When should an adult undergo a dual‑energy X‑ray absorptiometry (DEXA) scan and how are the T‑score results interpreted for osteoporosis or osteopenia?

DEXA Scan Indications and T-Score Interpretation

All women aged ≥65 years and men aged ≥70 years should undergo DEXA screening, while younger adults (women <65 years, men 50-70 years) require DEXA only if they have specific risk factors for osteoporosis. 1, 2

Who Should Get a DEXA Scan

Universal Screening Ages

• Women ≥65 years: Screen all, regardless of risk factors 1, 2
• Men ≥70 years: Screen all, regardless of risk factors 2

Younger Adults Requiring DEXA (Age ≥50 but below universal screening age)

DEXA is indicated if ANY of the following risk factors are present 2, 3:

• Prior fragility fracture (any low-trauma fracture after age 50)
• Chronic glucocorticoid use (≥5 mg prednisone equivalent daily for ≥3 months)
• Parental history of hip fracture
• Current smoking
• Excessive alcohol consumption (≥3 drinks/day)
• Rheumatoid arthritis
• Secondary causes: hyperthyroidism, hyperparathyroidism, hypogonadism, premature menopause, chronic kidney disease, malabsorption syndromes
• Medications causing bone loss: aromatase inhibitors, androgen deprivation therapy, anticonvulsants
• Low body weight (<127 lbs or BMI <21 kg/m²)

For postmenopausal women younger than 65 years, the USPSTF recommends a 2-step approach: first assess fracture risk using a clinical tool (like FRAX), then order DEXA if elevated risk is identified 1.

DEXA Measurement Sites

Measure BMD at three sites: lumbar spine (L1-L4), total hip, and femoral neck 2. Add one-third radius measurement if hyperparathyroidism is suspected or if spine/hip cannot be measured 2.

T-Score Interpretation

Diagnostic Categories (WHO Criteria)

The T-score represents how many standard deviations your BMD differs from a healthy young adult reference population 4, 5:

• Normal: T-score ≥ -1.0
• Osteopenia (Low Bone Mass): T-score between -1.0 and -2.5
• Osteoporosis: T-score ≤ -2.5 at lumbar spine, femoral neck, total hip, or one-third radius 2, 4

Use the lowest T-score from any measured site for diagnosis 2. The NHANES III reference database (young white women aged 20-29) is the standard for calculating T-scores at the hip 5.

Critical Interpretation Points

A T-score alone does not determine treatment need. Fracture risk depends on multiple factors beyond BMD 4, 3. For example:

• A 55-year-old woman with T-score -2.3 (osteopenia) may have lower fracture risk than a 75-year-old with the same T-score
• A patient with T-score -2.0 who has already sustained a fragility fracture has very high fracture risk despite not meeting the -2.5 threshold

The diagnosis of osteoporosis persists even if subsequent DEXA shows improvement above -2.5 3. Once diagnosed, the condition requires ongoing management.

Z-Score Context

The Z-score compares BMD to age-matched peers 5. A Z-score ≤ -2.0 suggests secondary causes of bone loss and warrants additional workup for underlying conditions 6.

Vertebral Fracture Assessment (VFA)

Consider VFA imaging during DEXA if T-score < -1.0 AND any of the following 2:

• Women ≥70 years or men ≥80 years
• Historical height loss >4 cm (>1.5 inches)
• Self-reported prior vertebral fracture (undocumented)
• Chronic glucocorticoid therapy

Vertebral fractures are often asymptomatic but increase future fracture risk 5-fold 3. VFA can detect these without additional radiation exposure.

Follow-Up DEXA Timing

Repeat DEXA every 1-5 years depending on clinical circumstances 2:

• 1-2 years: After starting new osteoporosis therapy, high-risk patients, or those on medications causing bone loss
• 2-3 years: Stable patients on treatment
• 3-5 years: Low-risk patients not on treatment

The USPSTF notes evidence on optimal screening intervals is limited 1. Always use the same DXA machine at the same facility when possible, as precision varies between devices 2.

Common Pitfalls

Artifacts that falsely elevate BMD 2:

• Lumbar spine: degenerative changes (osteophytes, facet sclerosis), aortic calcification, vertebral compression fractures, prior spinal surgery
• Hip: osteoarthritis, prostheses, prior hardware

When artifacts are present, exclude affected vertebrae or use alternative sites. Never rely on a single vertebra for diagnosis 2.

DXA underestimates volumetric bone density in short individuals and overestimates in tall individuals because it measures areal (2D) rather than volumetric (3D) density 2.