Bone mineral density (BMD) is a measure of the amount of minerals, primarily calcium and phosphorus, contained in a specific volume of bone. It is typically expressed in grams of mineral per square centimeter (g/cm²) and is used to assess the strength and density of a person's bones. BMD is a critical indicator of bone health and is often used to diagnose and monitor conditions such as osteoporosis, a disease characterized by low bone mass and deterioration of bone tissue, which can lead to an increased risk of fractures.

The measurement of BMD is usually done using a non-invasive imaging technique called dual-energy X-ray absorptiometry (DXA or DEXA). A DXA scan involves passing low-dose X-rays through bone and measuring the amount of X-ray that is absorbed by the bone. The denser the bone, the less X-ray is absorbed, and this information is used to calculate bone mineral density.

BMD measurements are typically taken at specific sites in the body, such as the spine, hip, or forearm, as these are common areas where fractures occur due to osteoporosis. The results are compared to the bone density of a young, healthy adult population to determine a T-score, which quantifies how an individual's BMD compares to that of a healthy young adult. A T-score below -2.5 indicates osteoporosis, and a T-score between -1 and -2.5 indicates osteopenia, a condition of low bone density that is less severe than osteoporosis.

Monitoring bone mineral density is important in the assessment of bone health, and it helps healthcare professionals make recommendations for lifestyle changes, medications, and other interventions to reduce the risk of fractures and bone-related problems.