The infamous T score and neglected Z score
Unraveling the mysteries of bone density test results
I bet most of the Better Bones community have heard of the infamous bone density T score. But I suspect that few understand what the T score means, and even fewer know about the hidden value of the Z score. Contrary to popular (mis)understanding, the T score does not measure fracture risk or indicate that you need bone drugs. So what then does the T score indicate?
Putting the T score in perspective
The T score measures how closely your bone density compares to that of an average 30-year-old of the same sex. This comparison is expressed in terms of the “standard deviation,” or SD, which you may recognize from a statistics class as being the amount that represents the typical distance above or below the mean for individual measurements. So, if your bone density differs from the average 30-year-old’s (which it probably will!), that difference will be characterized as “–1.5” if your bones are 1.5 SD below the mean, or “+1.2” if they’re 1.2 SD above the mean.
As the chart above shows, a T score of –1.0 is described as osteopenia, and –2.5 is described as osteoporosis — even though bone density bears little relationship to fracture risk (but that’s another story).
In this second chart (below), 68%, or around two-thirds, of the population falls within 1 SD of the mean, either lower or higher than the average, and another 32% (the remaining third) of the population falls more than 1 SD beyond the mean. Keep in mind that, starting in her mid-30s, the average woman will lose 35% of her cortical bone mass and 50% of her trabecular bone mass over her lifetime (assuming she doesn’t take steps to limit bone loss). So it is totally reasonable to expect that an older woman is not going to have the bone density of a 30-year-old woman, and that her T score will be a negative rather than a positive number.
The overlooked Z score
Elsewhere in your testing results, you will find out your Z score. The Z score compares an individual to others their own age and sex — which makes it a much more realistic assessment of how your bones are faring in their lifelong journey. Where an active, healthy 80-year-old woman, compared to a 30-year-old, might have a T score of –2.0, when compared to other 80-year-old women, she could very well have a Z score of +1.0 or even +2.0!
It’s a pity that the Z score, which “compares apples to apples,” gets so much less attention than the T score. It hardly seems realistic to be worried about having lower bone density that a 30-year-old — but there’s good reason to pay attention when your bone density is much lower than people your own age. For instance, if your Z score is –2.0 or more, it means that very few people your age have a bone density that low, and it signals the need for a complete medical workup looking for all possible cause of excessive bone loss.
Reference:
Hunter, D, and Sambrook, PN. Bone loss: Epidemiology of bone loss. Arthritis Res. 2000:2(6):441-445.