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What is a Bone Density test?

Bone density test, also known as dual-energy X-ray absorptiometry (DEXA), is an advanced method of X-ray technology used to measure bone loss. It is the standard test for measuring bone mineral density. The procedure uses a small dose of ionising radiation for producing images of the lumbar spine and hipbone. In children, the whole body may also be scanned. It is a simple, quick and noninvasive procedure to assess and diagnosis osteoporosis or other conditions that may lead to bone loss.

Previously, osteoporosis would be identified only after a fracture. This procedure provides more accurate information in such cases by calculating the risk of fractures in future. 

  1. Why is a Bone Density test performed?
  2. How do you prepare for a Bone Density test?
  3. How is a Bone Density test performed?
  4. What do Bone Density test results indicate?

Bone density tests are performed to diagnose osteoporosis; estimate bone loss and efficacy of osteoporosis medications, and to predict the risk of a future fracture. This test is recommended for women beyond the age of 65 years. Men are recommended to undergo testing after the age of 70 years. Young women and men of any age must undergo bone density test if they show risk factors, such as:

All calcium supplements should be discontinued for at least 24 hours prior to the test. If the person has undergone any procedure involving the intake of a contrast dye for barium studies, computed tomography or magnetic resonance imaging scan, he/she should wait for at least a week as the dye can interfere with results. All metal belts and buttons must be removed before the test.

This test can be performed in two ways. The most common and accurate method is a DEXA scan in which a low dose of X-ray radiation is used. There are 2 types of DEXA scans:

  • Central DEXA: In this procedure, the person lies on a table and the scanner is passed over the lower lumbar region and spine. Undressing is not required in most cases. No zippers should be included in the scan. This method can best predict the risk of hip fractures.
  • Peripheral DEXA: Smaller machines are used to measure bone density in wrist, fingers, leg and heels. Such machines are often seen in healthcare offices, medical camps, etc.

Both procedures are painless. The person only needs to remain still during the test.

Normal results: Two scores are received after the test: T score and Z score. T score compares the bone density with another healthy young adult of the same gender as the patient who undergoes the test.

  • T score of -1 and above: Indicates normal  bone density
  • T score of -1 to -2.5: Indicates low bone density and risk of osteoporosis
  • T score of -2.5 and above: Indicates osteoporosis

Z score compares the bone mass of other people of the same age, gender and size. A Z score below -2.0 indicates low bone mass caused due to factors other than ageing and requires additional investigations.

Abnormal results: An abnormal DEXA test does not help in detecting fractures. Along with pre-existing risk factors, it only predicts the risk of a fracture in the future. An abnormal DEXA scan may also be seen in the presence of other risk factors, such as:

While repeating the DEXA scan, it is recommended to perform the test at the same place. This helps in accurate comparison with previous reports. Even if it may not always be possible to undergo the test at the same place, it is necessary to compare the density scores. X-rays cannot replace bone density tests, because they cannot show osteoporosis until the disease is at an advanced stage. However, X-rays can be recommended along with a bone density scan to detect fractures in other parts of the body.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational perspective and is in no way a substitute for medical advice from a qualified doctor. 

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References

  1. American College of Radiology (ACR), the Society of Interventional Radiology (SIR), and the Society for Pediatric Radiology; Bone Densitometry (DEXA, DXA)
  2. UCSF health: University of California [internet]; Bone Density Scan
  3. Committee on Practice Bulletins-Gynecology, The American College of Obstetricians and Gynecologists. ACOG practice bulletin n. 129. Osteoporosis. Obstet Gynecol. 2012;120(3):718-734. PMID: 22914492
  4. osman F, de Beur SJ, LeBoff MS, et al; National Osteoporosis Foundation.Clinician’s Guide to Prevention and Treatment of Osteoporosis. Osteoporos Int. 2014;25(10):2359-2381. PMID: 25182228
  5. US Preventive Services Task Force, Curry SJ, Krist AH, Owens DK, et al. Screening for osteoporosis to prevent fractures: US Preventive Services Task Force recommendation statement. JAMA. 2018;319(24):2521-2531. PMID: 29946735