DXA is a fast, reliable, low-radiation method for assessing BMD, which is the arbiter for osteoporosis treatment eligibility and the best determinant of future fracture risk. • Web-based fracture risk calculators can utilise BMD data to provide further information for medical practitioners and patients. • The interval for serial BMD assessment for detecting development of osteoporosis should be planned according to the previous DXA findings

Lee, Joseph C.

Loh, Nelson K.

Crossref

Frequently asked questions on measurement of bone mineral densitometry

University of Queensland eSpace

VOLUME 4 • NUMBER 3 • SEPTEMBER 2012  J OURNAL OF PRIMARY HEALTH CARE 259
Frequently asked questions on measurement 
of bone mineral densitometry
1 Department of Nuclear 
Medicine, The Prince Charles 
Hospital, Queensland, 
Australia
2 School of Medicine, 
University of Queensland, 
Queensland, Australia
3 Department of Nuclear 
Medicine, Sir Charles 
Gairdner Hospital, Western 
Australia, Australia
CORRESPONDENCE TO:
Joseph C Lee
Staff Specialist Consultant 
Physician, Department of 
Nuclear Medicine, The 
Prince Charles Hospital, 
Queensland, Australia
Joseph_Lee@health.
qld.gov.au
Joseph C Lee MBBS, FRACP;1,2 Nelson K Loh BMedSci, MBBS, FRACP3
Introduction
Osteoporosis currently afflicts over half of 
women and almost 30% of men aged over 60 
years; this figure will grow as the population 
ages. Fractures—especially of the hip—result 
in considerable social and economic costs to the 
individual and to society, as well as leading to an 
appreciable mortality.
Bone mineral density measured with dual-energy 
x-ray absorptiometry is the ‘gatekeeper’ to osteo-
porosis treatments in New Zealand and the best 
way to estimate future fracture risk. Hence it is 
important to be familiar with some of the techni-
cal aspects of densitometry and its application in 
the community. Clarification of some of these 
areas may facilitate more effective utilisation.
What are the indications for bone 
mineral density (BMD) assessment?
The indications for BMD testing, as recommend-
ed on the Osteoporosis New Zealand website 
(http://www.bones.org.nz), are listed in Table 1. 
As pointed out on the site, BMD should only be 
measured if it will impact management.
What is the accuracy?
The precision of DXA is approximately 1.0–1.5% 
for spine and total hip BMD.1 DXA scanners 
have excellent long-term precision because their 
calibration is very stable and there are effective 
instrument quality control procedures (provided 
by the manufacturers) to detect any long-term 
drifts. The test–retest variation for spine and hip 
DXA is 2–3%, resulting in T-score accuracy errors 
of ± 0.5.2
What is the radiation dose?
The effective radiation dose is 1–10 µSv.3 Context 
is provided in Table 2.
Are spine or hip measurements better?
The best way of predicting fracture risk at any 
given site is to measure BMD at that site—for ex-
ample, DXA of the proximal femur predicts femo-
ral fracture better than measurements at other 
sites.4 Similarly, for vertebral or wrist fractures.
The spine is the best site for follow-up measure-
ments because treatment changes are usually the 
Table 1. Indications for bone densitometry 
Women over 60 years and men over 70 years with risk factors such as:
Glucocorticoid therapy
Parental history of a hip fracture 
Low body weight (< 58 kg) or body mass index (< 20 kg/m²) 
History of smoking or heavy alcohol intake
Premature menopause in women or hypogonadism in males 
Rheumatoid arthritis 
Malabsorption, chronic liver or renal disease 
Any woman over 65 years or man over 75 years considering specific measures to prevent osteoporosis
Any individual prescribed long-term glucocorticoids or other medications associated with osteoporosis, e.g. anti-
convulsants, aromatase inhibitors 
Women with a history of premature menopause 
