Kristen Gasnick, PT, DPT, is a medical writer and a physical therapist at Holy Name Medical Center in New Jersey.

Yaw Boachie-Adjei, MD, is a board-certified, double-fellowship Orthopedic Surgeon.

A bone density test is used to examine your bone mass, an indicator of your overall bone strength.

Assessing your bone density, or bone mass, is necessary for diagnosing osteopenia or osteoporosis, conditions that increase your risk of broken bones, but a bone density test generally cannot be used to diagnose arthritis. 

This article will review what a bone density test is used for, how it is performed, what the results mean, and how arthritis is diagnosed. 

Jose Luis Pelaez Inc / Getty Images

A bone density test is a procedure used to examine your bone density, or bone mass. The density, or mass, of your bones is an overall indicator of your bone strength. The greater your bone density, the thicker and stronger your bones are. 

A bone density test is used to diagnose osteoporosis, a condition that causes brittle bones at risk for breaking due to significantly low bone density. A bone density test can also diagnose osteopenia, a condition characterized by lower than normal bone mass that can lead to osteoporosis.

It is recommended that all women aged 65 and older and all men aged 70 and older have a bone density scan performed to screen for bone loss to help prevent fractures.

Bone density scans can be used to establish a baseline level of your bone density and track changes over time. If you already have osteoporosis or osteopenia, a bone density scan can be used to track how well your bones are responding to treatment.

The most common bone density test is dual energy X-ray absorptiometry scan, or DEXA scan. A DEXA scan is similar to getting an X-ray taken, but it uses two X-ray beams to produce a more detailed and sensitive reading.

During a DEXA scan, you will lie on your back on a table with your legs elevated on a padded platform. An X-ray scanner will pass over your spine and hips while another X-ray scanner scans beneath you. While the scan is taking place, you will need to hold very still in order to obtain an accurate image.

The DEXA scan will obtain bone density readings from your spine and hip—the two most commonly fractured bones—and generally takes less than 30 minutes. 

A DEXA scan measures bone density by grams per centimeter squared (g/cm2). This number indicates how dense bone cells are packed together in a specific area of bone. This bone density reading is then compared to a standardized value to determine if your bone density is within a normal range or lower than average. 

For postmenopausal women and men aged 50 and older, bone density values are reported as a T score. T scores are compared to a standardized bone density level of a healthy 30-year-old adult who has peak bone density levels.

 T scores indicate the following:

For women who have not undergone menopause and men under 50 years old, bone density values are reported as a Z score. Z scores are compared to bone density levels of other people of the same age and sex. A Z score of minus 2.0 or lower indicates low bone density that can be caused by factors other than aging such as side effects of medications, nutritional deficiencies, or thyroid problems. 

Because a DEXA scan only measures the thickness of bones, it cannot be used to diagnose arthritis, a condition that occurs within joints and causes the breakdown of cartilage.

In more severe forms of arthritis, abnormal thickening of bone, called sclerosis, can occur as a result of repeated friction within joints, similar to the formation of a callus of hardened skin. If arthritis occurs and causes sclerosis in the spine and hip, bone density can actually increase. 

Because bone density tests also primarily only examine the hip and spine, there is no way for them to provide information about other parts of the body where arthritis typically occurs, such as the knees and fingers. 

An X-ray of your affected joint is currently the most accurate way to diagnose arthritis. The Kellgren-Lawrence classification system is often used to categorize the extent of arthritis based on the severity of joint damage seen on an X-ray. 

According to this system, arthritis can be classified as:

A bone density scan is performed through dual energy X-ray absorptiometry (DEXA) that examines the thickness of your bones. Results from DEXA scans are compared to standardized values to determine if your bone density is lower than normal and if you may have osteopenia or osteoporosis.

While a bone density scan can examine the thickness of your bones, it cannot be used to assess arthritis in your joints which requires a standard X-ray.

While a bone density scan is used to diagnose osteopenia or osteoporosis, it cannot be used to diagnose arthritis. But regardless of what condition you may have, strengthening your bones is essential. Exercise can be incredibly beneficial to improve your bone density, the mobility of your joints, and the strength of surrounding muscles to support and protect your joints and bones.

If you have been diagnosed with osteopenia, osteoporosis, or arthritis, talk with your healthcare provider about available treatment options. 

Conditions like osteopenia and osteoporosis that are characterized by low bone density can be detected on a bone density test. 

Arthritis is not directly related to bone density, However, arthritis can develop in areas of abnormal thickening of the bones (sclerosis) within joints without adequate protective cartilage. 

A healthcare provider will order a bone density scan to check for bone loss to see if you are at an increased risk of broken bones due to low bone density. 

National Institutes Health. Bone Mass Measurement: What the Numbers Mean.

Kling JM, Clarke BL, Sandhu NP. Osteoporosis prevention, screening, and treatment: a review. J Womens Health (Larchmt). 2014 Jul;23(7):563-72. doi: 10.1089/jwh.2013.4611.

Kohn MD, Sassoon AA, Fernando ND. Classifications in Brief: Kellgren-Lawrence Classification of Osteoarthritis. Clin Orthop Relat Res. 2016 Aug;474(8):1886-93. doi:10.1007/s11999-016-4732-4

By Kristen Gasnick, PT, DPT Kristen Gasnick, PT, DPT, is a medical writer and a physical therapist at Holy Name Medical Center in New Jersey.

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