Complex sentence: The T-score is a comparison of a patient's BMD to that of a healthy thirty-year-old, and the Z-score is a comparison of a patient's BMD to the average BMD of their, sex, and race.
Split into simpler sentences: The T-score is a comparison of a patient's BMD to that of a healthy thirty-year-old. The Z-score is a comparison of a patient's BMD to the average BMD of their, sex, and race.