Bone Density Test — Purpose, Procedure & Normal Values | MyMedicPlus

A bone density test, most commonly performed as a DEXA (dual-energy X-ray absorptiometry) scan — also written DXA — measures bone mineral density (BMD) by transmitting two low-dose X-ray beams at different energy levels through skeletal tissue. The detector measures differential absorption to calculate the grams of mineral (primarily calcium hydroxyapatite) per square centimetre of bone area. Endorsed by the World Health Organization as the gold standard for osteoporosis diagnosis and fracture risk estimation, DEXA quantifies both osteoporosis (severe bone loss defined by T-score ≤−2.5) and osteopenia (low bone mass, T-score −1.0 to −2.5) at clinically important skeletal sites. Standard acquisition sites are the left femoral neck (hip) and the lumbar vertebrae L1–L4 (spine); the non-dominant forearm is used when hip or spine measurements are technically unreliable. Results are combined with clinical risk factors in the FRAX fracture risk assessment tool to estimate an individual's 10-year probability of a major osteoporotic fracture or hip fracture, guiding decisions about pharmacological treatment. The scan is performed by radiographers in radiology departments, orthopaedic clinics, and dedicated bone health centres.

DEXA is recommended for all women aged 65 years and above and men aged 70 and above as universal osteoporosis screening. Earlier scanning is indicated for individuals with significant risk factors: prolonged use of corticosteroids (≥5 mg prednisolone daily for ≥3 months), aromatase inhibitors or androgen deprivation therapy; a history of fragility fracture (fracture from a fall from standing height or less) after age 50; low body weight (BMI <18.5 kg/m²); current smoking; heavy alcohol consumption; premature menopause (under age 45) or primary ovarian insufficiency; hypogonadism in men; or secondary causes of bone loss including rheumatoid arthritis, inflammatory bowel disease, coeliac disease, hyperparathyroidism, hyperthyroidism, chronic kidney disease, and multiple myeloma. Serial DEXA scans monitor the skeletal response to osteoporosis treatment (bisphosphonates, denosumab, romosozumab, teriparatide) and track disease progression in untreated osteopenia.

No fasting, bowel preparation, or change to regular medications is required. Wear comfortable clothing without metal buckles, zips, belt clips, or underwire bras — you may be asked to change into a hospital gown to prevent metal artefact on the images. Avoid calcium supplements, antacids containing calcium (Tums, Rennie), and multivitamins for at least 24 hours before the scan, as recent oral calcium ingestion can falsely elevate spine BMD readings. Do not have a barium meal, barium enema, or radionuclide nuclear medicine scan in the 7 days preceding the DEXA, as residual contrast material interferes with image quality. Women of childbearing age should inform the radiographer if there is any possibility of pregnancy, as the scan uses ionising radiation (though at an extremely low dose). Remove wrist and ankle jewellery and inform the radiographer of any prior vertebral fractures or spinal implants, as these affect lumbar spine BMD calculation.

You lie flat and still on a padded DEXA table, fully clothed (or in a gown). A C-arm scanning system with an X-ray source below the table and a detector above it passes slowly along the length of your body. For the hip scan, your leg is positioned in internal rotation using a positioning device to standardise the femoral neck angle. For the lumbar spine, you lie with your legs raised on a foam positioning cube to flatten the lumbar lordosis. Two to three passes per site are acquired in approximately 2–5 minutes each. You do not need to hold your breath. The detector measures the attenuation of both X-ray beams — higher-energy and lower-energy photons are differentially absorbed by bone mineral versus soft tissue — allowing precise calculation of BMD. The full procedure is completely painless, requires no injection or contrast, and exposes you to less radiation than 30 minutes of natural background radiation. The radiographer confirms image quality before you leave; results are reviewed and reported by a radiologist or bone densitometry specialist.

DEXA results are reported as two scores. The T-score compares your BMD to that of a healthy young adult of the same sex at peak bone mass: T-score ≥−1.0 — Normal bone density; T-score between −1.0 and −2.5 — Osteopenia (low bone mass); T-score ≤−2.5 — Osteoporosis. The Z-score compares your BMD to age-matched and sex-matched peers: a Z-score below −2.0 is considered significantly below expected for age and warrants investigation for secondary causes of bone loss. The diagnostic T-score threshold applies specifically to postmenopausal women and men aged 50 and over at the femoral neck; different criteria apply in premenopausal women and younger men. Clinicians integrate the T-score with the FRAX algorithm — which incorporates age, sex, weight, height, smoking, alcohol, parental hip fracture history, and secondary causes — to calculate the 10-year probability of major osteoporotic fracture (hip, wrist, shoulder, or clinical spine fracture) and hip fracture alone. Treatment thresholds vary by country but generally align with a 10-year hip fracture risk ≥3% or major fracture risk ≥20% per National Osteoporosis Foundation guidance.

DEXA is one of the safest imaging procedures available. The radiation dose ranges from 1–10 microsieverts (μSv) per scan — equivalent to a few hours of natural background radiation and dramatically lower than a chest X-ray (100 μSv) or abdominal CT (10,000 μSv). The negligible radiation dose makes DEXA safe for repeated monitoring every 1–3 years as clinically indicated. DEXA is contraindicated only during pregnancy as a precautionary measure. Clinically important limitations include: vertebral osteoarthritis (osteophytes, endplate sclerosis, aortic calcification, and facet joint hypertrophy) all falsely elevate lumbar spine BMD, potentially masking true osteoporosis — an important issue in the older age group most affected. Vertebral fractures similarly elevate measured BMD at that level. Severe obesity, spinal implants, or hip prostheses prevent reliable measurement at the affected sites. BMD measures bone quantity but not bone quality (microarchitecture, collagen cross-linking, mineralisation homogeneity), so fracture risk cannot be fully predicted from BMD alone. Additional tools including TBS (trabecular bone score) and high-resolution pQCT may provide supplementary bone quality data. DEXA cannot diagnose the cause of bone loss — secondary causes require blood tests (calcium, vitamin D, PTH, thyroid function, testosterone, protein electrophoresis) and sometimes bone biopsy.