A Z-score matches your bone mineral density with individuals of the same age, gender, and ethnicity. The following formula is used to determine your Z-score:
Patient’s BMD – BMD age-matched normal reference
Standard deviation of age-matched reference
Z-scores are used to identify low bone mass in men under the age of 50, premenopausal women, and children. The International Society for Clinical Densitometry recommends classifying Z-scores of –2.0 and lower as “low bone mineral density for chronological age” or “below the expected range for age.” They suggest classifying scores above –2.0 as “within the expected range for age.” Z-scores are not helpful in identifying osteoporosis or osteopenia because your Z-score may remain constant throughout your life. Z-scores are more useful in evaluating bone density in children, who have not yet reached peak bone mass, and younger adults.
Very low Z-scores in postmenopausal women or men over 50, indicating a much lower bone mass than that of their peers, do require further investigation into the causes of bone loss related to secondary osteoporosis (the type of osteoporosis not associated with postmenopause or aging). If you have a Z-score lower than –1.5, your clinician should be looking for reasons why you have significant bone loss compared to your own age group. If you have a medical condition, for example, hyperthyroidism (one of the illnesses that can cause secondary osteoporosis) that has gone undetected, a low Z-score may point your clinician toward identifying and treating this reason for bone loss.