Precision of Dual Photon Absorptiometry Measurements

used to assess bone mass: simple radiography, neutron activation analysis, radiogammetry, and absorptiome try (1—8). In the early 1970s single photon absorptiom etry gained popularity because of the good precision and accuracy (@@-3%), and the low radiation exposure (@5 mrem). However, single photon absorptiometry is limited to the peripheral skeleton which, although cor related with the clinically important axial skeletal regions, is not optimal for diagnosis of axial skeletal disease (9—10). Dual photon absorptiometry allows the assessment of bone mineral of the lumbar spine and femoral neck. These instruments are useful for estab lishing present or future fracture risk and to monitor change. Spinal fracture risk is believed to be highly correlated with skeletal bone mineral, and good accu racy is important. The accuracy is @—5% (11). Since fracture risk is not believed to be significantly increased until bone mineral has decreased by 20% (bone mass is 80% of normal), an accuracy of 5% seems adequate and is not likely to significantly affect diagnosis (12,13). Precision is of primary importance in determining temporal changes in bone mineral to decide whether an observed difference is real or not. Temporal changes