DEXA Bone Density: Longevity-Medicine Target Ranges, Normal Values, and When to Treat

What T-Score and Z-Score Actually Mean

DEXA (dual-energy X-ray absorptiometry) measures bone mineral density (BMD) in grams per square centimeter at specific skeletal sites. The machine then expresses your result in two standard-deviation scores.

The T-score compares your BMD to a young-adult reference population at peak bone mass (typically a 30-year-old of the same sex). The Z-score compares you to age- and sex-matched peers. Both scores matter clinically, but for different reasons.

T-Score: The Diagnostic Anchor

The World Health Organization's 1994 classification, still used by the International Society for Clinical Densitometry (ISCD), defines three categories based on T-score alone in postmenopausal women and men aged 50 or older [1]:

• Normal: T-score at or above -1.0
• Osteopenia (low bone mass): T-score between -1.0 and -2.5
• Osteoporosis: T-score at or below -2.5
• Severe osteoporosis: T-score at or below -2.5 with one or more fragility fractures

These cutoffs were derived from data on postmenopausal White women. For premenopausal women, men under 50, and children, the ISCD recommends using Z-scores rather than T-scores for diagnosis.

Z-Score: The "Is Something Else Going On?" Signal

A Z-score below -2.0 means your BMD is lower than 97.5% of people your own age and sex. That signals a secondary cause, such as glucocorticoid use, hypogonadism, malabsorption, or hyperthyroidism, and warrants a full secondary workup before attributing bone loss solely to aging [2].

Which Skeletal Site Controls the Diagnosis

The ISCD 2023 official positions state that the lowest T-score among the lumbar spine (L1-L4), total hip, or femoral neck determines the diagnostic category [3]. A normal hip with an osteoporotic spine still earns an osteoporosis diagnosis. Peripheral devices (heel ultrasound, finger DXA) are screening tools only and cannot be used for WHO classification.

Longevity-Medicine Target Ranges: Why "Normal" Is Not Good Enough

Standard clinical thresholds tell you when bone disease exists. Longevity medicine asks a different question: what T-score minimizes lifetime fracture risk and preserves functional independence into the ninth decade?

The Fracture-Risk Math

Hip fracture carries a 30-day mortality rate near 5-8% and a one-year mortality rate of approximately 20-30% in adults over 70 [4]. Vertebral fractures double the risk of subsequent fractures within 12 months, a phenomenon called the "vertebral fracture cascade." Given that math, the longevity-medicine community sets its sights on T-scores well above the osteopenia floor.

A 2019 analysis published in the Journal of Bone and Mineral Research demonstrated that absolute fracture risk increases continuously across the entire T-score range, with no safe plateau even within the "normal" zone [5]. Adults with T-scores between -0.5 and -1.0 still carry a meaningfully higher 10-year fracture probability than adults with T-scores above 0.0.

The HealthRX longevity framework for bone health uses the following tiered targets, intended as optimization goals rather than diagnostic cutoffs:

| Age Band | Minimum Acceptable T-Score | Longevity Target T-Score | |---|---|---| | Age <50 | Z-score above -2.0 | Z-score 0.0 or above | | Age 50-64 | T-score above -1.5 | T-score 0.0 or above | | Age 65-74 | T-score above -2.0 | T-score -1.0 or above | | Age 75 and older | T-score above -2.5 | T-score -1.5 or above |

These targets are based on the continuous fracture-risk curves published by Kanis et al. In FRAX validation studies and on the bone-loss trajectories described in the Study of Women's Health Across the Nation (SWAN) [6].

Why Bone Mass Peaks Matter

Peak bone mass, typically reached between ages 25 and 35, sets the reservoir from which the rest of life draws. Each 10% increase in peak bone mass reduces osteoporosis risk by approximately 50% decades later, according to the International Osteoporosis Foundation consensus [7]. Longevity-oriented clinicians therefore screen adults in their 30s and 40s, not just after menopause or age 65, to intervene during the phase when lifestyle changes have the greatest marginal return.

Trabecular Bone Score as an Adjunct

Trabecular Bone Score (TBS) is a texture analysis extracted from the same lumbar spine DXA image. A TBS above 1.350 is considered normal; 1.200-1.350 is partially degraded; below 1.200 is degraded. The ISCD endorsed TBS in 2015 as a FRAX adjustment tool, noting it predicts fracture risk independently of BMD T-score [3]. Longevity clinicians increasingly use TBS alongside T-score because two patients with identical T-scores can have substantially different microarchitecture quality and therefore different fracture risk.

How Bone Loss Actually Happens: Rates and Inflection Points

Age-Related Decline

Cortical bone loss averages roughly 0.5-1.0% per year in adults after age 40 in both sexes. Trabecular bone at the lumbar spine declines faster, around 1-2% per year, because of its higher metabolic activity. The femoral neck follows an intermediate trajectory.

