Bone Density: What Do the Numbers Mean?

Joseph R. Anticaglia MD
Medical Advisory Board

The adult human skeleton has a total of 213 bones. Each bone goes through a lifelong process of making new, healthy bone tissue called bone modeling, and removing of old bone tissue labeled bone resorption. Bone loss happens when the body resorbs more bone than it makes.

Bone density, or bone mineral density (BMD), is the amount of mineral, such as calcium, collagen, and phosphorous, in bone tissue. Osteopenia and osteoporosis are conditions that adversely affect bone health and bone density.

In osteopenia, (low bone density), your bone density is abnormal but not decreased enough to cause your bones to break easily. It can lead to osteoporosis, a condition wherein the bone density is significantly compromised to the extent that your bones can break easily; there have been reports of bones breaking spontaneously.

“Four ladies were enjoying a game of bridge when one of them said, “Did you hear what happened to Jennifer?”

What happened?

“She”s in the hospital. Three days ago, she slipped, fell, and broke the bone in her right leg — the one that goes from the hip to the knee. Had to have surgery.”

Oh, my goodness! Is she all right?

“The surgery went well, but she”s in a lot of pain. Her doctor said the outside of the bone was as thin as an eggshell. It took little force to break the bone. The inside of the bone did not offer much support. Something I didn”t quite understand called low bone density.”

Bone Density Test

A bone density test measures bone mineral content and the degree of bone density (compactness). It is done to determine whether you have normal bone density, osteopenia or osteoporosis. The measurement may be done using X-rays, dual-energy X-ray absorptiometry (DEXA or DXA), or a special CT scan that uses computer software,

“T” Score”

Your T-score compares your bone density, or bone mass, to that of a young healthy adult of your sex. It is the number of units of measurements(standard deviation) that your score is above or below the average bone density. In a previous post, I discussed the T-score.

Fig-1 Credit Ben Godwin: How is Bone Density Measured

Your bone density is considered normal if the -score is –1 or above. A score between –1 to –2.5 is a sign of osteopenia. In this instance, the bone density is below normal and may progress to osteoporosis. A score of –2.5 and below indicates you have osteoporosis.

In Jennifer”s case, before being discharged from the hospital, she had a Bone Mineral Density test performed. The radiologist used the following classification system:

BMD Range WHO category
Above 120 mg/cm3 Normal
80 to 120 mg/cm3 Osteopenia
Below 80 mg/cm3 Osteoporosis

Credit American College of Radiology

Jennifer”s test results using the above classification averaged 75.7 for the first two lumbar bones (vertebrae). Her T-Score was (minus) –3.57.

Who Might Benefit from the Bone Density Test?

  • Drugs: People on certain drugs, like prednisone, or other steroid medications, taken over a lengthy period can interfere with bone formation and weaken your bones.
  • Age: Post-menopausal women not taking estrogen
  • Women over sixty-five and men over seventy
  • Fragility Fractures: People who break bones more easily, and more frequently, should consider having a bone density test.
  • Height: As you get older are you getting shorter? People who have lost 1.5-2 inches or more in height could benefit from the density test
  • Hormones: A drop in estrogen or testosterone levels makes you more susceptible to bone fractures.
  • Diseases: People with rheumatoid arthritis, for instance, have been associated with low bone mass (osteoporosis) and an increased risk of hip fracture.
  • People with a family history of hip fracture; in addition, those who smoke or drink excessively are more prone to have weakened.

Bone density peaks around 30 years of age. Women lose bone mas more rapidly than men. Bone density tests help the clinician diagnose osteopenia, osteoporosis, evaluate your risk for fracture, prescribe medication to make your bones stronger and tell whether the medication is working or not. We need strong, healthy bones not only to minimize the likelihood of fractures but to enable our body to function optimally.


  1. Shevaun Mackie Doyle, MD; Low Bone Density and Osteoporosis in Children; HSS, 6/26/2009
  2. Joseph R. Anticaglia MD; Osteoporosis: What to Know — What to Do; Doctor”s Column, HC Smart, January 2019
  3. Joseph R. Anticaglia MD; Osteoporosis Revisited; Doctor”s Column, HC Smart, September 2021


The key functions of the skeletal system are body support, facilitate movement and local motion, protect internal organs, provide blood cell formation, and storage of minerals. We need strong, healthy bones not only to minimize the possibility of fractures but to enable our body to function optimally.

This article is intended solely as a learning experience. Please consult your physician for diagnostic and treatment options.