Last updated: November 8, 2014
Synonyms: Single-photon absorptiometry, dual-energy x-ray absorptiometry (DEXA).
CPT- Code: DEXA axial skeleton (including hips/pelvis) 77079; Appendicular skeleton(e.g., radius) 77078
Description: Densitometry is a noninvasive procedure that uses a radiation source to quantitate bone mineral density. This imaging technique is widely used in the evaluation of osteoporosis and other metabolic bone disorders. Densitometry is far superior to radiography in estimating bone mass. Newer techniques for measuring bone density that do not use radiation [ultrasonography (US), MRI] are under evaluation.
Method: Bone is exposed to a radiation source (photons or gamma rays), and as the rays pass through the bone, the amount of attenuation is quantitated. Observed results are compared with gender- and age-specific normal values. The differences between observed and expected values may be used to assess the risk of fracture. The most common method currently in use is DEXA, which allows a shorter scanning time and is more precise than single-photon absorptiometry. Single-photon absorptiometry produces a radiation dose of 15 mrem and DEXA produces <5 mrem. Unlike some other methods, DEXA can be used to assess both trabecular and cortical bone.
Commonly, two sites (lumbar spine and hip) are evaluated. These are common sites of fracture, and each represents a different bone-type trabecular bone in the spine and cortical bone in the hip. The patient is positioned with the hips flexed to reduce the normal lumbar lordotic curvature and a posteroanterior view is obtained. Vertebral bodies with compression fractures should be avoided. The procedure usually takes 10 to 20 minutes. Intravenous contrast is not used.
Normal Values: Most equipment reports standards for normal individuals of both genders in age-specific categories. Results are expressed relative to both the age-matched, gender-matched group and to normal young controls of the same gender. The T score is most important amongst these and refers to the number of standard deviations the bone mineral density differs from gender-matched normal young individuals. Normal T scores are greater than -1.0.
Increased in: T scores may be increased in fractures and in rare bone disorders such as osteopetrosis or with heterotopic bone formation and ankylosis (e.g., ankylosing spondylitis).
Decreased in: T scores between -1.0 and -2.5 indicate osteopenia (low bone mass). Values equal to or less than -2.5 indicate osteoporosis and a significant risk for fracture. Bone mineral density may be low in postmenopausal women, hypogonadal men, patients with excess endogenous or exogenous glucocorticoids, immobilized patients (or limbs), hyperthyroidism, and primary hyper-parathyroidism.
Confounding Factors: Fractures, even in very thin bones, can cause false elevations in calculated bone density. Heavy calcification in the abdominal aorta can confound readings for the lumbar spine. Residual barium may produce significant artifact.
Indications: With the advent of numerous therapies to treat osteoporosis, diagnosis of this condition has become more important, but there is no universal agreement on when to perform densitometry. New guidelines for DEXA testing have been developed by the National Osteoporosis Foundation and include (a) all women aged 65 and older; (b) younger postmenopausal women with one or more risk factors; (c) postmenopausal women who present with fracture; (d) estrogen-deficient women at clinical risk of osteoporosis; (e) individuals with vertebral abnormalities; (f) individuals starting or receiving long-term glucocorticoid therapy; (g) individuals with primary hyperparathyroidism; (h) individuals while being monitored on osteoporosis drug therapy.
An initial two-site DEXA is reasonable for high-risk patients or individuals who have already sustained a fracture. After initiation of treatment, it is also reasonable to repeat the examination yearly to determine whether the chosen therapy has been effective.
Cost: The usual cost is $150–$350 for one or more sites. Medicare allots $126 for the entire examination, regardless of the number of sites. Medicare allows DEXA once every 2 years (or every 12 months if the patient is on steroids).
Bray VJ. Osteoporosis Screening Guideline. ISCS.org
Grossman JM, Gordon R, Ranganath VK, Deal C, et al. American College of Rheumatology 2010 recommendations for the prevention and treatment of glucocorticoid-induced osteoporosis. Arthritis Care Res 2010; 62):1515-26. PMID:20662044
National Osteoporosis Foundation. National Osteoporosis Foundation Releases New Clinical Recommendations for Low Bone Mass and Osteoporosis Incorporating Absolute Fracture Risk. Available at http://nof.org/news/174