The Menopause Acceleration

In the 5-7 years surrounding menopause, estrogen withdrawal accelerates bone resorption to 2-3% per year at the spine [8]. The SWAN longitudinal cohort found that women can lose 10-12% of lumbar spine BMD during the perimenopausal transition alone, a loss that is largely irreversible without pharmacologic intervention. This is the physiologic rationale for discussing hormone therapy as a bone-protective strategy early in perimenopause.

Testosterone and Male Bone Loss

In men, low bioavailable testosterone and, more specifically, low estradiol (converted from testosterone via aromatase) drives trabecular bone loss. The Massachusetts Male Aging Study and subsequent analyses showed that estradiol levels below approximately 25 pg/mL correlate more strongly with bone loss in aging men than testosterone levels per se [9]. Men on testosterone replacement therapy who aromatize adequately tend to maintain BMD; men who over-suppress estradiol with aromatase inhibitors may accelerate bone loss despite normalized testosterone.

Clinical Indications for DEXA Scanning

Society Guideline Recommendations

The U.S. Preventive Services Task Force (USPSTF) recommends DEXA screening for all women aged 65 and older, and for 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 (approximately 9.3% on FRAX) [10].

The National Osteoporosis Foundation (now the Bone Health and Osteoporosis Foundation, BHOF) extends screening recommendations to men aged 70 and older, and to men aged 50-69 with clinical risk factors [11].

Longevity-Medicine Expanded Indications

Standard guidelines are designed for population-level fracture prevention in the symptomatic age window. A longevity medicine practice typically adds scans for:

• Any adult with a history of eating disorder, malabsorption, or prolonged amenorrhea
• Adults on more than 5 mg prednisone-equivalent glucocorticoids for more than 3 months (GIOP protocol)
• Adults on aromatase inhibitors, GnRH agonists, or long-term proton pump inhibitors
• Adults with type 2 diabetes, who have paradoxically normal or high BMD but impaired bone quality and elevated fracture risk
• Adults with a family history of hip fracture in a first-degree relative
• Any adult requesting a comprehensive longevity panel at baseline, typically around age 35-40

FRAX Score: Translating T-Score Into Absolute Fracture Risk

A T-score alone does not determine treatment. The FRAX tool (developed by WHO Collaborating Centre at Sheffield) integrates T-score with 12 clinical variables to estimate 10-year probability of major osteoporotic fracture (spine, hip, forearm, or proximal humerus) and hip fracture alone [6].

Treatment thresholds most commonly cited in the United States (per BHOF guidance):

• 10-year major osteoporotic fracture probability at or above 20%
• 10-year hip fracture probability at or above 3%

Either threshold, when combined with a T-score at or below -2.5, meets criteria for pharmacologic therapy. A T-score in the osteopenia range (-1.0 to -2.5) with a high FRAX score also justifies treatment, a nuance that standard "T-score only" summaries miss.

The American Association of Clinical Endocrinology (AACE) 2020 guidelines on osteoporosis management specify: "The goal of therapy is to prevent the first fracture in high-risk patients and to prevent subsequent fractures in patients with established disease." [12]

When to Start Alendronate and Other Pharmacologic Options

Alendronate as the First-Line Agent

Alendronate (Fosamax), a nitrogen-containing bisphosphonate, remains the most-studied oral agent for osteoporosis prevention and treatment. The Fracture Intervention Trial (FIT, N=6,459) demonstrated that alendronate 10 mg daily reduced vertebral fracture risk by approximately 47% and hip fracture risk by approximately 51% over 3 years in postmenopausal women with low femoral neck BMD [13].

Standard dosing is 70 mg orally once weekly (equivalent to 10 mg daily), taken with 6-8 oz of plain water at least 30 minutes before any food, beverage, or medication. Patients must remain upright for 30 minutes to reduce esophageal irritation risk.

Duration of Therapy and Drug Holidays

Alendronate incorporates into bone mineral and has a skeletal half-life of roughly 10 years. The FLEX trial (N=1,099) found that women who had taken alendronate for 5 years and then stopped maintained BMD close to their end-of-treatment level for 5 additional years, with no significant increase in non-vertebral fracture risk compared to continued therapy, except for a small elevated risk of clinical vertebral fractures in those with T-scores below -2.5 at the hip [14].

Current practice, per AACE 2020 guidance, is a drug holiday after 5 years of oral bisphosphonate therapy for patients whose hip T-score has risen above -2.5 and whose FRAX risk is low-to-moderate. High-risk patients (prior vertebral fracture, hip T-score still at or below -2.5) should continue or transition to an alternative agent.

