Bone mineral density: testing for osteoporosis.

Primary osteoporosis is related to bone loss from ageing. Secondary osteoporosis results from specific conditions that may be reversible. A thoracolumbar X-ray is useful in identifying vertebral fractures, and dual energy X-ray absorptiometry is the preferred method of calculating bone mineral density. The density of the total hip is the best predictor for a hip fracture, while the lumbar spine is the best site for monitoring the effect of treatment. The T-score is a comparison of the patient's bone density with healthy, young individuals of the same sex. A negative T-score of -2.5 or less at the femoral neck defines osteoporosis. The Z-score is a comparison with the bone density of people of the same age and sex as the patient. A negative Z-score of -2.5 or less should raise suspicion of a secondary cause of osteoporosis. Clinical risk calculators can be used to predict the 10-year probability of a hip or major osteoporotic fracture. A probability of more than 5% for the hip or more than 20% for any fracture is abnormal and treatment may be warranted.