What is the equation to find the Z-score?

• A. Z = BMD / mean BMD
• B. Z = BMD - age mean BMD / age matched SD
• C. Z = age mean BMD - BMD / age matched SD
• D. Z = age matched SD / BMD

Answer: (B)

The Z-score is a statistical metric that allows for the comparison of an individual's bone mineral density (BMD) to the average BMD of a group that matches their age. The equation for calculating the Z-score is specifically aimed at determining how many standard deviations an individual's BMD is from the mean BMD of an age-matched population.

In the correct formula, the BMD of the individual is subtracted from the mean BMD of their age group, and then the result is divided by the standard deviation (SD) of that age-matched group. This approach provides a clear representation of how the individual's bone density compares to their peers, which is crucial for clinical assessment and diagnosis of conditions such as osteoporosis.

Using this method ensures that comparisons are valid and relevant since bone density can vary significantly with age. This calculation helps healthcare professionals understand whether an individual's BMD is below, above, or at the expected range for their age group.