BMD stands for bone mineral density, a measurement of how much calcium and other minerals are packed into your bones. It’s the standard way doctors assess bone strength and diagnose conditions like osteoporosis. A simple, painless scan gives you a score that indicates whether your bones are healthy, thinning, or at risk for fractures.
What BMD Actually Measures
Your bones aren’t solid like rock. They’re living tissue constantly being broken down and rebuilt. Minerals, primarily calcium, give bones their hardness and structural integrity. Bone mineral density quantifies the concentration of these minerals in a given area of bone. Higher mineral density means stronger, more fracture-resistant bones. Lower density means the internal structure has become more porous and fragile.
Throughout your life, your body removes old bone and replaces it with new bone. Until about age 30, you build bone faster than you lose it. After that, the balance gradually shifts. Bone loss outpaces new growth, and density slowly declines with age. How quickly that happens depends on genetics, hormones, diet, physical activity, and other health factors.
How a BMD Test Works
The gold standard for measuring bone mineral density is a DXA scan (sometimes written DEXA), which stands for dual-energy X-ray absorptiometry. It uses two low-dose X-ray beams at different energy levels. Because bone and soft tissue absorb these beams differently, the scanner can isolate how dense your bones are with high precision.
During the scan, you lie on a padded table while a scanning arm passes over your hips and spine. A second scanner passes beneath you at the same time. The combined images are sent to a computer that calculates your bone density. The whole process takes about 10 to 20 minutes, involves no injections, and exposes you to far less radiation than a standard chest X-ray.
The hip and lower spine are the most common sites tested because they’re the areas most vulnerable to osteoporotic fractures. In some cases, your wrist, hand, or foot may be scanned instead using a smaller portable device called a peripheral DXA scanner.
Preparing for the Scan
Preparation is minimal. Wear loose, comfortable clothing without metal zippers, buttons, or buckles, since metal interferes with the X-ray images. You’ll need to remove any jewelry. If you take calcium supplements or over-the-counter vitamins, stop them 24 hours before your appointment. No fasting or other restrictions are needed.
Understanding Your T-Score
Your BMD result is reported as a T-score, which compares your bone density to that of a healthy 30-year-old adult at peak bone mass. The score is expressed as a number above or below zero:
- T-score of -1.0 or higher: Normal, healthy bone density.
- T-score between -1.0 and -2.5: Osteopenia, meaning bone density is below normal but not yet in the osteoporosis range. Think of it as a warning zone.
- T-score of -2.5 or lower: Osteoporosis, meaning bones have lost enough density to be significantly more fragile and fracture-prone.
Every full point drop in T-score roughly doubles fracture risk. So a score of -2.0 carries meaningfully more risk than -1.0, even though both fall short of the osteoporosis threshold.
For younger adults, premenopausal women, and men under 50, doctors use a Z-score instead. This compares your density to others your same age and sex rather than to peak bone mass. A very low Z-score in a younger person suggests something beyond normal aging is affecting the bones, such as an underlying medical condition or medication side effect.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends routine BMD screening for all women 65 and older. Postmenopausal women younger than 65 should also be screened if they have risk factors such as low body weight, a parent who fractured a hip, smoking, excess alcohol use, or long-term use of corticosteroids or certain other medications.
For men, there isn’t enough evidence yet for a blanket screening recommendation. However, men with specific risk factors (long-term steroid use, low testosterone, heavy alcohol use, or a history of fractures) are often tested on a case-by-case basis. These guidelines apply to adults 40 and older who haven’t already been diagnosed with osteoporosis or had a fragility fracture.
How Often to Repeat the Test
Bone density changes slowly, and most DXA scanners can’t reliably detect small shifts over short periods. The general recommendation is to wait at least two years between scans, because changes over shorter intervals are often smaller than the scanner’s margin of error.
For healthy women 67 and older whose initial scan shows normal bone mass, recent evidence suggests retesting may not be needed for up to 10 years, as long as no new risk factors develop. On the other hand, someone starting treatment for osteoporosis may get a follow-up scan at two years to check whether the treatment is working. Repeating the scan only makes sense when the result would change what you or your doctor decide to do next.
What Causes Low Bone Density
Age is the single biggest factor. As you get older, bone breaks down faster than it rebuilds. But several other forces accelerate that process.
Hormonal changes play a major role. Estrogen helps protect bone, so women lose density rapidly in the years following menopause. Younger women who lose their periods due to extreme exercise, eating disorders, or hormone imbalances face the same risk. Low testosterone in men has a similar, though less dramatic, effect.
Diet matters throughout life. Chronically low intake of calcium and vitamin D weakens bones starting in childhood. Excessive dieting and poor protein intake also contribute to bone loss. Weight-bearing exercise (walking, running, lifting weights) stimulates bone growth, so a sedentary lifestyle works against you.
Certain medications are known to thin bones over time. Long-term corticosteroids, commonly prescribed for asthma, rheumatoid arthritis, and autoimmune conditions, are among the biggest culprits. Anti-seizure medications and some cancer treatments also increase risk. Medical conditions including thyroid disorders, gastrointestinal diseases that impair nutrient absorption, rheumatoid arthritis, anorexia nervosa, and HIV/AIDS can all contribute to bone loss as well.
What a Low Score Means for You
A BMD diagnosis of osteopenia doesn’t mean you’ll inevitably develop osteoporosis. It means your bones are thinner than ideal and you’d benefit from taking steps to slow further loss: regular weight-bearing exercise, adequate calcium and vitamin D through food or supplements, and avoiding smoking and excessive alcohol. Your doctor may also evaluate whether any medications you’re taking could be contributing.
An osteoporosis diagnosis carries a higher fracture risk, particularly in the hip, spine, and wrist. Treatment typically involves prescription medications that either slow bone breakdown or stimulate new bone formation, combined with the same lifestyle measures. The goal is to stabilize or improve density enough to reduce fracture risk over time. Falls become a bigger concern at this stage, so practical steps like removing tripping hazards at home and improving balance through exercise become part of the picture.
Your T-score is one piece of the puzzle. Doctors also consider your age, fracture history, family history, and overall health when deciding how aggressively to treat low bone density. Two people with the same T-score can have very different actual fracture risks depending on these other factors.