CT scans ordered for other reasons may also detect osteoporosis

Computed tomography scans ordered for other reasons may be an acceptable method of detecting osteoporosis without exposing a patient to additional radiation, according to a new study.

In a cross-sectional study at one U.S. academic medical center, researchers used computed tomography (CT) scans performed for other clinical indications to compare bone mineral density (BMD) assessment on CT versus dual-energy X-ray absorptiometry (DXA). CT-attenuation values of trabecular bone between the T12 and L5 vertebral levels were measured in Hounsfield units (HU), with emphasis on the L1 measures. BMD was measured by DXA as the reference standard. The study results appeared in the April 16 Annals of Internal Medicine.

A total of 1,867 adults (2,063 CT-DXA pairs) had CT and DXA during a six-month period. Most patients (81%) were women, and the mean age was 59.2 years.

Patients with osteoporosis on DXA had significantly lower CT-attenuation values at all vertebral levels (P<0.001). A CT-attenuation threshold of 160 HU or less at the L1 vertebra was found to be 90% sensitive and a threshold of 110 HU was found to be over 90% specific for distinguishing between osteoporosis and osteopenia and normal BMD. At L1 CT-attenuation thresholds less than 100 HU, positive predictive values for osteoporosis were 68% or more, while negative predictive values were above 99% at a threshold above 200 HU. One hundred nineteen patients had at least one moderate to severe vertebral fracture, and of these, 62 (52.1%) had false-negative DXA results while 97% had an L1 or mean T12 to L5 vertebral attenuation of 145 HU or lower.

The authors noted that the potential benefits and costs of the different CT-attenuation thresholds were not assessed and that DXA is itself not a perfect reference standard for osteoporosis. However, they concluded, “abdominal CT images obtained for other reasons that include the lumbar spine can be used to identify patients with osteoporosis or normal BMD without additional radiation exposure or cost.”

The authors of an accompanying editorial said they believed the current results would be best used to identify patients who are at high risk for fracture because of densitometric or clinical osteoporosis. They noted that this approach may seem conservative but is justified because a significant proportion of CT scans already report incidental findings, many of which are never followed up. “Systematically adding more information to reports already replete with incidental findings that are not being acted on should be undertaken with trepidation,” the editorialists wrote.

They also said that tolerance for false-positive results should be low and that a threshold yielding 90% specificity, a positive likelihood ratio of 6 and a post-test probability of approximately 70% should be used.

“Although sensitivity would suffer,” they wrote, “radiologists and patients would be assured low rates of false-positive results and minimization of issues related to testing cascades and potential liability (for radiologists) and of harms related to additional radiation, labeling, and the ‘hassle factor’ (for patients).”

The editorialists said that the authors of the current study had established the evidence to justify this use of conventional CT imaging and that “it is now up to the rest of us to safely and cost-effectively translate this new knowledge into everyday clinical practice.”