in postmenopausal women receiving calcium supplement relative to those treated with hormonal antiresorptive therapy.