How to Read a Bone Density Scan: Understanding T-Score vs. Z-Score

Your guide to understanding T-score vs. Z-score and other common terminology in your bone density scan.

Your bones are alive, constantly forming and breaking down bone. While it may seem like bones are purely solid, they are actively composed of an intricate scaffold of bony struts (called trabeculae). In healthy people, bone density peaks in teenage years and decreases as people age. A healthy diet and exercise can help slow this decay.

A bone density scan is used to measure how porous or dense your bones are. This scan may also be called a bone mineral density (BMD) test or a dual-energy X-ray absorptiometry (DEXA/DXA) scan. Denser calcified bone tissue is usually stronger because it has more bony struts and fewer pockets or holes in it. Bones with fewer dense calcified struts are weaker and more likely to fracture.

Osteoporosis is the name of the bone disease affecting people with abnormally low bone density, which can predispose patients to fractures from low-impact injuries like falls. It is very common; approximately 10 million Americans and 2 million Canadians have osteoporosis. If your doctor has recommended you get a DXA scan or bone density test, you’re not alone: all adults over the age of 65 should have annual bone density testing to determine whether they’re at risk of osteoporosis.

A bone density scan is an important diagnostic tool that both you and your doctor can access. Being able to review the radiologist’s report for your bone density scan lets you have more informed discussions with your healthcare provider about your results, but understanding the terminology in your report is key.

This article clarifies the common terms you will come across when reviewing your bone density scan report, so you can have a more complete understanding of your results before your follow-up appointment.

What does DXA stand for?

DXA or DEXA stands for dual-energy X-ray absorptiometry and is the most common and effective method used to measure bone density. A DXA scan uses X-ray beams at 2 different energies to calculate bone density.

What’s T-score vs. Z-score?

Your bone density measurement is interpreted by comparing your BMD value (in grams per cubic centimeter) to others of the same sex using 2 statistical tests that use the bell curve: the T-score and Z-score.

Understanding T-score

The T-score range or number on your bone density test report indicates how much your bone density differs from the bone density of an average, healthy 30-year-old. Your T-score number is measured in units of Standard Deviation (SD). If you remember that bell curve from school, the greater the SD, the greater your bone mass deviates from the average.

For example, if your bone density report shows:

• 0 SD T-score, your bone density is equal to that of an average, healthy 30-year-old, or zero standard deviations away from the average.
• +1 or +2 SD T-score, your bone density is greater than the average score, indicating you are one or two standard deviations away from average bone density for an average, healthy 30-year-old.
• -1 or -2 SD T-score, your bone density is less than the average score, indicating you are one or two standard deviations away from average bone density for an average, healthy 30-year-old.

T-scores can also help your doctor determine whether you have osteopenia or osteoporosis, serious conditions that can lead to increased fracture risk.

• A T-score of -1 to -2.5 SD indicates osteopenia, which is defined as low bone density compared to the average population.
• A T-score of -2.5 SD or more indicates osteoporosis, which means you have lower bone mass than approximately 95% of healthy 30-year-olds of your biological sex.

Understanding Z-score

The Z-score range or number on your bone density test compares your bone density to the average bone density of people your own age and sex and uses the same units of measurement (SD) as the T-score. For example, a -1 or -2 SD Z-score indicates that your bones are less dense than what’s average for your age and sex.

Z-scores are only used to analyze the bone density of children, young adults, pre-menopausal women and men under 50 years of age. Z-scores, along with analysis of lifestyle factors, can also help indicate secondary osteoporosis, a type of osteoporosis caused by certain illnesses, medications or lifestyle factors.

While the T-scores and Z-scores of one bone density test cannot show whether you are losing bone mass, they do compare you to an average. Your doctor may recommend blood tests to track markers of bone density loss over time.

What does FRAX mean?

One primary concern for those with osteopenia or osteoporosis is an increased risk of fractures. The Fracture Risk Assessment Tool (FRAX) from the World Health Organization calculates your risk of suffering a ‘fragility fracture’ (i.e. fracture due to a fall from standing) in the next decade by analyzing various risk factors, including bone density.

Your doctor will use FRAX to determine your risk of fracture by considering the following:

• Age
• Sex
• BMI (Body Mass Index measured using weight-to-height ratio)
• BMD in the hip
• Fracture history
• Secondary osteoporosis
• Medication use, particularly steroids
• Parental hip fracture
• Rheumatoid arthritis
• Current cigarette smoking
• Alcohol intake

After analysis, your doctor will determine your FRAX percentage, which indicates the likelihood you will develop a fracture in either your hip, arm or spine within the next ten years. This is mainly used to determine what medication may be most helpful for you.

• 10% or less FRAX score. A normal FRAX score indicating the risk of fragility fracture in the next ten years is less than 10%.
• 10% to 20% FRAX score. A moderate FRAX score indicating the risk of major fragility in the next ten y2ears is between 10% and 20%.
• 20% or higher FRAX score. A high FRAX score strongly suggests the risk of fragility fracture in the next 10 years is 20% or higher.

Understanding your treatment options

If you do receive an osteoporosis diagnosis, there are several treatment options available, such as:

• Medication. There are multiple medications available, which can help slow bone loss or promote bone growth. For example, bisphosphonates are most commonly prescribed and can be taken orally or via IV infusion. Talk to your doctor about this and other options.
• Lifestyle changes. Frequent weight-bearing exercises are important to help strengthen bones while improving balance and strength. This is key to preventing falls (i.e., the more active and fit you are, the lower your chances of falling and breaking a bone).
• Diet changes and supplements. Calcium is the main mineral found in bones, therefore, a balanced diet needs calcium to maintain strong, healthy bones. Getting the required amount of calcium from diet alone can be difficult, so you’ll likely take calcium supplements, as well as additional vitamin D, which helps your body better absorb calcium.

How PocketHealth can help you understand your bone density scan

Waiting for our follow-up appointment to discuss the results from your bone density test can be nerve-wracking, but you can easily access your images and results with PocketHealth, often before your follow-up appointment. With PocketHealth you can quickly access your results as soon as the radiologist has completed their report, and store and share your results directly from your device.

If you encounter some complex medical terminology in your bone density scan report, PocketHealth Report Reader can help. Report Reader provides definitions for the medical terms in your report, making it easier to comprehend your results, and spotlights any follow-up recommendations so you can feel informed and confident when speaking to your doctor at your next appointment. Access your records here.

“With Report Reader, I get explanations for words [in my report] that I don’t understand. I can now discuss my health with my family doctor with more confidence and have a better understanding of what is going on.”

– PocketHealth Patient

Stay on top of your bone health—and your bone density results

If your doctor has recommended you get a bone density scan, you may have questions about your results, including the differences between T-scores and Z-scores, BMD and how to determine your personal fracture risk. You’ve gained a lot of important information by reading this article, but you can learn more by accessing your bone density test images and results through PocketHealth.

Reviewing your bone density scan report before your follow-up appointment can help you prepare questions and engage confidently with your physician when discussing the results. Once you understand the terminology and results outlined in your report, you’ll become a more informed participant in your healthcare journey.

How PocketHealth works

Learn more about how to use PocketHealth to securely access and share your bone density results.

 

Medical content review provided by Ben Fine, MD. Any health-related information contained in this post is intended to provide general education only and is not medical advice. This should not be used as a substitute for the advice you receive from your healthcare provider.