A measurement of how much mineral (mostly calcium) is packed into your bones—the standard test for osteoporosis.Bone mineral density (BMD) is measured by dual-energy X-ray absorptiometry (DXA) at the lumbar spine, femoral neck, and total hip, expressed as T-scores (standard deviations from young adult mean). T-score ≤ −2.5 defines osteoporosis; −1.0 to −2.5 defines osteopenia. BMD is a surrogate endpoint—fracture reduction is the clinically meaningful outcome. Anabolic peptides (teriparatide, abaloparatide) increase BMD more rapidly than antiresorptives, but BMD gains do not always proportionally predict fracture risk reduction.