DEXA Bone Density: When to Order This Test

What a DEXA Scan Measures

A DEXA scan quantifies bone mineral density (BMD) in grams per square centimeter (g/cm²) at the lumbar spine, total hip, and femoral neck. The scanner uses two X-ray beams at different energy levels to separate bone from soft tissue, producing a precise measurement with a coefficient of variation of 1% to 2% at the spine and 1.5% to 3% at the hip 1. The entire scan takes 10 to 20 minutes and delivers a radiation dose of roughly 1 to 10 microsieverts, which is less than the exposure from a standard chest radiograph 2.

Results are reported as two scores. The T-score compares a patient's BMD to the mean BMD of a healthy 30-year-old reference population. The Z-score compares BMD to age-matched and sex-matched peers. The World Health Organization established diagnostic thresholds using femoral neck T-scores from a young white female reference database: a T-score at or above -1.0 is normal, between -1.0 and -2.5 indicates osteopenia, and at or below -2.5 defines osteoporosis 3. These thresholds apply to postmenopausal women and men aged 50 and older. For premenopausal women, men under 50, and children, the International Society for Clinical Densitometry (ISCD) recommends using Z-scores, with a Z-score of -2.0 or below classified as "below the expected range for age" 4.

Who Should Be Screened: USPSTF and Society Guidelines

The U.S. Preventive Services Task Force (USPSTF) issued a Grade B recommendation in 2018: screen for osteoporosis with DEXA in all women aged 65 and older, and in postmenopausal women younger than 65 whose 10-year fracture risk (calculated by FRAX or a comparable tool) is equal to or greater than that of a 65-year-old white woman with no additional risk factors, which corresponds to a 10-year major osteoporotic fracture probability of approximately 9.3% 5. The USPSTF found insufficient evidence to recommend for or against screening in men.

The Endocrine Society and the American Association of Clinical Endocrinologists (AACE) extend screening recommendations to men aged 70 and older, and to men aged 50 to 69 with clinical risk factors including prior fragility fracture, glucocorticoid therapy exceeding 5 mg prednisone daily for three or more months, hypogonadism, or a body weight below 70 kg 6. The AACE 2020 guidelines state: "All postmenopausal women and men aged 50 years and older who present with a fragility fracture should undergo BMD testing and vertebral fracture assessment" 7.

Younger adults warrant DEXA when specific clinical situations arise: organ transplant recipients, patients initiating aromatase inhibitor therapy for breast cancer, individuals on androgen deprivation therapy for prostate cancer, and anyone with conditions known to accelerate bone loss (celiac disease, inflammatory bowel disease, type 1 diabetes, hyperthyroidism, or anorexia nervosa) 8.

Clinical Triggers That Justify Ordering a DEXA

Beyond age-based screening, several clinical scenarios should prompt a DEXA order. Fracture history matters most. A vertebral compression fracture or a low-trauma fracture of the hip, wrist, or humerus after age 50 is an indication for immediate DEXA regardless of the patient's age or sex 7.

Medication-related bone loss is the second most common trigger. Glucocorticoid-induced osteoporosis develops rapidly: the American College of Rheumatology (ACR) 2022 guidelines recommend DEXA within 6 months of initiating prednisone at 2.5 mg/day or more if the anticipated duration is 3 months or longer 9. Aromatase inhibitors (letrozole, anastrozole) reduce BMD by 2% to 3% annually at the lumbar spine. The American Society of Clinical Oncology recommends baseline DEXA before starting these agents 10.

Other triggers include:

• Radiographic evidence of vertebral deformity or incidental osteopenia
• Height loss exceeding 1.5 inches (4 cm) from peak adult height
• Chronic kidney disease stages 4 to 5 (to distinguish renal osteodystrophy from primary osteoporosis)
• GLP-1 receptor agonist therapy when rapid weight loss exceeds 10% of body weight, since bone density can decline with significant caloric restriction 11
• Testosterone replacement therapy monitoring in men with documented hypogonadism and baseline osteopenia

Understanding Your T-Score and Z-Score

The T-score drives diagnosis. A result of -1.0 or higher is normal. A T-score of -1.5, for example, means your BMD sits 1.5 standard deviations below the young adult mean. The WHO classification system was validated in a 2004 meta-analysis by Johnell et al. (N=39,000+) showing that each standard deviation decrease in femoral neck BMD roughly doubles hip fracture risk 12.

Z-scores serve a different purpose. In premenopausal women and men under 50, a Z-score of -2.0 or below signals that bone loss is greater than expected for the patient's age, prompting investigation for secondary causes: vitamin D deficiency, hyperparathyroidism, celiac disease, multiple myeloma, or Cushing syndrome 4. The ISCD Official Positions state: "In premenopausal women, Z-scores, not T-scores, are preferred. A Z-score of -2.0 or lower is defined as 'below the expected range for age.' A Z-score above -2.0 is 'within the expected range for age'" 4.

Context changes interpretation. Degenerative disc disease, aortic calcification, and vertebral compression fractures can falsely raise lumbar spine BMD. When spine results seem discordant with hip results, the clinician should rely on the hip measurement or consider lateral spine imaging 1.

What High or Low Results Mean Clinically

A T-score above -1.0 does not eliminate fracture risk. The majority of fractures in postmenopausal women actually occur in individuals with osteopenia rather than osteoporosis, simply because the osteopenic population is much larger 13. This is why FRAX exists.

