🔬 DEXA Scan

A DEXA scan is a low-radiation X-ray that measures Bone Mineral Density (BMD). It is the gold standard for diagnosing osteoporosis and osteopenia. By comparing a patient's bone density to that of a healthy young adult (T-score), clinicians can assess the risk of future fractures and decide on the necessity of bone-sparing medications. It is most commonly performed on the hip and lumbar spine and is interpreted alongside clinical risk factors using tools like FRAX.

Indications include women >65 and men >75, or younger patients with risk factors: previous fragility fracture, low BMI (<19), long-term systemic corticosteroid use (over 3 months), parental history of hip fracture, or secondary causes (e.g., Rheumatoid Arthritis, malabsorption, premature menopause, or prolonged secondary amenorrhea). It is also used to monitor BMD in patients already on treatment (usually every 2-3 years) and to assess bone health in patients with conditions like Primary Hyperparathyroidism or those undergoing androgen deprivation therapy for prostate cancer.

Dual-energy X-ray Absorptiometry (DEXA or DXA) uses two X-ray beams with different energy levels to measure Bone Mineral Density (BMD) in g/cm². After subtracting the absorption of soft tissue, the bone mineral component can be determined. Usually, the lumbar spine and the proximal femur (hip) are scanned, as these are the most clinically significant sites for fracture. The procedure is quick (10-20 mins), non-invasive, and involves a very low dose of radiation (equivalent to less than a day’s background radiation).

A T-score of -1.0 SD or higher is considered normal. This indicates that the patient's bone density is within one standard deviation of the peak bone mass of a healthy young adult. Normal findings suggest a low risk of fragility fracture, although lifestyle advice (adequate calcium/vitamin D intake and weight-bearing exercise) should still be provided, especially as patients age and bone density naturally declines.

The T-score is the most critical value for clinical decision-making; it compares the patient's BMD to that of a healthy young adult (peak bone mass). The Z-score compares the patient to an age-matched, sex-matched, and ethnicity-matched population. In pre-menopausal women and younger men, the Z-score is more clinically relevant than the T-score. Clinicians use DEXA results alongside clinical risk factors via the FRAX or QFracture tools to determine the 10-year probability of a major osteoporotic fracture. An 'amber' result on FRAX usually indicates the need for a DEXA scan.

Results are reported as a T-score and Z-score. A T-score of -1.0 to -2.5 SD defines Osteopenia. A T-score of -2.5 SD or below defines Osteoporosis. 'Severe' or 'established' osteoporosis is defined as a T-score below -2.5 SD in the presence of one or more fragility fractures. A significantly low Z-score (e.g., <-2.0) indicates that bone loss is much greater than expected for the patient's age and should prompt a search for secondary causes of osteoporosis, such as hyperparathyroidism, malabsorption, or steroid use.

The DEXA scan is the definitive tool for diagnosing osteoporosis and assessing fracture risk. Identifying low bone mineral density (BMD) allows for the initiation of pharmacological treatments (e.g., bisphosphonates, denosumab) and lifestyle interventions (weight-bearing exercise, Vitamin D/Calcium optimization) to prevent debilitating fragility fractures of the hip, spine, and wrist. It is a cornerstone of the NICE FRAX (Fracture Risk Assessment Tool) pathway, helping to decide which patients require medical intervention.

One major pitfall is ignoring the Z-score in younger patients; a T-score might look 'low' but be normal for their age, whereas a very low Z-score demands a secondary cause workup. Another is scanning only one site; spinal density can be misleadingly high in the elderly due to degenerative changes, while the hip density is a more reliable predictor of fracture risk. Patients should also be reminded not to take calcium supplements on the morning of the scan, as un-dissolved tablets in the GI tract can interfere with the image.

DEXA is a 2D projection of a 3D structure; it can be falsely elevated by spinal osteoarthritis (osteophytes), vertebral compression fractures (which make the bone look 'denser' in that area), or vascular calcification (e.g., aortic calcification). It cannot distinguish between osteoporosis and osteomalacia (vitamin D deficiency), and it provides information on bone quantity but not bone 'quality' (microarchitecture). It is also less accurate in patients with extreme BMIs (very obese or very thin) due to tissue attenuation issues.

For exams, remember the WHO criteria: -1.0 to -2.5 is osteopenia, <-2.5 is osteoporosis. If a patient over 75 has a clinical fragility fracture, some guidelines (like SIGN) allow for the diagnosis of osteoporosis without a DEXA, but NICE generally recommends DEXA to establish a baseline. Remember to calculate FRAX scores first; DEXA is often the mid-step in the 'FRAX -> DEXA -> Re-calculate FRAX' pathway. Be aware of secondary causes of osteoporosis (SHATTERED mnemonic: Steroids, Hyperthyroidism/Hyperparathyroidism, Alcohol/Ascites, Thin, Testosterone low, Early menopause, Renal/Liver failure, Erosive/inflammatory disease, Dietary/Diabetes).