Is repeat bone density scan necessary?

Currently, re-testing bone mineral density (BMD) is a controversial issue. Some experts argue that re-measurement of BMD is not necessary, because a single measurement of BMD can predict fracture up to 25 years. However, others argue that re-measurement of BMD is necessary (1), because the change in BMD is an important indication of fracture risk. I belong to the second group of experts. So, let me explain my view ...

Everyone working in the field of osteoposis knows that BMD is a key measure of bone health. Indeed, low BMD is the most important and the most reproducible risk factor for fracture, with each standard deviation decreasing in BMD being associated with 2-3 fold increase in fracture risk. This magnitude of association is actually equivalent to that of between blood pressure and stroke. Therefore, in practice clinicians use BMD to diagnose "osteoporosis".

Now, a question of practical/clinical interest is: if an individual has already had a BMD measurement, is it necessary for the individual to have a second measurement? Some experts argue that there is no need to repeat BMD measurement, because -- they argue -- a single BMD can predict the risk of fracture over 20-25 years. This seems to make sense in many cases whose BMD is in the "normal" range. For those individuals with normal BMD, they have low risk of fracture anyway.

However, for individuals whose BMD is close to the osteoporotic level, what should we do? Now, in postmenopausal women and elderly men, BMD progressively declines with advancing age. The rate of decline is highly variable between individuals, but the average is about 0.5 to 0.8 per cent per year. That means if an individual with a current BMD [by GE-Lunar] of, say, 0.71 g/cm^2 (which is just above the osteoporotic threshold of 0.70) will likely to have osteoporosis over 1-2 years. I consider that these individuals should have a repeat BMD measurement.

Moreover, our study has previously shown that the rate of longitudinal bone loss is an important risk factor for fracture. Two individuals may have the same baseline BMD, but the one with excessive bone loss is at a much greater risk of fracture than the one with stable BMD. We cannot ignore the contribution of bone loss to fracture!

I consider that the decision of whether to repeat BMD measurements cannot be made without some knowledge of an individual's initial BMD measurement. Moreover, the rate of bone loss is related to age and sex, and the presence of comorbidities. Older people tend to lose more bone mass than younger people. Women are known to have a greater rate of bone loss than men. Thus, the decision of the time to repeat needs to be personalized according to an individual's profile.

It turns out that the two most important factors in the profile are age and initial BMD measurement. My student, Dr. Steven Frost, and I have developed a model for determining the time for repeat BMD measurements for each individual (2). Based on age and initial BMD measurement, we determined the time when an individual risk will reach the level of 20% risk of having osteoporosis or fracture. The result is a nomogram which can be translated into a table as follows:

Table 1: Estimates for determining the optimal time (in 0.5-yr increments) to reach the lower limit of the 90% CI of (A) 10% and (B) 20% risk of osteoporosis or fracture in women. Source: from (2).

Table 2: Estimates for determining the optimal time (in 0.5-yr increments) to reach the lower limit of the 90% CI of (A) 10% and (B) 20% risk of osteoporosis or fracture in men. Source: from (2).

Clinical case 1. An 80-yr-old women had a BMD T-score of −2. Her 5-year osteoporosis or fracture risk is 0.45. Moreover, table 1 suggests that the time to reach 10% risk of fracture or osteoporosis for the woman is 2 yr (90% CI: 1.5–2.1 yr). Thus, the woman would be advised to return for bone densitometry in 1.5 yr.

Clinical case 2. A 75-yr-old man had a BMD T-score of −1.5. Based on our paper (1) the 5-yr risk of osteoporosis or fracture is ∼0.18. Table 2 suggests that the time to reach 10% risk of osteoporosis or fracture is 2.7 yr (90% CI: 2.4–3.1 yr). Therefore, the suggested time for repeating BMD measurement is ∼2.5 yr.

Thus, as you can see from the above cases, repeat BMD measurement is necessary for some people. However, let me make my view clear: when it comes to the time to repeat BMD measurement, there is no yes/no categorical answer; it has to be determined on the basis of an individual's risk profile, and we have a model (2) to help guide the decision.

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(1) http://www.amjmed.com/article/S0002-9343(17)30640-X/pdf

(2) http://onlinelibrary.wiley.com/doi/10.1359/jbmr.090514/full