FRAX integrates BMD with clinical risk factors (age, sex, BMI, prior fracture, parental hip fracture, glucocorticoid use, rheumatoid arthritis, smoking, alcohol intake of 3+ units daily) to calculate 10-year probabilities of major osteoporotic fracture and hip fracture 14. The National Osteoporosis Foundation (now the Bone Health and Osteoporosis Foundation) recommends pharmacologic treatment when the 10-year probability of hip fracture reaches 3% or the probability of major osteoporotic fracture reaches 20% 15.

Low T-scores trigger a treatment cascade. At T-scores between -1.0 and -2.5, FRAX risk assessment determines whether to treat. At T-scores of -2.5 or below, pharmacotherapy is indicated. First-line options include oral bisphosphonates (alendronate 70 mg weekly, risedronate 35 mg weekly) or intravenous zoledronic acid 5 mg annually 6. For very high-risk patients (T-score below -3.0, or T-score below -2.5 with prior vertebral fracture), anabolic agents such as romosozumab 210 mg subcutaneous monthly for 12 months or teriparatide 20 mcg subcutaneous daily for up to 2 years may be used first, followed by an antiresorptive 16.

Dr. Clifford Rosen, a senior scientist at Maine Medical Center Research Institute, noted in the New England Journal of Medicine: "The decision to treat should not rest on the T-score alone but on the integration of fracture probability, patient preferences, and the risk-benefit profile of available therapies" 17.

How to Improve Bone Density

Pharmacologic therapy produces measurable BMD gains. In the HORIZON trial (N=7,765), zoledronic acid 5 mg IV annually increased lumbar spine BMD by 6.7% and total hip BMD by 6.0% over 3 years compared to placebo, reducing hip fracture risk by 41% (HR 0.59; 95% CI 0.42 to 0.83) 18. Romosozumab produced a 13.3% increase in lumbar spine BMD at 12 months in the FRAME trial (N=7,180) 16.

Non-pharmacologic strategies form the foundation of any bone health plan. Calcium intake should reach 1,000 to 1 to 200 mg daily (diet plus supplement if needed), and vitamin D should be maintained at 30 to 50 ng/mL through supplementation of 1,000 to 2,000 IU daily for most adults, though some patients need higher doses based on serum 25-hydroxyvitamin D levels 19. The Endocrine Society 2011 guideline on vitamin D recommends: "Adults aged 19 to 70 years require at least 600 IU/day of vitamin D to maximize bone health, but to raise serum 25(OH)D above 30 ng/mL may require at least 1,500 to 2,000 IU/day" 19.

Weight-bearing exercise and resistance training are evidence-backed interventions. A meta-analysis by Howe et al. in the Cochrane Database (N=4,320) found that combined weight-bearing and resistance exercise produced statistically significant improvements in spine BMD compared to no exercise 20. Walking alone was less effective than progressive resistance training for the hip.

Patients on hormone therapy (estrogen for postmenopausal women, testosterone for hypogonadal men) typically see BMD stabilization or modest gains. The Women's Health Initiative (WHI) demonstrated that conjugated equine estrogen plus medroxyprogesterone acetate reduced hip fracture risk by 34% (HR 0.66; 95% CI 0.45 to 0.98) over 5.6 years of follow-up 21.

When to Repeat the DEXA Scan

The ISCD recommends repeat DEXA when the result is expected to influence clinical management. For patients starting osteoporosis pharmacotherapy, a follow-up scan at 1 to 2 years confirms treatment response 4. A BMD increase or stable BMD supports treatment continuation. A decline exceeding the least significant change (LSC) for the scanner (typically 3% to 5% at the spine, 4% to 6% at the hip) may indicate nonadherence, malabsorption, or the need to switch therapy.

For untreated patients with normal baseline results, the optimal rescreening interval depends on the initial T-score. A 2012 study by Gourlay et al. in the New England Journal of Medicine (N=4,957 women aged 67+) found that the estimated time for 10% of women to make the transition to osteoporosis was approximately 16.8 years for those with normal BMD (T-score -1.0 or higher), 4.7 years for those with mild osteopenia (T-score -1.01 to -1.49), and 1.1 years for those with advanced osteopenia (T-score -2.00 to -2.49) 22. This data supports longer screening intervals for women with normal baseline scans.

Medicare covers DEXA every 24 months for qualified beneficiaries, and more frequently if medically necessary (for example, after starting or changing osteoporosis therapy). Most commercial insurers follow the USPSTF recommendations for coverage.

Special Populations and Considerations

GLP-1 receptor agonist patients. Rapid weight loss from semaglutide or tirzepatide (10% to 20% of body weight over 68 to 72 weeks) raises concern about concurrent bone loss. The STEP-1 trial (N=1,961) reported that semaglutide 2.4 mg produced 14.9% mean weight loss at 68 weeks versus 2.4% with placebo 23. While bone density outcomes were not a primary endpoint, data from earlier weight-loss studies suggest that roughly 1% to 2% of BMD is lost for every 10% of body weight reduction 11. Patients with pre-existing osteopenia who begin GLP-1 therapy should have a baseline DEXA and a follow-up scan at 12 months.

Transgender patients on hormone therapy. Trans women on estradiol and trans men on testosterone require bone density monitoring per the Endocrine Society 2017 clinical practice guideline, which recommends DEXA screening if risk factors are present or if hormone therapy is discontinued 24.

Patients on proton pump inhibitors (PPIs). Long-term PPI use (more than one year) has been associated with a modest increase in hip fracture risk. A 2019 meta-analysis (N=244,109) found a pooled relative risk of 1.26 (95% CI 1.18 to 1.35) for hip fracture among PPI users 25. Consider DEXA in patients on chronic PPIs who also have other risk factors.

Baseline DEXA before initiating denosumab 60 mg subcutaneous every 6 months documents starting BMD and enables monitoring of treatment response at 2-year intervals 6.