Objectives: To quantify between-day reliability of upper-body diaphyseal measurements (radius, ulna, humerus) using peripheral Quantitative Computed Tomography (pQCT).

Methods: Fourteen males (age: 25.8±2.3 years,) underwent repeat pQCT scans (one to two days apart) at mid-shaft ulna (60%), mid-shaft radius (60%) and mid-shaft humerus (50%) cross-sections of the non-dominant limb. Intraclass correlation coefficients (ICC) and coefficients of variation (CV) were determined for musculoskeletal morphology variables.

Results: Reliability was excellent (ICC: 0.76–0.99; CV: 1.3–7.3) at all sites for bone mass, stress-strain index, endocortical and pericortical radius, endocortical volumetric bone mineral density (vBMD), muscle area, total area, non-cortical area, and cortical area. Reliability was good to excellent (ICC: 0.58–0.80; CV: 0.6–3.7) for polar vBMD and mid-cortical vBMD; fair to excellent (ICC: 0.30–0.88; CV: 0.5–8.0) for muscle density and cortical density; and fair to good (ICC: 0.25–0.60; CV: 3.4–7.6) for pericortical vBMD. Average reliability across the three sites was excellent (ICC ≥0.77; CV ≤8.0).

Conclusions: Overall between-day reliability of pQCT was excellent for the mid-shaft ulna, radius and humerus. pQCT provides a reliable and feasible body composition and skeletal morphology assessment tool for upper limb longitudinal investigations in scientific and clinic settings.