Bone densitometry: T-score, Z-score and how to report by WHO and ISCD criteria

Bone densitometry (DXA) seems like the simplest exam to report: the machine delivers the numbers and the classification comes out on its own. In practice, it is where the most errors that change management appear. A Z-score used in a postmenopausal woman, osteoporosis diagnosed on a fractured vertebra, "bone loss" that is just machine variation. A good densitometry report is one that picks the right number for the right patient.

This text summarizes the rules of the World Health Organization (WHO) and the official positions of the International Society for Clinical Densitometry (ISCD) that come up most often in daily practice.

T-score or Z-score: it depends on who the patient is

The T-score compares the patient's density with that of young adults of the same sex. The Z-score compares it with people of the same age and sex. Each serves a group:

• T-score: postmenopausal women or women in the menopausal transition and men aged 50 years or older. This is where the WHO classification applies.
• Z-score: premenopausal women, men younger than 50 years, and children and adolescents. In these groups, the WHO classification does not apply.

In those who use the Z-score, a value of -2.0 or below is described as "below the expected range for age", and not as osteoporosis. In children, the diagnosis of osteoporosis also requires a history of a clinically significant fracture.

The WHO classification

For those who use the T-score, the classification is made by the lowest T-score among the valid sites:

• Normal: T-score of -1.0 or above.
• Low bone mass (osteopenia): T-score between -1.0 and -2.5.
• Osteoporosis: T-score of -2.5 or below.
• Established (severe) osteoporosis: T-score of -2.5 or below with a fragility fracture.

Which sites are valid

The diagnosis uses the lumbar spine (L1 to L4), the femoral neck and the total hip. Ward's triangle and the isolated trochanter are not part of the diagnosis. The 33% radius (distal third) of the nondominant forearm is used when the spine or hip cannot be measured or interpreted, in hyperparathyroidism and in obesity above the weight limit of the machine's table.

When to exclude a vertebra

Osteoarthritis, osteophytes, fracture, aortic calcification and surgical material falsely increase the density of the vertebra. The ISCD advises excluding the affected vertebra and keeping at least two in the analysis. A practical sign of an altered vertebra is a difference of more than 1.0 in T-score relative to the neighboring vertebra. If only one valid vertebra remains, the spine should not be used for diagnosis.

The report should state which vertebrae were considered. "Lumbar spine (L1 to L3)" is information that the next exam needs in order to compare the same segment.

Comparing with the prior study

Every measurement has a precision error. For this reason the ISCD recommends that each practice calculate its own least significant change (LSC). A difference smaller than the LSC cannot be called a gain or a loss. Rules for comparing safely:

• Compare the density in g/cm², not the T-score.
• Compare the same site and the same vertebrae.
• Only compare studies from the same machine (or with documented cross-calibration).
• State the change in g/cm² and as a percentage, and whether it exceeds the LSC.

What the report needs to include

• Machine and sites assessed, with the vertebrae considered.
• Density (g/cm²), T-score and/or Z-score for each site, according to the profile.
• WHO classification (or the Z-score phrase) based on the lowest valid value.
• Technical limitations that interfere with the result.
• Comparison with the prior study, when available, using the LSC.

Common errors

• Using the Z-score in a postmenopausal woman, or the T-score in a young woman.
• Calling a low Z-score in a young patient osteoporosis.
• Keeping a fractured or severely arthritic vertebra in the analysis.
• Comparing T-scores instead of g/cm², or studies from different machines.
• Diagnosing by Ward's triangle.

How Laudário helps

In the Laudário densitometry module:

• The system chooses the T-score or the Z-score according to the patient profile.
• The WHO classification and the ISCD criteria are applied in the diagnostic impression.
• Recommendations on their own tab, separate from the findings.

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