The T-score is a widely used parameter to assist in the interpretation of BMD results. It measures the departure of the patient's BMD value from the mean BMD for a young adult healthy population in units of the population standard deviation (SD). The Z-score is similar in concept to the T-score, with the exception that the mean BMD and SD for a healthy age- and sex-matched population are used as reference values instead of the mean BMD and SD for a young normal group.⁴ Z-scores are not used to define osteoporosis, since they would not reflect the increasing prevalence of osteoporosis with age. For example, elderly patients may have a Z-score of zero, based on comparison to their own age group, but a T-score that would put them in the osteoporotic category. Z-scores are useful if they show that a patient's BMD is significantly below an age-matched group; this finding should prompt a more aggressive search for a secondary cause of osteoporosis.⁵