A bone density scan gives a person a Z-score and a T-score. T-scores compare bone density with that of a healthy person, whereas Z-scores use the average bone density of people of the same age, sex, and size as a comparator.

Although both scores can be useful, most experts prefer using Z-scores for children, teenagers, premenopausal females, and younger males. These scores are helpful for diagnosing secondary osteoporosis, which stems from underlying medical conditions, rather than primary osteoporosis, which usually results from aging.

Bone density scans are painless and quick, and they require little preparation. The results can tell a doctor whether a person has osteopenia or osteoporosis or is at risk of developing either condition.

Keep reading to learn more about bone density scans, the difference between T-scores and Z-scores, and what Z-scores mean in terms of osteoporosis.

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According to the Bone Health and Osteoporosis Foundation (BHOF), Z-scores compare a person’s bone density with the average bone density of those of the same age, sex, and body size.

These scores are in the form of standard deviations. This mathematical term measures how close a number is to the average. A low standard deviation means that the number is close to the average, and a high standard deviation means that it is further from the average.

This table shows the meanings of different Z-scores:

Z-scoreMeaning
+1–2Bone density is higher than in others of the same age, sex, and body size.
0Bone density is the same as in others of the same age, sex, and body size.
-1Bone density is lower than in others of the same age, sex, and body size.
-2Doctors consider scores higher than this to be normal.
-2.5This score or lower indicates secondary osteoporosis.

Dual-energy X-ray absorptiometry (DEXA) scans use a low dose of ionizing radiation to measure bone density.

If doctors need to measure a person’s bone density, they will likely use a DEXA scan. These scans measure bone mass, and doctors compare the results with established norms to provide a score.

What they do

The most common type of DEXA scan is the central DEXA. This scan measures bone density at the hip and lower back.

Uses of central DEXA scans include:

  • diagnosing osteoporosis
  • assessing a person’s risk of fractures
  • evaluating the body’s response to an osteoporosis treatment

Unlike central DEXA scans, peripheral scans usually play a role in screening. They identify people who may need further bone scans.

Peripheral scans measure bone density in the:

  • heel
  • finger
  • wrist
  • forearm

What to expect

DEXA scans are painless and quick. A central DEXA scan involves the following steps:

  1. The person will lie on their back on a padded table, either placing the legs out straight or resting them on a padded platform.
  2. One scanning machine will pass over the lower spine and hips, and another scanning machine will pass underneath.
  3. During the scanning, the person will need to be very still, and the technician might ask them to hold their breath.
  4. The images go to a computer where a healthcare professional can view them.

Little preparation is necessary for the central DEXA scan. However, a doctor may ask a person to refrain from taking calcium supplements 24–48 hours prior to the test. They may also ask the individual to avoid wearing metal jewelry or clothes with metal parts, such as buttons or buckles.

A peripheral scan is simpler and only involves a small, portable machine. A person will place their foot, finger, hand, or forearm in the device, and it will provide a reading within a few minutes.

As the scans use low doses of radiation, which could harm a developing baby, this procedure is not advisable during pregnancy. If someone thinks that they might be pregnant, they should tell the doctor, who will use other diagnostic methods.

T-scores reflect how bone density compares with that of a typical, young, healthy person, whereas Z-scores use the bone density of those with similar characteristics for comparison.

Healthcare professionals may provide DEXA scan results via T-scores and Z-scores. T-scores reveal how high or low a person’s bone density is compared with that of a typical healthy 30-year-old. The lower the scores, the lower the bone density.

T-scoreMeaning
-1.0 or highernormal
-1.0 to -2.5 osteopenia, which indicates that the bone mass is low but not low enough to classify as osteoporosis
-2.5 or below osteoporosis

The BHOF notes that Z-scores can be misleading because older adults commonly have low bone density. In other words, a normal Z-score only indicates that the bone density is comparable to that of others of the same age, sex, and body size. However, as older adults tend to have low bone density, people with normal Z-scores could have osteoporosis.

Most experts usually advise the use of Z-scores for children, teenagers, premenopausal females, and males under the age of 50 years.

These scores help diagnose secondary osteoporosis, which is osteoporosis due to a clinical disorder rather than aging — the cause of primary osteoporosis. Research in 2016 reports that a Z-score of less than -2.5 indicates the secondary type.

The causes of secondary osteoporosis are potentially reversible. They may stem from one or more of the following:

Aside from these conditions, certain medications can cause bone loss. They include:

  • levothyroxine (Synthroid), a drug that treats low thyroid
  • tamoxifen (Nolvadex), a synthetic drug that treats breast cancer and infertility in females
  • lithium, a drug that treats mood disorders
  • heparin, a drug that thins the blood

Doctors use Z-scores to diagnose osteoporosis in children, teenagers, premenopausal females, and younger males. A score of -2.5 or less suggests that a person may have secondary osteoporosis, which does not result from aging.

A doctor can measure a person’s bone density with a DEXA scan. If the scan reveals a low bone mass, the doctor can prescribe medication and recommend lifestyle changes.