Comparison of forearm bone mineral density between lumbar spine and hip: a useful tool to screen osteoporosis in female patients with connective tissue disease

Abstract

Objective To compare the forearm bone mineral density (BMD) between lumbar spine and left hip BMD by dual-energy X-ray abosorptionmetry (DXA), and explore the diagnostic value of the forearm BMD in female connective tissue disease (CTD) patients. Methods In the study, 181 female patients with established CTD underwent DXA of the lumber, left hip and non-superiority forearm DXA at the same time. We compared BMD at different sites, and the diagnostic cut-off value of abnormal axial BMD by forearm BMD was explored. Data analysis was performed by independent-sample t test and Pearson's correlation test. The area under the receiver operating characteristic curve (AUC) and calibration of predictions were assessed. Sen-sitivity and specificity were calculated to determine the correlation between cases of osteoporosis detected by the axial DXA scan and forearm. Results ① The mean age of the 181 female patients was (50±16) years. Thirteen (7.2% ) patients had the fragile fracture history. Based on their axial DXA data and fracture history, 69(38.1%) patients had normal BMD (T score≥-1.0), 112(61.9%) had abnormal BMD. The mean forearm BMD was (0.37±0.12) g/cm2 and T score was -2.1±1.7. ② Compared with abnormal axial BMD group, forearm BMD in normal axial group was significantly decreased [(0.33 ± 0.12) g/cm2vs (0.42 ± 0.09) g/cm2, t=5.40, P<0.01)], forearm T score was also significantly decreased [(-1.9 ± 1.8) vs (-1.2 ± 1.3), t=6.23, P<0.01]. T score of the forearm BMD was positively correlated with axial BMD T score, both at lumber and left hip respectively (r= 0.489, 0.571, P<0.01). After correction by age and BMI, T score of the forearm remained positively correlated with axial T score by partial correlation analysis (Pearson partial vertebral r=0.478, P<0.01; left hip r=0.572, P< 0.01). ③ The area under the ROC curve for diagnosis of lumber and hip osteoporosis was 0.757 (95%CI: 0.638, 0.876) and 0.615(95% CI: 0.481, 0.749) respectively. ④ The sensitivity and specificity for identifying osteoporosis in lumber was 86.5% and 50.1% respectively when the T-score threshold of forearm was defined as-3.52; however, the sensitivity and the specificity for identifying osteoporosis at left hip was 66.2% and 76.2% when T score threshold of forearm was defined as-2.5. Conclusion Our study has confirmed that DXA measurement performed of forearm analysis is capable of screening osteoporosis defined by axial BMD in female CTD patients. Key words: Forearm bone mineral density; Osteoporosis; Connective tissue disease