6.1 T-Score vs Z-Score Methodology & Reference Databases
Key Takeaways
A T-score expresses BMD relative to a young-adult reference. A Z-score compares BMD with a reference appropriate for the patient’s age.
ISCD recommends the NHANES III White female reference for femoral neck and total hip T-scores in men and women of all ethnicities. Spine T-scores use manufacturer references.
The standardized score depends on the reference mean and standard deviation. The score alone cannot establish a patient’s fracture probability or prior bone loss.
Use age-appropriate Z-scores in pediatric reports. In children with short stature or growth delay, adjust PA spine and total body less head results using their site-specific methods.
Measure first, then select the reference population
Dual-energy X-ray absorptiometry (DXA) uses two X-ray energy spectra to estimate bone mineral content (BMC) and projected bone area. Areal bone mineral density (BMD) is BMC divided by projected area. Its unit is grams per square centimeter. A scan report may express that same measurement as a T-score or a Z-score, depending on the patient and the clinical question. Both are standardized scores expressed in standard deviation units.
A standardized score requires a BMD measurement and a mean and standard deviation for the corresponding skeletal site and reference population. A negative score means the measured BMD is below that reference mean. A difference of one score unit corresponds to one standard deviation for that reference, so a fixed score difference does not convert to the same BMD difference for every site and database.
T-score calculation and adult references
The T-score compares BMD with the young-adult reference. Subtract the young-adult reference mean from the measured value and divide by the young-adult standard deviation (SD).
The International Society for Clinical Densitometry (ISCD) recommends a uniform White female normative reference for adult T-scores in men and women of all ethnicities. For the femoral neck and total hip, manufacturers use the Third National Health and Nutrition Examination Survey (NHANES III) reference. The young-adult White female data from ages 20–29 supply the femoral neck reference used for the World Health Organization (WHO) international osteoporosis standard. At the lumbar spine, use the manufacturer’s own young-adult reference. At the one-third radius, use a valid young-adult reference when the measurement is appropriate for diagnosis. Use local population reference data for Z-scores only.
Changing the reference changes the score without changing the patient’s measurement. Suppose BMD is 0.680 g/cm². With an illustrative young-adult mean of 0.880 g/cm² and SD of 0.100 g/cm², the T-score is −2.0. Raising only the reference mean to 0.930 g/cm² gives −2.5 because the numerator becomes more negative. Changing the SD can also alter the result. Keep the site and reference parameters fixed when solving an exam calculation. The common reference makes the diagnostic threshold consistent. Individual fracture probability still depends on age and other risk factors.
Z-score calculation and interpretation
Use the same calculation with the mean and SD of a population appropriate for the patient's age and sex. Use population-specific references when adequate data exist, and use the patient’s self-reported ethnicity for the Z-score calculation.
ISCD prefers Z-scores for premenopausal women and men younger than 50. A Z-score of −2.0 or lower is below the expected range for age. A higher Z-score is within the expected range for age. ISCD permits WHO criteria during the menopausal transition. Pediatric reports use Z-scores with the appropriate pediatric reference and record any adjustment for growth. In short stature or growth delay, use height Z-score or bone mineral apparent density for PA spine and height Z-score for total body less head.
| Clinical question | Use |
|---|---|
| How does BMD compare with the young-adult reference for this site? | T-score in a population where the densitometric classification applies |
| How does BMD compare with an appropriate reference for the patient’s age? | Z-score |
| Has the patient lost bone between studies? | Compare absolute BMD from technically comparable studies against the facility’s least significant change |
| Is the patient at high risk of fracture? | Interpret the scan in the context of fracture history, age, medications, and a suitable risk assessment where indicated |
A low Z-score prompts attention to factors such as low peak bone mass, glucocorticoid exposure, hypogonadism, or malabsorption. A score by itself cannot establish the cause. A Z-score within the expected range for age also cannot rule out a secondary disorder. The absolute BMD from one scan and its scores describe where the patient is today. Serial BMD is the measurement used to assess change.
Worked example using one BMD and two references
A 65-year-old postmenopausal woman has a valid femoral neck BMD of 0.650 g/cm². For this calculation, use the stated reference inputs. The age-matched values are illustrative.
| Reference | Mean (g/cm²) | Standard deviation (g/cm²) |
|---|---|---|
| Young adult | 0.858 | 0.120 |
| Age matched | 0.730 | 0.115 |
- T-score: , reported as −1.7. This femoral neck result is in the low bone mass range. Review all valid sites before assigning the patient's overall DXA classification.
- Z-score: , reported as −0.7. Her measurement is relatively closer to the age-matched reference mean. This result does not explain why her BMD is low relative to the young-adult reference.
- Check your arithmetic: The numerator and denominator have the same units, which cancel. The patient’s measured value is below both means, so both scores are negative. For the report, round the calculated score directly to one decimal place.
ISCD Adult Official Positions · ISCD Pediatric Official Positions
Under the ISCD Adult Official Positions, which reference is used for femoral neck and total hip T-scores in a 62-year-old Black man?
The NHANES III White female young-adult reference.
The NHANES III African American male reference database matched for sex and race.
The densitometer manufacturer's proprietary young adult male multi-ethnic database.
An age-matched African American male database utilizing Z-score standard deviation equivalents.
A 58-year-old postmenopausal woman undergoes DXA scanning. Her measured lumbar spine (L1-L4) BMD is 0.792 g/cm². The manufacturer's young adult female reference mean BMD is 1.050 g/cm² with a reference standard deviation of 0.110 g/cm². What is her calculated lumbar spine T-score?
-2.14
-2.35
-2.58
-1.85
Which DXA report should use a Z-score in place of a T-score under the applicable ISCD positions?
Femoral neck DXA in a 74-year-old man.
Lumbar spine DXA in a 60-year-old postmenopausal woman.
Lumbar spine DXA in an 11-year-old girl with systemic juvenile idiopathic arthritis.
One-third radius DXA in a 65-year-old man whose hip replacements and lumbar fusion make the central sites unusable.
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