Osteoporosis is defined by a BMD ≤ 2.5 SD below the young adult reference population. Standard dual-energy X-ray absorptiometry (DXA) scans for osteoporosis involve the femoral neck and lumbar spine, but alternative sites like the one-third radius (1/3 R) are only used when these sites are inaccessible. This study assessed the correlation and level of agreement between BMD at the 1/3 R, femoral neck, and lumbar spine to evaluate its diagnostic utility. Data from 43,801 patients referred for DXA scans in northwest England were analysed. Of these, 437 underwent 1/3 R scans. Demographic comparisons between patients with and without forearm scans were conducted. The primary analysis included patients with scans at the 1/3 R, lumbar spine, and bilateral femoral regions;(n = 183). Spearman's correlation assessed BMD relationships, Cohen's kappa analysed osteoporosis classification agreement, and Bland-Altman plots evaluated measurement bias. The cohort had a mean age of 65.7 years (SD 12.9), with 83.3% female and 41.2% reporting fractures. Patients who underwent 1/3 R scans (n = 437) were older, heavier, and had a higher body mass index (BMI). Correlation analysis showed only moderate associations between 1/3 R and femoral/lumbar spine BMD ;(r = 0.29 to 0.36, p < 0.001). Cohen's kappa demonstrated only slight agreement for 1/3 R, femoral neck and lumbar spine T-scores (κ = 0.14-0.29). Bland-Altman analysis revealed that 1/3 R scans systematically underestimated BMD relative to femoral and lumbar sites, with mean biases of -0.7 for femoral sites and -1.53 for lumbar spine. The 1/3 R BMD showed poor agreement and systematic underestimation compared to central sites, limiting its reliability for osteoporosis diagnosis. Future research should explore alternative peripheral weight-bearing sites and novel diagnostic technologies to assess BMD where central sites cannot be scanned.