Reading a DEXA Scan

Really, there are only four important numbers, and you can ignore two of them.

• First, identify the percentage of normal bone density for the patient's age. Most DEXA (Dual Energy X-ray Absorptiometry) scans have this listed; many but not all QCTs do. This is helpful in your explanation to patients, but doesn't really affect diagnosis or treatment. Tells you what you think it does: where a person is compared to others of their own age. (This is one of the numbers you can ignore, if you want).
• Second, find the Z-score, which is the standard deviations from normal for that patient's age group. (This is the other number you can ignore).
• Third, find the percentage of bone density compared to young adult normals. This number has a powerful impact on patients. 90% and above is considered normal. (Tell patients that this is the the amount of bone they have compared to what they had or should have had at age 40. Of course, we don't really know what the density was in an individual without measurements.).
• Fourth, find the T-score, which is the number of standard deviations from young adult normals. This is the key number. You can't make a diagnosis without it!

T-Score

The T-score shows where your patient is compared to the population. In other words, an Irish woman with a small frame stacks up differently than an African American woman with a larger frame, and you want to know how they compare to people of their own sex, race, age, height and weight.

The T-score predicts fracture risk: For every -1 standard deviation the fracture risk doubles.

T-score = 0 has average risk for a normal 40 year old.
T-score = -1 has twice the risk.
T-score = -2 has 4 times the risk
T-score = -3 has 8 times the risk.

Notice that the T-score decreases by -1 for about every 10% of bone lost (i.e., a person with 90% of young adult normal bone density will tend to have a T-score of about -1).

The other big reason to notice the T-score is that the World Heath Organization defines osteoporosis on the basis of T-scores.

T -1 or higher = Normal
T -2.5 to -1 = Osteopenia
T below -2.5 = Osteoporosis
T below -2.5 + fragility fracture = Severe osteoporosis
(some would categorize anyone with any T-score and a fragility fracture as severe osteoporosis)

Areas of Confusion

When the patient is in or near the age of the control group of normals.
Suggestion: Go with the T-score.

Greater than or Lesser than in T-scores.
Mathematically, you can use either term either way when dealing with negative numbers, so I prefer to use the words higher or lower for clarity.

All of the above can be applied to any measurement of bone density, including DEXA, QCT, ultrasound, radiographic absorptiometry, etc.

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