Bone Mineral Density Scan (Bone Densitometry or DXA Scan)

Dr Elke Hendrich

What are the generally accepted indications for a BMD scan?

The following indicate an increased risk for bone loss (osteoporosis) and might require BMD assessment to confirm osteoporosis.1 Although BMD scanning can be carried out for any of these risk factors, only some satisfy Medicare re batable indications, as specified in the MBS handbook. These indications are generally applicable to patients at increased risk of fracture and include:

Medicare rebatable indications for bone density screening

  • Age =70 years.
  • Medical conditions and medications that might cause secondary osteoporosis:
    • Endocrine (e.g. hypogonadism, Cushing syndrome, hyperparathyroidism, hyperthyroidism or thyroxine excess).
    • rheumatoid arthritis;
    • malabsorption and vitamin D deficiency (e.g. coeliac disease, Crohn’s disease);
    • chronic kidney disease (CKD), chronic liver disease;
    • drugs, especially oral or inhaled corticosteroids (e.g. 7.5 mg/day for =3 months), thyroxine;
    • patients who have sustained a low impact fracture (defined as a fracture due to a fall from standing height);
    • vertebral fracture (loss of height >3 cm or >20%, kyphosis, back pain).

Other conditions predisposing to fracture and bone loss, and that render a DXA scan worthwhile, but might not be rebatable, include:

  • low BMI or frailty;
  • family history of osteoporosis, especially if there is a history of hip fracture;
  • frequent falls;
  • poor mobility, housebound;
  • proximal myopathy;
  • high/excessive alcohol intake;
  • smoking;
  • medications: aromatase inhibitors, anticonvulsants, anti-androgen therapy.

What are the prerequisites for having a BMD scan done?

The only distinct prerequisites for a BMD scan is that the individual should not have had a previous radiological or nuclear medicine investigation in the last week and that they are known not to be pregnant (see what are the absolute contraindications for a BMD scan).

What are the absolute contraindications for a BMD scan?

Pregnancy is the only absolute contraindication. Pregnancy is contraindicated because of the ionising radiation involved in the study. Documentation of the last normal menstrual period or the results of a pregnancy test should be provided.

Bilateral hip replacements or bilateral hip pins or screws would prevent the hip sites from being scanned. Unilateral metallic surgical implants in the hip necessitate the opposite hip being scanned. Similarly, metallic rods or spinal fusion devices in the lumbar spine would preclude scanning at this site. Forearm scans are carried out where either the hip or spine sites are unavailable.

DXA scans are safe for patients with permanent pacemakers and implantable defibrillators.

What are the relative contraindications for a BMD scan?

  • Weight >120–130 kg is a relative contraindication and depends on the manufacturer of the DXA scanner. Newer scanners can accommodate weights up to 200 kg.
  • Carrying out a BMD scan within the week after other radiological investigations using contrast media; for example, barium meals/enemas, IVPs (intravenous pyelograms) or CT scans might interfere with the results. Recent nuclear medicine investigations with persistent radioactivity could interfere with the X-ray detector of the machine and produce inaccurate results. Scheduling of appointments should take these tests into account.
  • Inability to transfer from a wheelchair to the scanning table is a relative contraindication, as the scanner table height cannot be adjusted.

What are the adverse effects of a BMD Scan?

There are no apparent adverse effects of a BMD scan.

Is there any specific post-procedural care required following a BMD Scan?

There is no specific post-procedural care required.

Are there alternative imaging tests, interventions or surgical procedures to a BMD scan?

Although the currently accepted gold standard for assessing bone density is the DXA scan, an assessment of bone density can also be made by CT scan of the lumbar spine, or by heel ultrasound. There are significant limitations of both of these latter two methods.

CT scan of the lumbar spine is accurate and reproducible; however, it involves significantly higher doses of ionising radiation, and provides information about only one region. The hip and spine are composed of different types of bone, which are variably susceptible to medical or therapeutic insults. Having information about two sites is therefore preferred.

Heel ultrasound is a useful screening method to assess bone structure and density. It does not use ionising radiation, but requires formal DXA assessment for osteoporosis in those patients who have been identified as potential high fracture risk after heel ultrasound measurements.

Further information about BMD scan:

WHO criteria for BMD results are based on T score values; that is, measurements compared with normal young same sex population:

  • <-1 SD below the mean: normal
  • -1 to <-2.5 SD below the mean: osteopenia
  • >-2.5 SD below the mean: osteoporosis

BMD scanning is also indicated for follow up or monitoring of response to therapy aimed at preventing fracture and bone loss, or monitoring of progress according to medical condition or therapy. The frequency of repeated studies covered by Medicare in Australia varies according to the underlying condition, for example 12-monthly intervals for hypogonadism and glucocorticoid exposure, and 24-month intervals for the remainder.

There are a number of manufacturers of DXA scanners. It is important that follow-up DXA scans be carried out on the same scanner. Scans from different manufacturers, and to some extent those from the same manufacturer at different sites, cannot be directly compared.

Although diffuse bone loss in the elderly is commonly due to osteoporosis, the results of other investigations and clinical presentations need to be taken into account. For example, diffuse metastatic disease or myeloma could also present with vertebral body fractures and reduced BMD. Reduced bone loss in the younger adult warrants further investigation or specialist referral.

Useful websites about BMD

WHO Fracture Risk Assessment Tool:
www.shef.ac.uk/FRAX/

References

  1. RACGP: Guidelines for Preventive activities in General Practice 8th Edition
    www.racgp.org.au/your-practice/guidelines/redbook/

Last saved on 26 September 2016.

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