A normal bone density T-score is -1.0 or higher, meaning your bone density is within one standard deviation of a healthy young adult’s. A normal Z-score is above -2.0, meaning your bones are within the expected range for someone your age and sex. These two scores measure different things, and which one matters most depends on your age and whether you’ve gone through menopause.
T-Score Ranges Explained
Your T-score compares your bone density to that of a healthy 20- to 30-year-old at peak bone mass. Peak bone mass is typically reached by the end of your second decade of life, then holds relatively steady until around age 50 before gradually declining. The World Health Organization sets the diagnostic thresholds:
- Normal: T-score of -1.0 or higher
- Mild osteopenia: T-score between -1.01 and -1.49
- Moderate osteopenia: T-score between -1.50 and -1.99
- Advanced osteopenia: T-score between -2.00 and -2.49
- Osteoporosis: T-score of -2.50 or lower
Each full point on the scale represents one standard deviation of bone density. A T-score of -2.0 means your bones are two standard deviations less dense than the young adult reference. The further below zero your score falls, the lower your bone density and the higher your fracture risk.
What a Z-Score Tells You
A Z-score compares your bone density to other people of the same age, sex, and ethnicity rather than to a young adult at peak bone mass. This makes it a better measure of whether your bone loss is unusual for your stage of life. A Z-score above -2.0 is considered “within the expected range for age.” A Z-score of -2.0 or lower is classified as “below the expected range for age.”
When a Z-score drops that low, it raises suspicion that something beyond normal aging is driving bone loss. Your doctor may investigate secondary causes such as vitamin D deficiency, overactive parathyroid glands, certain blood cancers, or medications that accelerate bone thinning. The Z-score essentially flags whether your bones are weaker than they should be given how old you are.
When Each Score Is Used
T-scores are the standard for diagnosing osteoporosis in postmenopausal women and men over 50. For these groups, comparing bone density to a young adult baseline gives the clearest picture of fracture risk.
Z-scores are preferred for premenopausal women and men under 50. The International Society for Clinical Densitometry specifically states that osteoporosis cannot be diagnosed in men under 50 based on bone density alone. For younger people, a low T-score might simply reflect naturally smaller or lighter bones rather than a disease process, so the age-matched Z-score provides more useful context. Z-scores are also the standard for children and adolescents, sometimes with additional adjustments for height and weight.
Where Bone Density Is Measured
A DEXA scan (dual-energy X-ray absorptiometry) measures bone density at several sites: the lumbar spine (L1 through L4), the femoral neck (the narrow part of the hip bone), the total hip, and sometimes the forearm near the wrist. Your diagnosis is based on the lowest T-score at any of these locations, not an average across them. If your spine scores -1.2 but your femoral neck scores -2.6, you would be diagnosed with osteoporosis based on the hip reading.
When both hips are scanned, the lowest T-score from either the right or left femoral neck or total hip is used. The femoral neck is considered the reference site for population-level studies, but all measured sites factor into your individual diagnosis. Different sites can produce meaningfully different scores in the same person, which is why the scan covers multiple areas.
How T-Scores Factor Into Fracture Risk
A T-score alone doesn’t fully predict whether you’ll break a bone. A tool called FRAX calculates your 10-year probability of a major osteoporotic fracture by combining your femoral neck bone density with clinical risk factors like age, prior fractures, smoking, steroid use, and family history. A 65-year-old woman with a T-score of -2.5 but no other risk factors has a modestly elevated fracture probability. Add a prior fracture to that same T-score, and the risk increases substantially.
This matters because two people with the same T-score can have very different fracture risks. Bone density and clinical risk factors are only weakly correlated with each other, so combining both gives a more accurate picture than either one alone. Your doctor may use your FRAX result to decide whether medication is warranted, particularly if your scores fall in the osteopenia range where the treatment decision is less clear-cut than it is for full osteoporosis.
How to Read Your DEXA Report
Your scan results will list T-scores and Z-scores for each measured site. A few things to keep in mind when reviewing them:
Scores at different sites will rarely match. The spine and hip lose bone at different rates and can be affected differently by arthritis, compression fractures, or even aortic calcification, all of which can artificially inflate a spine reading. If your spine T-score looks surprisingly good compared to your hip, it may not reflect true bone health at that site.
Small changes between scans don’t always mean your bones got better or worse. DEXA machines have a precision error of roughly 1 to 2 percent, so a shift from -1.4 to -1.5 over two years could be measurement noise rather than real bone loss. Ask about the “least significant change” for your specific scanning center to know what size shift is meaningful.
If you’re a postmenopausal woman with normal bone density (T-score of -1.0 or higher), research from a large study published in the New England Journal of Medicine found that the estimated time before transitioning to osteoporosis was roughly 15 years. Women starting with moderate osteopenia transitioned much sooner, around 5 years. This gives a sense of how urgently repeat testing is needed based on where your numbers fall.