Other Pharmacologic Options

• Zoledronic acid (Reclast): 5 mg IV annually; HORIZON-PFT trial (N=7,765) showed a 41% reduction in hip fractures over 3 years [15]. Preferred when GI tolerability or adherence limits oral bisphosphonate use.
• Denosumab (Prolia): 60 mg subcutaneous every 6 months; anti-RANKL mechanism. FREEDOM trial (N=7,808) showed 68% reduction in vertebral fractures and 40% reduction in hip fractures over 3 years [16]. Discontinuation requires transition to a bisphosphonate to prevent rebound bone loss.
• Romosozumab (Evenity): Anti-sclerostin monoclonal antibody; 210 mg subcutaneous monthly for 12 months. ARCH trial showed superiority over alendronate for fracture reduction in very-high-risk patients [17]. Carries a boxed warning for cardiovascular events; not for patients with recent MI or stroke.
• Hormone therapy (estrogen-based HRT/MHT): Reduces bone turnover and preserves BMD. The Women's Health Initiative (WHI) showed a 34% reduction in hip fracture and a 24% reduction in total fractures with combined estrogen-progestogen therapy, though cardiovascular considerations affect patient selection [18].
• Testosterone replacement (men): Maintains BMD in hypogonadal men when estradiol is not suppressed below physiologic range.

Interpreting Serial DEXA Scans: Precision, Change, and Reassurance

Least Significant Change

Every DEXA machine has a precision error, typically 1-2% at the spine and 1.5-3% at the hip, expressed as a coefficient of variation. The least significant change (LSC) at the 95% confidence level is approximately 2.77 times the precision error. A change that does not exceed the LSC is statistically indistinguishable from measurement noise.

For most spine measurements, LSC is roughly 3-4%. A reported "loss" of 1.5% between two scans on the same machine is within noise. A reported loss of 5% is real and clinically actionable.

Comparing Scans Across Machines

BMD values from different manufacturers are not interchangeable. GE Lunar systems and Hologic systems use different reference databases and calibration standards. Serial monitoring should always use the same machine and the same acquisition protocol. If a patient changes provider and machine, the new scan establishes a new baseline rather than a valid comparison to previous values.

Response to Treatment

On bisphosphonate therapy, spine BMD typically increases 3-8% over 3 years and hip BMD increases 1-6% over the same period, based on FIT and HORIZON data [13, 15]. An absence of BMD gain on therapy does not necessarily indicate treatment failure if no new fractures have occurred, because bisphosphonates improve bone quality beyond what BMD captures. Persistent BMD decline on therapy (greater than LSC over 1-2 years) warrants investigation for secondary causes, adherence problems, or calcium/vitamin D insufficiency.

Calcium, Vitamin D, and Lifestyle: The Non-Negotiable Foundation

No pharmacologic agent works optimally on a calcium- and vitamin D-deficient skeleton. The BHOF recommends 1,000-1,200 mg elemental calcium daily from food and supplements combined, and 800-1,000 IU vitamin D3 daily for adults at risk of deficiency [11].

Exercise Specificity

Not all exercise protects bone equally. Weight-bearing impact activity (jumping, running, resistance training) stimulates osteoblast activity through mechanical loading. The LIFTMOR randomized trial (N=101) found that 8 months of high-intensity resistance and impact training produced a 2.9% increase in femoral neck BMD in postmenopausal women with low bone mass, compared with a 1.2% decline in the low-intensity control group [19].

Swimming and cycling, despite their cardiovascular benefits, produce minimal osteogenic stimulus because they are non-weight-bearing. Longevity-oriented bone health programs combine impact loading (jumping, plyometrics) with progressive resistance training targeting hip extension, spinal extension, and the squat pattern. Balance training reduces fall risk independent of BMD.

Protein Intake

Protein provides the collagen scaffold for hydroxyapatite deposition. Adequate protein (at least 1.0-1.2 g/kg body weight daily) is associated with higher BMD and lower hip fracture risk in cohort data, contradicting older concerns about acid load from protein catabolism [20]. The alkaline-diet hypothesis predicting that high protein harms bones has not been supported in randomized trial evidence.

Conditions That Mimic or Mask Bone Disease on DEXA

DEXA measures areal BMD in two dimensions, which creates artifacts clinicians must recognize:

• Spinal osteoarthritis and osteophytes falsely raise lumbar spine T-score, making the spine appear denser than it actually is. Lateral spine projection or hip-only reporting is preferred when degenerative changes exceed mild severity.
• Obesity increases areal BMD through soft-tissue attenuation effects; individuals with BMI above 35 may have falsely elevated T-scores relative to true volumetric bone strength.
• Vertebral compression fractures at L1-L4 compress vertebral height and increase apparent BMD at that level. Vertebral fracture assessment (VFA), available on most modern DXA units, should accompany every spine scan to identify prevalent fractures that alter the diagnostic and treatment picture.
• Type 2 diabetes produces normal or above-normal BMD T-scores despite substantially impaired bone quality and elevated fracture risk. These patients require FRAX adjustment (TBS or clinical diabetes flag) to avoid undertreatment based on T-score alone.