Z-score

A Z-score is one of the two numbers on a bone density (DEXA) scan report. It compares your bone density with what is expected for someone of your own age and sex; the NHS adds ethnicity to the comparison. The other number, the T-score, compares your bone density with that of a healthy young adult. Both are given in standard deviations, a measure of how far your result sits from the expected value (NHS).

A DEXA scan uses low-dose X-rays, usually of the hip and lower spine, and is more effective than an ordinary X-ray at identifying low bone density (NHS; NHS).

Z-score vs T-score

The T-score is the one used to diagnose osteoporosis. The World Health Organization classes T-scores as follows (NHS):

• above -1: normal
• between -1 and -2.5: mildly reduced bone density compared with peak bone mass
• at or below -2.5: osteoporosis

The Z-score answers a different question: is your bone density what you’d expect for your age? The NHS says a Z-score below -2 means your bone density is lower than it should be for someone of your age, and that Z-scores are usually used for children and people under 30 who are still growing (NHS).

The International Society for Clinical Densitometry goes further. For women before the menopause and men under 50, it says Z-scores, not T-scores, are preferred, and it describes a Z-score of -2.0 or lower as “below the expected range for age”. It also says osteoporosis can’t be diagnosed in men under 50 on the basis of bone density alone (ISCD).

What does a low Z-score mean?

It means your bones are less dense than expected for your age, and it usually leads a doctor to ask why. An international working group noted that low bone density in a young adult may simply reflect a naturally low peak bone mass, for example in lean but otherwise healthy people. True osteoporosis in younger adults is usually linked with a chronic condition that affects bone, and fragility fractures, such as vertebral fractures, without any such condition may point to a genetic or unexplained cause (Ferrari et al., 2012).

Neither score is a fracture forecast on its own. Someone with low bone density may never break a bone, and someone with average density may have several fractures, which is why doctors weigh the scan alongside age, sex, previous falls and other risk factors (NHS).

Why a desk worker might see a Z-score

The NHS says people under 50 may need a scan if they have risk factors, such as smoking or a previous broken bone (NHS). The NHS lists not getting much exercise, low calcium or vitamin D, being underweight, smoking, drinking too much alcohol and taking steroid tablets among the things that raise the chance of osteoporosis (NHS).

Long days of sedentary behaviour aren’t on that list as such, but not getting much exercise is. A UK consensus recommends resistance and impact exercise to maximise bone strength in people with osteoporosis, and activities that improve strength and balance to reduce falls (Brooke-Wavell et al., 2022).

When to see a doctor

See a GP if you think you may be at risk of osteoporosis and are worried, if you have back pain that isn’t getting better after a few weeks, or if your spine becomes curved or your posture hunched. Call 999 or go to A&E if you have severe pain after a fall or injury (NHS). If you’ve had a scan, the doctor who requested it is the right person to explain what your Z-score means for you.