Double duty: Routine abdominal CT could serve as osteoporosis screen

Abdominal CT images taken for any reason, that include the lumbar spine, can be used for the additional purpose of identifying patients with osteoporosis, according to a study published April 16 in Annals of Internal Medicine.

Osteoporosis, while prevalent and treatable, is underdiagnosed and undertreated, making it a prime target for screening efforts, according to Perry J. Pickhardt, MD, of the University of Wisconsin School of Medicine and Public Health, Madison, and colleagues.

“Retrieval of [bone mineral density (BMD)] data available on body CT examinations ordered for other indications requires no additional cost, patient time, equipment, software, or radiation exposure, and these data can be retrospectively acquired,” wrote the authors. “It could therefore expand population screening efforts for osteoporosis.”

The findings were based an analysis of CT-derived BMD assessments compared with dual-energy x-ray absorptiometry (DXA) measures more commonly used to define osteoporosis. Data from 1,867 adults undergoing CT and DXA exams within a six-month period over the 10-year study were included, with CT-attenuation values measured in Hounsfield units (HU) tracked for trabecular bone. CTs were conducted to assess vertebral fractures.

Pickhardt and colleagues wrote they placed an emphasis on the L1 vertebral level because it is easily identified as the first non-rib-bearing vertebra and included on all routine abdominal and thoracic CT scans.

Results confirmed that CT attenuation values were significantly lower at all vertebral levels for patients with osteoporosis as defined by DXA measures. A threshold of 160 HU or less at L1 was 90 percent sensitive and a threshold of 110 HU was more than 90 percent specific for distinguishing osteoporosis from osteopenia and normal BMD, according to the authors. Positive predictive values for osteoporosis were 68 percent or greater for L1 thresholds of less than 100 HU, while negative predictive values were 99 percent at thresholds greater than 200 HU.

“Although optimal implementation of this method of CT screening for osteoporosis remains to be determined, our data suggest ways that it could be used in practice, depending on clinical objectives,” wrote Pickhardt and colleagues. They suggested that persons identified with very low BMD could be used for rapid identification of high-risk cohorts who require further evaluation, or a high BMD could be used to exclude osteoporosis and make DXA unnecessary in such patients. They are currently assessing an automated system of CT BMD using PACS data.

In an accompanying editorial, Sumit R. Majumdar, MD, MPH, of the University of Alberta in Edmonton, Canada, and William D. Leslie, MD, MSc, of the University of Manitoba in Winnipeg, Canada, wrote the study has many strengths, but urged a low tolerance of false-positives—or what they dubbed “incidentaloporosis.”

Majumdar and Leslie suggested limiting CT-attenuation-based osteoporosis screening to conservatively rule in patients at high risk for fracture on the basis of densitometric or clinical osteoporosis, and use an L1 threshold of less than 110 HU as a diagnostic cutoff to insure low rates of false positives.  

“Pickhardt and associates have laid all the groundwork needed to justify using conventional CT imaging to detect incidental osteoporosis—it is now up to the rest of us to safely and cost-effectively translate this new knowledge into everyday clinical practice,” wrote Majumdar and Leslie.