A comparison of a peripheral DXA system with conventional densitometry of the spine and femur

Because of the perceived high cost of dual-energy X-ray absorptiometry (DXA ) studies of the spine and femur, there is renewed interest in small, low-c ost X-ray devices for scanning the peripheral skeleton. We have compared fo rearm bone mineral density (BMD) measurements (distal and ultradistal sites ) performed on a DTX-200 (Osteometer MediTech, Hoersholm, Denmark) with spi ne (L1-L4) and femur (femoral neck and total hip sites) scans performed on a QDR-4500 (Hologic Waltham, MA) in 172 white UK women aged 22-84 yr with a view to establishing differences caused by inconsistent reference ranges a nd different age-related changes in BMD. All BMDs were expressed as T-score s using the manufacturers' reference ranges for the forearm and spine, and the National Health and Nutrition Examination Survey (NHANES) ranges for th e femur. Linear regression between peripheral and axial sites gave correlat ion coefficients r = 0.71-0.74 and roof mean standard errors (RMSE) 0.88-1. 14 in T-score units. Subjects were divided into the following five age grou ps: <40 yr; 40-49 yr; 50-59 yr; 60-69 yr and greater than or equal to 70 yr . A large systematic difference between distal and ultradistal T-scores (me an Delta T = 0.59, SEM = 0.05) was found affecting all age groups. When the mean difference in T-score between each forearm site (distal, ultradistal) and each axial site (spine, femoral neck, total hip) was examined for prem enopausal subjects (n = 58) the mean difference between forearm and axial T -score showed a consistent negative offset (Delta T = -0.41 to -0.48) for t he distal forearm site and a consistent positive offset (Delta T = +0.30 to +0.37) for the ultradistal site. When interpreting results in postmenopaus al women, age-related T-score changes in the forearm were in close agreemen t with the femoral neck region of exterest (ROI), but systematic difference s were found between the forearm and the spine and total hip sites. The two forearm and three axial sites were compared to evaluate the number of post menopausal subjects identified as osteoporotic on the basis of the World He alth Organization (WHO) Study Group criteria (T-score <-2.5). Although fore arm and spine T-scores identified approximately equal numbers of subjects a s osteoporotic (distal 38/114; ultradistal 31/114; spine 30/114), the two f emur sites identified fewer subjects as osteoporotic (femoral neck 25/114; total hip 16/114). The number for the total hip site was statistically sign ificantly smaller than the spine and forearm sites. In conclusion, we have identified systematic differences between T-score results for a peripheral and an axial DXA device that may have a significant effect on the interpret ation of BMD measurements.