Bone Density and Musculoskeletal Testing: DEXA and Beyond

DEXA: What It Measures and When

Dual-energy X-ray absorptiometry measures bone mineral density at the hip and lumbar spine, and modern machines also give total and regional body composition including lean mass and visceral fat estimates.

Results are reported as T-scores, standard deviations from a young adult reference. Above minus 1 is normal, minus 1 to minus 2.5 is low bone mass, and minus 2.5 or below meets the criterion for osteoporosis. Z-scores, comparing to age-matched peers, are used in younger adults and children.

Who should have one. Guideline positions generally include women from 65 and men from 70, earlier with risk factors, anyone after a fragility fracture at any age, anyone on long-term glucocorticoids, and people with conditions or medications affecting bone. Fracture risk calculators combining clinical factors with or without bone density help decide.

Limitations. Bone density explains only part of fracture risk, since bone quality and microarchitecture also matter and are not captured. Degenerative change and vertebral fracture can artificially raise spine readings in older adults, which is why hip measurements are often more reliable there. Comparisons should use the same machine, since between-machine differences exceed real change.

For body composition, DEXA is a reasonable lean mass measure and it is not the appropriate tool for tracking small changes month to month, given precision limits.

The Functional Tests That Cost Nothing

Grip strength. Measured with a hand dynamometer, which is inexpensive. It associates with all-cause mortality, cardiovascular events, disability and cognitive decline in large prospective analyses, at effect sizes that make it one of the more remarkable simple measures in medicine. It is used as a sarcopenia criterion.

Gait speed. Timed over a short distance at usual pace. Speeds below roughly 0.8 metres per second are associated with adverse outcomes, and gait speed predicts survival across many cohorts. Requires a stopwatch and a corridor.

Sit-to-stand. Repetitions from a chair without using arms in 30 seconds, or the time for five repetitions. Reflects lower body strength and power, and is used in sarcopenia and frailty assessment.

Timed up and go. Standing from a chair, walking three metres, turning and returning. A composite of strength, balance and mobility, associated with fall risk.

Single-leg stand time. Balance measure associated with fall risk and, in some cohorts, with mortality.

Getting off the floor. Not standardised and functionally decisive. Difficulty here predicts a specific loss of independence.

These take about ten minutes in total, require almost no equipment and carry outcome associations comparable to expensive measurements. That combination is rare.

Blood Markers Relevant to Bone

The secondary-cause screen matters because a meaningful proportion of osteoporosis has an identifiable contributor: hyperparathyroidism, thyroid excess, coeliac disease, low testosterone in men, glucocorticoid use, kidney disease or multiple myeloma. Identifying one changes management, and it is routinely omitted when bone loss is attributed to age.

Bone turnover markers are for monitoring therapy rather than for screening, and they have substantial biological variation.

Joint and Soft Tissue Assessment

Osteoarthritis is a clinical assessment. History and examination establish it, and imaging often adds little because radiographic change correlates poorly with symptoms. Many people with significant radiographic change have no pain and vice versa.

When imaging helps. Where a specific structural question changes management, where symptoms are atypical, or before a procedure. Not to confirm what the examination already shows.

The overdiagnosis problem. Imaging findings in asymptomatic people are common: disc degeneration, rotator cuff changes and meniscal tears are frequently found in people with no symptoms and increase with age. Attributing symptoms to an incidental finding leads to unnecessary intervention, and this is one of the better-documented harms of over-imaging.

Ultrasound for tendons. Useful in specific contexts and, again, findings in asymptomatic tendons are common.

What is more useful than imaging for most people: a functional assessment of what you can and cannot do, tracked over time, and a strength assessment of the muscles around the joint.

A Sensible Testing Schedule

From midlife, annually, at home: grip strength, sit-to-stand in 30 seconds, single-leg stand time, and whether you can get off the floor unassisted. Ten minutes, and the trend is the point.

Once, and repeated per clinical advice: DEXA, according to guideline age thresholds or earlier with risk factors. Same machine for any repeat.

With bone concern: vitamin D, corrected calcium, parathyroid hormone, alkaline phosphatase, thyroid function, renal function, coeliac serology, and testosterone in men. This is the secondary-cause screen.

After any fragility fracture, at any age: bone density assessment and secondary-cause screening. This is the single most commonly missed opportunity in this Pillar, particularly in men and in people under 65.

Skip: imaging to confirm clinically obvious osteoarthritis, bone turnover markers for screening, and DEXA repeated at short intervals, where real change is smaller than measurement variation.

Escalate clinically for: any fracture from a low-impact fall, height loss or new spinal deformity suggesting vertebral fracture, unexplained bone pain, joint swelling with morning stiffness over an hour, or a rapid decline in functional test performance.

Why the Functional Tests Deserve More Attention

There is an asymmetry in this Pillar worth stating directly. A DEXA scan measures a risk factor. Grip strength and gait speed measure the thing the risk factor threatens, and they do so with comparable outcome prediction, at no cost, repeatable at home, sensitive to intervention.

They are also motivating in a way a T-score is not. A person who can do 14 sit-to-stands this year and did 11 last year has evidence their training works, on a timescale that sustains behaviour. A bone density scan every few years does not provide that feedback.

The reason they are rarely used is that nobody sells them. A dynamometer costs less than a month of most supplements and predicts more than most tests in this Journal, which is a reasonable summary of how Pillar 7 works generally.