DXA and Bone Mineral Density

A story on NPR on the emergence of osteopenia as a bone disease due in part to the efforts Merck inspired a closer look at dual x-ray absorptiometry (DXA) and bone mineral density (BMD) measurements.

Introduction

DXA has 3 major roles:

• Diagnosis of osteoporosis
• Monitoring response to treatment
• Assessment of fracture risk

T-scores using WHO criteria are used for patients older than 50 years of age. For patients under 50 years of age, the Z-score should be used for interpretation. Children's DXA scans should also be interpreted using Z-scores, since BMD measurements in children are affected to a greater extent by bone size.

Measurement Sites

Hip BMD is the most reliable measurement for predicting hip fracture risk, while all DXA sites are more or less equally effective at predicting an osteoporotic fracture at any site.

Spine BMD is best used for monitoring treatment response. This is because treatment changes tend to be greatest in the spine and spine BMD measurements have low precision error. However, spine DXA has limited sensitivity, and its use in monitoring patients is more controversial than its use for the diagnosis of osteoporosis and prescription of treatment. Therefore, it is recommended that follow-up scans be obtained no sooner that every 1-2 years.

For spine measurements to be diagnostic, at least 2 vertebral bodies must be used. In addition, DXA precision is decreased in obese patients and those who have large weight changes between scans.

Peripheral measurement sites (as opposed to the central hip and spine sites) can also be used, but with caution. Peripheral measurements other than "33% radius" (distal 1/3 of the radius shaft) cannot be interpreted with T-scores and WHO criteria. This is because when reference values for the different BMD measurement sites are plotted as a function of age, the hip, spine and 33% radius decline at the same rate. Reference values for other sites decline at different rates. For example, reference values for the heel decline at a much slower rate and can lead to underdiagnosis of osteoporosis if the -2.5 SD is used.

Risk Stratification

For a given hip T-score, fracture risk varies greatly according to age and other risk factors. The FRAX (Fracture Risk Assessment Xool [that's Tool with an X] initiative estimates the 10-year probability of the patient sustaining an osteoporotic fracture based on a number of risk factors. In addition, it uses "health economic criteria to set thresholds for intervention based on the costs of treatment, savings to health services, and the contribution of fracture prevention to patients' quality of life."

FRAX risk factors include:

• Country or geographic region
• Ethnic origin (US only)
• Age
• Gender
• BMI
• Previous history of fracture (after age 50)
• Parental history of hip fracture
• Current smoking habit
• Current or past use of corticosteroids
• Rheumatoid arthritis
• Secondary osteoporosis
• Alcohol intake ≥3 units daily
• Hip BMD (femoral neck was used in the development of the algorithm)

The FRAX web site can also be used without the hip BMD to determine who may benefit most from DXA.

Treatment Guidelines

Treatment Criteria from the National Osteoporosis Foundation 2008 Guidelines recommend treatment in postmenopausal women and men age 50 and older regardless of ethnicity who meet one or more of the following criteria:

• A previous hip or vertebral fracture
• T score −2.5 or less at the femoral neck, total hip, or spine
• T score between −1.0 and −2.5 at the femoral neck, total hip, or spine and one or more of the following:
  • a. Other previous fractures
  • b. A secondary cause of osteoporosis associated with a high risk of fracture
  • c. 10-year fracture risk as assessed by FRAX of 3% or more at the hip or 20% for a major osteoporosis-related fracture (humerus, forearm, hip, or clinical vertebral fracture)

References

Blake GM, Fogelman I. An update on dual-energy x-ray absorptiometry. Semin Nucl Med. 2010 Jan;40(1):62-73.