The World Health Organization (WHO) has established specific guidelines about what constitutes abnormal bone mineral density.
These guidelines are based on how far off the patient's results are from those of the average healthy young woman, using something t scores.
Osteoporosis is defined as a t score of less than -2.
5, which corresponds to a marked degree of bone thinning.
If you have a t score of less than -2.
5, you are in the bottom 1% in terms of bone density.
Osteopenia is bone thinning that is not as severe as osteoporosis, and is defined as t scores between -1.
0 and -2.
If you have a t score in this range that means that your bone density is so low that only 16% of women would have such a result.
As bone thinning progresses you could go from having osteopenia to osteoporosis.
The National Osteoporosis Foundation (NOF) recommends BMD testing for postmenopausal women who have a risk factor for osteoporosis (smoking, drinking, lack of exercise, weight less than 127 pounds, family history of osteoporotic fracture, or prior vertebral fracture), and all women over the age of 65.
I believe the logic of this measure is deeply flawed -- to judge older women by applying standards for young women doesn't make any sense.
The t scores are calculated by comparing how much a woman deviates from the bone density of a healthy young woman.
That's like having a 70 year old run a 100 yard dash against a 20 year old, and then if he loses, saying that the older man has a disease.
Bone density normally declines with age, and therefore there is no reason to think that this is necessarily a cause for concern.
Yet today if you have an abnormal BMD test, defined as a t score of greater than 2.
5, the NOF guidelines include advice to "talk to your doctor" and provide a description of medications used to prevent fractures.
For example, if you are a woman who gets BMD testing and follows the WHO criteria, there is a 50% chance you will be diagnosed with osteoporosis at the age of 72 (t score less than -2.
5), and a good chance your doctor will recommend medication treatment.
Your risk of having osteopenia (t score less than -1.
0), for which your doctor may recommend medication to "prevent" osteoporosis, is 50% by age 52.
In other words, according to the guidelines, half of postmenopausal women should be taking medication for osteoporosis.
However, as I show in the next section, recommendations for so many women to take bone medications don't make any sense.
Also, even though BMD predicts fracture, and medications increase BMD, that doesn't mean they necessarily prevent fracture (at least not the ones that cause loss of function or pain).
This is the old A equals B and B equals C therefore A equals C.
Osteopenia causes fracture, and medication reduces osteopenia, therefore medication reduces fracture.
Right? Not exactly.
Normally there are two kinds of cells that regulate bone turnover, osteoblasts and osteoclasts.
The osteoblasts are actively increasing bone by laying down calcium and phosphate.
Osteoclasts, in turn, are chewing up bone on the other end.
In young people there is a balance of these two activities.
When you get older, however, osteoblast activity declines, and so your bones get thinner.
Popular osteoporosis medications get in between the bone and the osteoclast and prevent the osteoclast from breaking down bone.
This leads to an increase in bone mineral density.
But what doctors and drug makers often don't tell you is that the medications used to treat osteoporosis, which I discuss shortly, increase the laying down of calcium on the outer, cortical bone, which is more densely packed.
The inner bone, called trabecular bone, is less dense, but forms a lattice like network that is more important for the strength of the bone.
With aging there is a loss of trabecular bone, therefore there is less area for calcium to be laid down.
Therefore calcium is preferentially laid down on the outer, cortical bone.
This may increase your BMD score, but it won't necessarily reduce your risk of fracture.
And there are some particularly nasty side effects to deal with, which I will discuss below.
The research studies don't show much benefit for women without a prior history of fracture, and any benefit goes away after a few years.
Based on that I say don't get your BMD checked, and exercise, quit smoking, and do other things to prevent fractures instead.
If you have a history of fracture you can talk to your doctor.