DEXA Scan for Bone Density

Dual-energy X-ray absorptiometry — universally abbreviated as DEXA or DXA — is the clinical standard for measuring bone mineral density (BMD) and diagnosing osteoporosis. This page covers how the technology works, the clinical scenarios that prompt its use, and the thresholds that guide interpretation and follow-up decisions. Understanding DEXA scan results is relevant to anyone managing osteoporosis and its endocrine connections, as bone density is directly regulated by hormones including estrogen, testosterone, parathyroid hormone, and cortisol.

Definition and scope

DEXA scanning is a low-dose radiographic technique that measures the absorption of two X-ray energy beams — one high-energy and one low-energy — as they pass through bone and soft tissue. The differential absorption between the two beams allows software to isolate bone mineral content from surrounding tissue, producing a quantitative density measurement in grams per square centimeter (g/cm²).

The United States Preventive Services Task Force (USPSTF) recommends screening for osteoporosis with bone measurement testing in women aged 65 and older, and in younger postmenopausal women whose 10-year fracture risk is equal to or greater than that of a 65-year-old white woman with no additional risk factors (USPSTF Osteoporosis Screening Recommendation, 2018). The National Osteoporosis Foundation (now the Bone Health and Osteoporosis Foundation, or BHOF) extends clinical guidance to men aged 70 and older and to men aged 50–69 with risk factors.

DEXA is distinct from quantitative computed tomography (QCT), which generates a volumetric BMD in mg/cm³ and is more sensitive to trabecular bone loss but carries a radiation dose roughly 10 to 100 times higher than standard DEXA. Peripheral DEXA devices (pDXA), which measure the wrist or heel, are used for population screening but are not interchangeable with central DEXA results for diagnostic classification.

The regulatory context for endocrinology in the United States assigns CPT code 77080 to central DEXA scans of the axial skeleton (spine and hip), the most diagnostically weighted measurement sites. Medicare Part B covers this procedure every 24 months for qualifying beneficiaries, as specified under the Medicare Bone Mass Measurement Act (42 U.S.C. § 1395m(d)).

How it works

A standard central DEXA examination follows a defined sequence:

1. Positioning: The patient lies supine on a padded table. For lumbar spine measurement, the legs are elevated on a positioning block to flatten the lumbar lordosis. For hip measurement, the foot is internally rotated approximately 15–25 degrees and held in a positioning device to align the femoral neck perpendicular to the beam.
2. Scan acquisition: The X-ray arm passes over the target region, typically the lumbar spine (L1–L4 vertebrae) and the proximal femur (total hip and femoral neck). Scan time ranges from approximately 10 to 30 seconds per site depending on equipment model.
3. Image analysis: Proprietary software identifies bone edges and calculates BMD for each region of interest (ROI). The software compares the measured BMD against a reference database.
4. T-score and Z-score generation: The resulting scores are the primary clinical outputs.

The T-score compares a patient's BMD to the mean peak bone mass of a young adult reference population of the same sex. The Z-score compares BMD to an age-matched and sex-matched reference population.

The World Health Organization (WHO) diagnostic classification uses T-scores at the spine, total hip, or femoral neck (WHO Technical Report, 1994):

• Normal: T-score ≥ −1.0
• Low bone mass (osteopenia): T-score between −1.0 and −2.5
• Osteoporosis: T-score ≤ −2.5
• Severe osteoporosis: T-score ≤ −2.5 with one or more fragility fractures

The International Society for Clinical Densitometry (ISCD) specifies that in premenopausal women, men under 50, and children, the Z-score — not the T-score — is the appropriate diagnostic metric. A Z-score ≤ −2.0 is classified as "below the expected range for age" (ISCD Official Positions 2019).

Radiation exposure from a central DEXA scan is approximately 1–6 microsieverts (μSv), equivalent to a fraction of the natural background radiation received in a single day. By comparison, a standard chest X-ray delivers approximately 100 μSv.

Common scenarios

DEXA scans are ordered across a range of clinical situations, with endocrine conditions representing a major driver of testing:

• Postmenopausal women and older men: Estrogen and testosterone deficiency accelerate bone resorption, making age-related screening the most frequent indication.
• Glucocorticoid-induced osteoporosis: Patients taking oral glucocorticoids (e.g., prednisone) at a dose of ≥ 5 mg/day for ≥ 3 months are at elevated fracture risk. The American College of Rheumatology (ACR) 2017 guidelines recommend DEXA for this population at treatment initiation (ACR Glucocorticoid-Induced Osteoporosis Guidelines, 2017).
• Primary hyperparathyroidism: Excess parathyroid hormone preferentially resorbs cortical bone at the distal radius; DEXA including a forearm site is recommended for this population. See hyperparathyroidism and calcium disorders for the broader diagnostic workup.
• Hypogonadism: Low sex hormone states — including testosterone deficiency in men and hypothalamic amenorrhea in women — are established risk factors for low BMD.
• Thyroid disorders: Both untreated hyperthyroidism and overtreatment with levothyroxine suppress TSH and accelerate bone turnover.
• Cushing's syndrome: Chronic cortisol excess is among the most potent causes of secondary osteoporosis.
• Fracture risk assessment: DEXA data feeds directly into the FRAX algorithm (developed by the University of Sheffield and endorsed by the WHO), which calculates a 10-year probability of major osteoporotic fracture and hip fracture, incorporating BMD alongside 11 clinical risk factors.
• Treatment monitoring: Serial DEXA scans at 1–2 year intervals track response to pharmacologic therapy such as bisphosphonates or denosumab. The full spectrum of medications for osteoporosis is evaluated in part through longitudinal BMD data.

Decision boundaries

Interpretation of DEXA results requires distinguishing between the scan's numerical outputs and the clinical decision to intervene. A T-score alone does not determine treatment; the BHOF and ISCD position that pharmacologic therapy is appropriate when:

• A hip or spine T-score is ≤ −2.5, or
• A low-bone-mass T-score (−1.0 to −2.5) is combined with a FRAX 10-year probability of major osteoporotic fracture ≥ 20% or a hip fracture probability ≥ 3%

These thresholds are specific to the United States FRAX model calibrated to US fracture and mortality rates.

DEXA vs. QCT — key contrasts:

Artifacts that can falsely elevate lumbar spine BMD — including osteophytes, aortic calcification, vertebral compression fractures, and posterior element sclerosis — are common in older adults and can produce normal-appearing T-scores despite genuine bone fragility. ISCD positions state that fewer than 2 evaluable lumbar vertebrae renders the lumbar spine BMD unreliable, necessitating reliance on hip measurements or alternative sites.

Referral to an endocrinologist or metabolic bone specialist is standard when Z-scores fall unexpectedly low for age, when BMD declines despite treatment, or when secondary causes of bone loss require systematic investigation through the endocrinology specialty network.

References

• USPSTF Osteoporosis Screening Recommendation (2018)
• WHO Technical Report Series 843: Assessment of Fracture Risk (1994)
• ISCD Official Positions on Bone Densitometry (2019)
• [ACR Clinical Practice Guideline on Glucocorticoid-Induced Osteoporosis (2017)](https://www.rheumatology.org/Practice-Quality/Clinical-Support/Clinical

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