A Z-score is given by bone density testing, which is a crucial tool used to screen for and diagnose osteoporosis. This technology measures your bones' mass and mineral content to assess their overall health.
When testing is complete, the results allow you to compare the density of your bones with established norms and predict your risk of a future fracture.This article will explain bone density testing and detail what a Z-score means.
:max_bytes(150000):strip_icc()/bone-scan-GettyImages-165564261-83942d4c3fcb4700934ec8701afdd333.jpg "Understanding Your Z-Score in Osteoporosis Testing (1)")
DEXA Bone Density Scan
Dual-energy X-ray absorptiometry (DEXA) scans are considered the gold standard for assessing the density of a person’s bones. This technology passes both high- and low-intensity X-ray beams through various bones to determine their overall mass.
The test is usually performed on the thigh (femur) bones and bones in the lumbar (lower back) portion of the spine. Occasionally, other peripheral areas like the forearm (radius), finger, or heel (calcaneus) bones may also be assessed.
A DEXA scan detects weakening bones earlier than a standard X-ray, allowing fracture-preventing treatments to be initiated sooner. The test also uses small amounts of radiation, comparable to a normal X-ray, making it a relatively safe screening tool. Usually, the scan takes about 15 to 20 minutes to complete.
Who Should Get Tested?
According to the U.S. Preventive Services Task Force (USPSTF) recommendations, all women aged 65 and older should undergo regular bone density testing to screen for osteoporosis.
(Note that for cited research or health authorities, the terms for sex or gender from those sources are used.)
The same group also concluded that postmenopausal women under age 65 should also have their bones screened. This is because after menopause, a person has lower estrogen levels and is at a higher risk of developing osteoporosis.
The current research is mixed on whether screening for osteoporosis is beneficial in men. A healthcare provider can assess your situation and evaluate whether a DEXA scan is necessary. Osteoporosis screening for transgender people may also be recommended on an individual basis.
Several risk factors elevate your likelihood of getting osteoporosis and could necessitate earlier testing. These include:
- Fractures after age 50
- Long-term use of corticosteroid medications
- Low dietary intake of calcium or vitamin D
- Cigarette smoking
- Alcohol use disorder
- Petite build
What Is a Z-Score?
After you have completed a DEXA test, a healthcare provider will typically discuss your results with you. You may be given a Z-score for the evaluated bones during this conversation.
A Z-score compares the density of your bones to the established norms for someone the same age, sex, race, height, and weight as you. This comparison helps establish the overall strength of the structures that were tested.
Z-Score Meanings and Average Range
Z-scores tell you how far (in standard deviations) your bone density deviates from the average age-matched norms.
This particular data point is usually more closely analyzed when a DEXA scan is performed on a child, young adult, premenopausal female, or a male under age 50. A low Z-score can indicate secondary osteoporosis, which occurs when another medical condition or treatment causes the disease.
Z-scores more than 2.0 standard deviations below the age-matched average signal your healthcare provider to look for medical conditions or medications that could be causing low bone density.
Causes of Secondary Osteoporosis
Several health conditions and medications can lead to secondary osteoporosis. Among the most common are:
- Central nervous system (CNS, the brain and spinal cord) conditions (stroke, Parkinson’s disease, multiple sclerosis)
- Endocrine or metabolic disorders (type 1 diabetes, hyperparathyroidism, hyperthyroidism, adrenal insufficiency)
- Human immunodeficiency virus (HIV) or AIDS
- Gastrointestinal disorders (celiac disease, gastric bypass surgery, inflammatory bowel syndrome)
- Rheumatoid arthritis or systemic lupus erythematosus
- Medications including anticonvulsants, chemotherapeutics, methotrexate, heparin, or corticosteroids
Z-Score vs. T-Score
In addition to a Z-score, people who undergo a DEXA scan also usually receive a T-score for each bony region that was tested. This score compares the density of your bone to that of a healthy 25- to 35-year-old of the same sex and ethnicity.
The T-score is much more widely utilized when diagnosing osteoporosis and is classified in the following manner:
| T-Score Classifications | |
|---|---|
| T-Score | Classification |
| 0 to minus 0.9 | Normal bone |
| minus 1.0 to minus 2.4 | Osteopenia |
| minus 2.5 or more | Osteoporosis |
Summary
DEXA scans use high and low-intensity X-ray beams to assess your bone density. This is considered to be the gold standard screening tool when diagnosing osteoporosis. This scan generates a Z-score that compares your bone to that of an average, healthy individual of the same age, gender, race, height, and weight. This data point is valuable when screening for secondary osteoporosis.
If you are over 65 or have other risk factors for osteoporosis, it is important to ask a healthcare provider whether a DEXA scan is appropriate. This quick and painless test will assess the quality of your bones and provide a helpful comparison to the bones of other people your age.
It can also provide an early warning that a fracture is likely and inform your healthcare provider about whether further treatment is needed.
Frequently Asked Questions
What could cause a low Z-Score?
A low Z-Score (more than 2.0 standard deviations below the average) typically indicates that secondary osteoporosis is present. This version of the disease occurs when a concurrent medical condition causes the density of your bones to thin. Certain medications can also be responsible for this bone deterioration.
Learn MoreCauses and Risk Factors for Osteoporosis
How can you improve your Z-Score and bone density?
Smoking cessation, drinking alcohol in moderation, reducing caffeine intake, and consuming appropriate amounts of calcium and vitamin D can help improve your bone density and raise your Z-score. Regular weight-bearing exercise and resistance training can also be beneficial.
In addition, medications like bisphosphonates, Evista (raloxifene), Forteo (teriparatide), or Prolia (denosumab) have been shown to stabilize or enhance bone mass.
What is a good Z-Score?
A Z-score of zero indicates that your bone density is equivalent to that of a healthy, age-matched individual. The closer your score is to zero, the better the overall health of your bone is. Scores that are 2.0 standard deviations or more beneath this age-matched average are considered to have secondary osteoporosis.
Learn MoreStages of Osteoporosis
6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Johns Hopkins Medicine. Bone densitometry.
Centers for Disease Control and Prevention. Radiation in healthcare: bone density (DEXA scan).
US Preventive Services Task Force, Curry SJ, Krist AH, et al. Screening for osteoporosis to prevent fractures: US Preventive Services Task Force recommendation statement.JAMA. 2018;319(24):2521. doi:10.1001/jama.2018.7498
UCSF Gender Affirming Health Program.Bone health and osteoporosis.
Jeremiah MP, Unwin BK, Greenawald MH, Casiano VE. Diagnosis and management of osteoporosis.afp. 2015;92(4):261-268.
American Bone Health. Understanding bone density results.
:max_bytes(150000):strip_icc()/IMG_36371-36db884b1cb041febec9cf44c1c6a1db.jpg "Understanding Your Z-Score in Osteoporosis Testing (2)")
By Tim Petrie, DPT, OCS
Petrie is a board-certified orthopedic specialist who has practiced as a physical therapist for more than a decade.
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