T Scores and Z Scores

Contents

1. Where the reference point comes from
2. How the T score is read
3. When the Z score is used instead
4. What a score cannot say
5. The firmest ground is after menopause

Interpreting a bone density report, which Candace Cooley has done as part of her DEXA work, means weighing two numbers rather than one. A bone density result is not an absolute quantity on the report; it is a comparison with a reference group, and the T score and the Z score name two different comparisons. The National Institute of Arthritis and Musculoskeletal and Skin Diseases defines the T score as the difference between a person’s bone mineral density and zero, where zero stands for the density of a healthy young adult, and the Z score as the difference between that person’s density and the average for healthy people of the same age, ethnicity and sex.¹

Where the reference point comes from

The origin of the T score lies in the shape of a lifetime of bone. The Royal Osteoporosis Society explains that density rises while the skeleton grows, peaks at around age 30 and then declines, faster after menopause, in both women and men.⁶ The young adult at that peak is the yardstick. The New York State Department of Health adds the statistical detail: the T score compares a measured density with the average of young healthy adults of the same sex and is expressed in standard deviations above or below that average.³ A score of minus 2, in other words, means density two standard deviations below the young adult mean. Because nearly everyone loses some bone after 30, a falling T score with age is expected. The Royal Osteoporosis Society notes that a result in the low bone density range is common in older adults, and that a small decline between two scans can even count as a good result.⁶

The Z score answers a different question. Instead of asking how a skeleton compares with its own best years, it asks how it compares with its peers. MedlinePlus lists the matching factors as age, weight, sex and ethnic or racial origin.²

Figure

The two scores side by side

	T score	Z score
Compared with	A healthy young adult of the same sex	People of the same age, sex, weight and ethnic origin
Used for	Women past menopause, men 50 and older	Premenopausal women, men under 50, children
Main job	Classifying normal, low density or osteoporosis	Flagging bone lower than expected for age

How the T score is read

The T score is the number used to classify women past menopause and men 50 and older. The Bone Health and Osteoporosis Foundation gives the bands: minus 1.0 or above is normal, between minus 1.0 and minus 2.5 is low bone mass, also called osteopenia, and minus 2.5 or below is osteoporosis, provided no other condition explains the low reading.⁴ The more negative the number, the weaker the bone and the higher the chance of a break. NIAMS states the same relationship plainly: the lower the T score, the higher the fracture risk.¹

The middle band is often misunderstood. The New York State guide stresses that low bone mass is not a disease but a condition in which density sits below the young adult average.³ A second Bone Health and Osteoporosis Foundation page adds that a low reading does not on its own show that bone is being lost; some people never have normal density, because of genetics, body size or other conditions, and a score of minus 2.2 simply sits lower than one of minus 1.2.⁵

Figure

Reading a T score

1. 01
   Minus 1.0 or aboveNormal bone density; examples are 0.9, 0 and minus 0.9.
2. 02
   Minus 1.0 to minus 2.5Low bone mass, or osteopenia; examples are minus 1.1, minus 1.6 and minus 2.4.
3. 03
   Minus 2.5 or belowOsteoporosis, once other causes are ruled out; examples are minus 2.6 and minus 3.3.

When the Z score is used instead

For premenopausal women, men under 50 and children, NIAMS explains, the report gives a Z score.¹ A young skeleton compared with a peak it has barely left would tell little; compared with peers, it can reveal a problem. The New York State guide calls a Z score below minus 2.0 a warning sign that bone mass is lower than expected for age, or may be falling quickly, and says it can prompt further tests or referral to a specialist.³ The Royal Osteoporosis Society adds that a low Z score can point to another condition affecting the bones, such as osteomalacia or osteogenesis imperfecta, and that it is mainly useful in children and young adults.⁶

What a score cannot say

Neither number is a complete account of a skeleton. The Bone Health and Osteoporosis Foundation notes that osteoporosis can be diagnosed with a T score better than minus 2.5 when a fracture has already happened or a fracture risk calculation shows high risk, and that a score of minus 2.5 or below can reflect a different disease, such as osteomalacia or multiple myeloma.⁴ According to the New York State guide, a single test shows only current density; it cannot show whether bone is being lost, which takes a repeat test, usually about two years later.³ The Royal Osteoporosis Society warns that someone in the osteoporosis range may not benefit from medicine while someone in the osteopenia range may, which is why scores are combined with a full fracture risk assessment such as FRAX.⁶ How those numbers apply to any one woman is a conversation for her and her own clinician. The scan that produces the scores is described under dual energy X ray absorptiometry, and the condition they classify under osteoporosis.

Figure

Two thresholds from the New York State guide

Z score below which bone mass is lower than expected for age

minus 2.0

usual wait before a repeat test can show bone loss

2 years

The firmest ground is after menopause

The scores are on firmest ground in the group they were built for. NIAMS and MedlinePlus both reserve the T score for women past menopause and men 50 and older, the populations in which density is used to diagnose osteoporosis and estimate fracture risk.¹² In that group the New York State guide describes bone mineral density as one of the best single predictors of a future fracture.³ The evidence is thinner at the edges: in younger adults, where the Z score is a prompt for investigation rather than a diagnosis, and in anyone whose spine or history makes a single number misleading. Younger adults are tested far less often; the New York State guide lists premenopausal women and men under 50 as candidates for a density test only in rare cases.³ Repeat measurement is where both scores earn their keep: the New York State guide treats a comparison between two tests as the only way to show loss, and the Royal Osteoporosis Society recommends the same machine each time so that a change in the number reflects the bone rather than the equipment.³⁶ The years when the T score first begins to fall are covered in bone and the menopause transition.