Bone densitometry (or bone densitometry) is an imaging test that helps determine bone density. The technique uses an X-ray beam: it consists of measuring the attenuation undergone by the beam after it passes through bone tissue (generally at the level of the spine and a hip).

Bone densitometry is used to diagnoseosteoporosis, a condition that results in loss of bone density and an increased risk of fractures.

Osteoporosis mainly affects postmenopausal women (about 8 to 18% of women over 50 and 5% of men in the same age group are affected).

Several situations can lead the doctor to prescribe a bone densitometry.

In France, the Haute Autorité de Santé considers that measuring bone density is only useful in people with risk factors for osteoporosis:

• in postmenopausal women who have a history of corticosteroid treatment, a body mass index less than 19 (thinness), who had their menopause before 40 years or a history of fracture of the neck of the femur without major trauma;
• in those with a condition which can induce osteoporosis (history of prolonged hypogonadism, untreated hyperthyroidism, hypercorticism, primary hyperparathyroidism, osteogenesis imperfecta);
• in those who have been on systemic corticosteroid therapy for at least 3 months;
• in front of signs of osteoporosis: in case of discovery of a vertebral fracture or fragility fracture.

Bone densitometry is a painless test and no preparation is necessary.

The patient is lying on an x-ray table and the machine (an x-ray tube) takes two measurements: preferably the lower back (lumbar spine) and the upper end of the femur. The dose of x-rays used is lower than for an x-ray.

The result of the bone densitometry is expressed by the T-score, which is the difference between the measured bone density and the theoretical bone density of a young adult of the same sex, at the same bone site (lower back, upper extremity of the femur or wrist).

The results are:

• T-score> – 1: normal density
• — 2,5 < T-score ≤ - 1: ostéopénie
• T-score ≤ – 2,5: osteoporosis (said to be severe in the event of a fracture)

Depending on the results, the doctor may confirm the diagnosis of osteoporosis or estimate the risk of a fracture. He may then decide to prescribe drug treatment to prevent fractures. In all cases, treatment will only be prescribed after correcting any deficiency in calcium and / or vitamin D, by adjusting food intake and / or drug supplementation.

When the result of the first osteodenstiometry is normal or shows osteopenia, and the initiation of treatment has not been followed, a second examination may also be offered 3 to 5 years later, depending on the appearance of new risk factors.