How Often Should You Check Bone Mineral Density?

Matthew F. Watto, MD; Paul N. Williams, MD


July 21, 2021

This transcript has been edited for clarity.

Matthew F. Watto, MD: Welcome back to The Curbsiders. I'm Dr Matthew Watto, here with my good friend, Dr Paul Williams. Today we are going to talk about osteoporosis from a podcast that was brilliantly titled "Bone Up on Osteoporosis" with internist Dr Carolyn Crandall, who is an expert in osteoporosis and does a lot of research on the topic.

One thing Dr Crandall told us, which was just a simple fact that I wasn't aware of, is that we're now calling osteopenia low bone mass or low bone density. That's when someone has a T-score of minus -1.0 to -2.5 on their DEXA scan. Paul, I don't know about you, but do you find the definition of fragility fracture to be a little confusing? Are you unsure about what qualifies as a fragility fracture?

Paul N. Williams, MD: The definition itself is not so confusing, but I feel like there are a lot of clinical circumstances where there's sort of a gray area and you're not sure whether it's a low-impact fracture or a site that qualifies as a fragility fracture.

Watto: If a patient comes to you with a new fracture, you really want to know whether it qualifies as osteoporosis. Fingers and toes don't qualify. But the AACE guidelines define a fragility fracture as a low trauma fracture — eg, falling from standing or even lower height; fracturing the spine, hip, proximal humerus, pelvis, or distal forearm. We asked specifically about ankles, and it sounds like ankle is a little controversial. It's up to you; you can call that a fragility fracture if you want to.

Tell us, what did you take home from this podcast about the DEXA scan, which we spent a fair amount of time talking about?

Williams: We talked about the practical considerations for interpreting the DEXA scan, which I appreciated. For diagnosis, you use the lowest score on the DEXA report, so you don't have osteoporosis and osteopenia (low bone density) at the same time. It's determined by the lowest score. If a patient has osteoporosis in any site on the DEXA, they have osteoporosis.

In terms of the frequency of DEXA monitoring, it's like everything else — it depends. It's not something you want to do on a yearly basis. In fact, you don't even really need to measure bone density while the patient is on a standard course of therapy — 5 years of oral bisphosphonates or 3 years of intravenous therapy with zoledronic acid or denosumab.

So you get the DEXA, make the diagnosis, and treat. After that standard course, you repeat the DEXA scan. The problem with yearly DXA scans is that you see changes that are within the standard error of measurement. You can actually expect changes in bone mineral density of 3%-6% at the hip or 2%-4% at the spine that are just due to precision error of the test itself and do not reflect actual clinical changes. Patients will misunderstand, thinking bone density has decreased further after only a year. That's probably not true. It's just the variation in the machine. It's a bit more nuanced than just looking at the numbers.

To reduce this error, make sure that the same machine is used to measure bone density each time. The DEXA report should comment specifically on whether there was a significant change from previous testing.

Watto: Surveillance by repeat scan after treatment can aid decisions about a drug holiday, but beyond the holiday, we don't really know what to do. If the BMD starts to drop off or if the patient experiences a fracture, we start a new course of therapy. But that's not very well defined.

One thing I want to caution the audience about is denosumab, which is often prescribed to patients with renal disease. Bisphosphonates are not recommended in patients with eGFR < 30 mL/min and advanced chronic kidney disease. These patients end up on denosumab, but we don't know how long patients can be on that. The longer they're on it, the higher their risk for an atypical femur fracture, although that's still less than their risk of fracturing without some type of therapy. What to do after treatment, we don't know. It's an evidence-free zone. It's almost like a destination therapy.

Williams: The drop off in BMD after stopping denosumab is a little bit concerning, but there isn't strong guidance.

Watto: She mentioned that if someone misses a denosumab shot (during the pandemic, for example) and they have adequate renal function, she recommends putting them on oral bisphosphonates to prevent that loss of bone density that we know occurs pretty quickly.

We talked about the workup. You asked about the 24-hour urine collection. She said that some guidelines recommend it, but she doesn't routinely order it.

Williams: Exactly right. She made the point that it's recommended, but it may not always be done in practice.

Watto: This came up when we were talking about the workup. If the patient has had a fragility fracture, and you get a DEXA scan and find that they have osteoporosis, do we have to do more of a workup? Of course, you're going to check a comprehensive metabolic panel, thyroid, calcium, vitamin D. But beyond that, she at least thinks about some of these other clinical syndromes that could mimic osteoporosis or that could be associated with fractures — multiple myeloma, celiac disease, thyroid, chronic steroid use. Cushing's is another one that she looks for. Does the patient have cushingoid features? If so, she will selectively test some patients but not all-comers.

With respect to the 24-hour urine, I didn't really understand why it would be ordered. It sounds like the reason is you are trying to evaluate the adequacy of calcium intake and absorption. We talked about the "more is better" enthusiasm for calcium and vitamin D supplementation that doesn't pan out clinically, and there are potential harms associated with it. As long as patients are meeting their requirements though diet and other ways, you don't necessarily need to supplement them.

We really got into the weeds on all of this, and it was a great discussion with Dr Crandall. You can listen to the full podcast at Bone Up on Osteoporosis in Primary Care with Dr. Carolyn Crandall. You can also visit The Curbsiders to join our mailing list and get a PDF copy of our show notes every week.

Thank you for watching.

The Curbsiders are a national network of students, residents, and clinician educators from across the country, representing 15 different institutions. They "curbside" experts to deconstruct various topics in the world of medicine to provide listeners with clinical pearls, practice-changing knowledge, and bad puns. Learn more about their contributors and follow them on Twitter.

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