What is the Difference Between T-Score and Z-Score Osteoporosis

The key difference between T-score and Z-score osteoporosis is that T-score compares a patient’s bone density with healthy young individuals of the same sex. At the same time, Z-score compares the bone density of a patient with the bone density of people of the same age and sex as the patient.

Osteoporosis occurs when the natural process of bone breakdown exceeds the rate of bone replacement. Primary osteoporosis is related to bone loss due to aging, while secondary osteoporosis occurs due to specific conditions that can be reversible. Bone density is a good indication to determine the risk of getting osteoporosis. Dual-energy X-ray absorptiometry (DEXA) can usually be used to assess the bone density. DEXA scan or ultrasound will give two scores called T-score and Z score, representing how close people are to average peak bone density.

CONTENTS

1. Overview and Key Difference
2. What is T-Score Osteoporosis
3. What is Z-Score Osteoporosis
4. Similarities – T-Score and Z-Score Osteoporosis
5. T-Score vs. Z-Score Osteoporosis in Tabular Form
6. Summary – T-Score vs. Z-Score Osteoporosis

What is T-Score Osteoporosis?

A bone density test is like any other medical test usually performed in laboratories. T-score on a bone density test such as DEXA shows how much a person’s bone mass differs from the bone mass of an average healthy 30-year-old adult. Therefore, mathematically T-score is a standard deviation calculated to know how much a result varies from the average or mean. In this test, the results for the entire population will be distributed around an average score.

In the results, a T-score of -1 or greater for bone density is considered normal. On the other hand, a T-score between -1 and -2.5 for bone density indicates osteopenia but still not osteoporosis. If an individual has a T-score of -2.5 or lower for bone density, it indicates osteoporosis, even if they have not experienced bone fractures. Therefore, a negative T-score of –2.5 or less defines osteoporosis.

What is Z-Score Osteoporosis?

The Z-score compares a person’s bone density to the average bone density of people of the same age and gender. For example, for a 60-year-old female, the Z-score compares her bone density to the average bone density of 60-year-old females. Usually, a Z-score helps in diagnosing secondary osteoporosis. Z score is more commonly used for children, young adults, pre-menopausal women, and men under 50.

In the results, a +1 to 2 Z-score means the bone density is higher than in others of the same age, sex, and body size. 0 Z-score means the bone density is the same as in others of the same age, sex, and body size. A -1 Z-score means bone density is lower than in others of the same age, sex, and body size. Moreover, the Z score of -2.5 or less indicates secondary osteoporosis. Hence, a negative Z-score of –2.5 or less raises suspicion of a secondary cause of osteoporosis.

What are the Similarities Between T-Score and Z-Score Osteoporosis?

• T-score and Z-score are two scores of DEXA scan or ultrasound to determine osteoporosis.
• Both scores are based on mathematical calculation.
• Both scores are based on comparison or standard deviation.
• The WHO standard accepts both.

What is the Difference Between T-Score and Z-Score Osteoporosis?

The T-score compares the bone density of a patient with healthy young individuals of the same sex, while Z-score compares the bone density of a patient with the bone density of people of the same age and sex as the patient. Thus, this is the key difference between T-score and Z-score. Furthermore, the T-score is mainly used to determine primary osteoporosis, while the Z-score is primarily used to determine secondary osteoporosis.

The following table summarizes the difference between T-score and Z-score.

Summary – T-Score vs. Z-Score Osteoporosis

Osteoporosis is a condition that occurs when the creation of new bone is less fast than the loss of old bone. Therefore, osteoporosis weakens bones, making them fragile or more likely to break. The loss of bone density is a good marker of osteoporosis, which can be measured by technologies such as DEXA. DEXA uses two types of scores to determine people’s bone density: T-score and Z-score. T-score compares the bone density of a patient with healthy young individuals of the same sex, while Z-score compares the bone density of a patient with the bone density of people of the same age and sex as the patient. This is the summary of the difference between T-score and Z-score Osteoporosis.

Reference:

1. “Diagnosing Primary Osteoporosis: It’s More than a T Score.” CCJM.
2. “Z-Scores for Bone Density: Chart, Meaning, and More.” Medical News Today.

Image Courtesy:

1. “DEXA scan screen ALSPAC” By Nick Smith photography – ALSPAC web site (CC BY-SA 3.0) via Commons Wikimedia
2. “Osteoporosis in Bones” By Scientific Animations.com – (CC BY-SA 4.0) via Commons Wikimedia