Bone Mineral Density Measurement
Dual energy x-ray absorptiometry (DEXA or DXA) is the preferred method for measurement of BMD. It costs more than single-photon absorptimetry and calcaneal ultrasound and has less radiation exposure than quantitative CT. Its specificity and sensitivity is superior to the other tests.
Results of BMD measurements are reported as raw density in gm/cm2. This is converted, based on normative data, to T- and Z-scores. T-score is a standard deviation of the patient's value compared to young adult female controls. Z-score is a standard deviation of the patient's value compared to age and sex matched controls. T-scores are most useful for postmenopausal women and men over age 65. They should be used more cautiously in other populations.
Osteoporosis was historically defined as a T-score <= 2.5 standard deviations below the mean. The WHO now defines osteoporosis as "a disease characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to enhanced fragility and a consequent increase in fracture risk." For treatment purposes, a patient's fracture risk can be estimated using the FRAX calculator.