How to Read a Bone Density Scan, From T-Scores to Z-Scores

A bone density scan, done by dual-energy X-ray absorptiometry (DXA), measures how much mineral your bones hold at the lower spine and the hip. The report gives each site three numbers: the bone mineral density (BMD) itself, in g/cm², and two scores that compare it with other people. The T-score is the one used to diagnose osteoporosis after menopause and in older men.

This article is general information, not medical advice. A bone density report is best read with the clinician who ordered it.

T-Score: Compared With a Young Adult

A T-score says how many standard deviations your BMD is above or below the average for healthy young adults. The World Health Organization’s criteria classify it as:

T-score	Category
−1.0 or higher	Normal
Between −1.0 and −2.5	Low bone mass (osteopenia)
−2.5 or lower	Osteoporosis

Osteoporosis with a fragility fracture (one from a fall from standing height or less) is called severe or established osteoporosis. In a meta-analysis by Marshall and colleagues, each standard deviation lower raised fracture risk by about 1.5 to 2.6 times, depending on the site measured and the fracture, so a T-score of −2.0 is a meaningful difference from −1.0 even though both are “osteopenia”. Kanis and colleagues proposed the femoral neck, compared with a reference of young women from the US NHANES III survey, as the international reference standard.

Z-Score: Compared With People Your Age

A Z-score compares your BMD with the average for people of your own age and sex. The International Society for Clinical Densitometry (ISCD) recommends Z-scores, not T-scores, for premenopausal women, men under 50, and children, where a Z-score of −2.0 or lower is described as “below the expected range for age”. A low Z-score at any age can prompt a look for a cause of bone loss beyond ageing, such as a medicine or a hormonal condition.

Which Site Counts

The scan reports the lumbar spine (usually L1 to L4), the total hip, and the femoral neck, the narrow part below the ball of the hip joint. By ISCD’s positions, the diagnosis uses the lowest T-score of the three. The spine can read falsely high in older adults because of arthritis or other calcification, so the hip often matters more with age.

Comparing Scans

• Repeat scans on the same machine, ideally at the same centre, since BMD differs between manufacturers.
• Compare BMD in g/cm², not T-scores, from one scan to the next.
• A change only counts if it’s bigger than the centre’s least significant change, which is often a few percent.
• Bone changes slowly; scans are typically one to two years apart or more.

Who Should Be Screened

The US Preventive Services Task Force recommends screening women aged 65 or older, and postmenopausal women younger than 65 with risk factors for osteoporosis. It found the evidence insufficient to recommend for or against screening men. A tool such as FRAX combines BMD with other risk factors to estimate your 10-year fracture risk.

Following It in Kaizora

Kaizora keeps each site’s Bone Mineral Density, T-Score, and Z-Score as separate metrics on the Body page, so the spine and hip each have their own history. Log a scan’s values with Log at the time of the scan, with the scanner as the device, so a scan on a different machine is labelled. With several scans, the chart of a site’s BMD shows the trend that the categories alone can hide.

The Short Version

• T-score compares you with young adults: −2.5 or lower is osteoporosis.
• Z-score compares you with your own age group and is used before menopause and under 50.
• The lowest of the spine, total hip, and femoral neck sets the diagnosis.
• Compare BMD between scans on the same machine.

Sources

• Kanis JA et al., “A reference standard for the description of osteoporosis,” Bone 42, no. 3 (2008): 467–475. doi:10.1016/j.bone.2007.11.001
• LeBoff MS et al., “The clinician’s guide to prevention and treatment of osteoporosis,” Osteoporosis International 33 (2022): 2049–2102. doi:10.1007/s00198-021-05900-y
• Marshall D, Johnell O, Wedel H, “Meta-analysis of how well measures of bone mineral density predict occurrence of osteoporotic fractures,” BMJ 312, no. 7041 (1996): 1254–1259. doi:10.1136/bmj.312.7041.1254
• International Society for Clinical Densitometry, Official Adult Positions (2023).
• US Preventive Services Task Force, Osteoporosis to Prevent Fractures: Screening.
• FRAX fracture risk assessment tool, University of Sheffield.