Postmenopausal women or older men with a history of minimal trauma fracture 
(from Osteoporosis New Zealand—www.bones.org.nz)
VIEWPOINT
J PRIM HEALTH CARE
2012;4(3):259–261.
260 VOLUME 4 • NUMBER 3 • SEPTEMBER 2012  J OURNAL OF PRIMARY HEALTH CARE
and 250 000 person-years of follow-up.8 The 
Garvan Institute fracture risk calculator (http://
garvan.org.au/promotions/bone-fracture-risk/cal-
culator) requires fewer data and emphasises past 
fractures and falls.
A recent study found that the Garvan Institute 
calculator overestimated hip fracture rates and 
FRAX (with BMD measurements) underestimat-
ed osteoporotic and hip fracture rates.9 However, 
BMD should continue to be used to calculate 
fracture risk which informs treatment decisions.
How can BMD results be 
used to plan treatment?
BMD and/or fracture risk scores are used as the 
arbiters for osteoporosis treatment as treatment is 
considered cost-effective at certain thresholds of 
fracture risk. For example, Special Authority for 
Subsidy is granted under the Pharmac schedule10 
for alendronate according to the criteria in Ta-
ble 4. By comparison, the threshold for treatment 
is set at a hip fracture risk in the next 10 years of 
4% in the United Kingdom and Sweden.11
Why are there discrepancies 
between hip and spine T-scores?
A number of factors can spuriously affect the 
T- (and Z-) score at the hip and spine, including 
Table 3. Clinical risk factors included in FRAX algorithm—
www.shef.ac.uk/frax
Country of geographic origin
Ethnic origin
Age
Sex
Weight
Height
Previous history of fracture (after age 50)
Parental history of hip fracture
Current smoking status
Corticosteroid therapy (current or past)
Rheumatoid arthritis
Secondary osteoporosis
Alcohol intake (≥3 units daily)
Hip BMD
Table 4. Pharmac Special Authority for subsidy criteria for alendronate10
One significant osteoporotic fracture demonstrated radiologically and  T-score ≤ -2.5
One significant osteoporotic fracture demonstrated radiologically, and either the 
patient is elderly, or densitometry scanning cannot be performed because of major 
logistical, technical or pathophysiological reasons
Two significant osteoporotic fractures demonstrated radiologically
T-score ≤ -3.0
Ten-year risk of hip fracture (according to FRAX or Garvan calculator) ≥ 3%
Patient has had a Special Authority approval for zoledronic acid (underlying cause—
osteoporosis) or raloxifene
The patient is receiving systemic glucocorticosteriod therapy (≥ 5 mg per day 
prednisone equivalents) and has already received or is expected to receive therapy for 
at least three months; and
•	 T-score ≤ -1.5; or
•	 one significant osteoporotic fracture demonstrated radiologically; or
•	 Special Authority approval for zoledronic acid (underlying cause—
glucocorticosteroid therapy) or raloxifene
Table 5. Recommended intervals for serial DXA for detecting development of 
osteoporosis14
Normal BMD or ‘mild osteopenia’ (T-score not less than -1.5) 15 years
‘Moderate osteopenia’ (T-score between -1.5 and -2.0) 5 years
‘Advanced osteopenia’ (T- score between -2.0 and -2.5) 1 year
Table 2. Effective radiation dose of DXA compared to urban background and other 
medical procedures3
DXA 1-10 μSv
Chest x-ray 30 μSv
Mammogram 130 μSv
Annual background (urban environment) 2000 μSv
CT pelvis 5000 μSv
largest at this site and the precision error is lower 
than at other sites.5,6
How do the fracture risk 
calculators compare?
The WHO Fracture Risk Assessment Tool 
(FRAX) algorithm (available at http://www.shef.
ac.uk/frax) accounts for 14 risk factors (as listed 
in Table 3) in calculating 10-year probabilities of 
hip and major osteoporotic (humerus, forearm, 
hip or clinical vertebral) fractures.7 It incorporates 
data from 12 independent fracture studies, over 
60 000 men and women (from four continents) 
VIEWPOINT
VOLUME 4 • NUMBER 3 • SEPTEMBER 2012  J OURNAL OF PRIMARY HEALTH CARE 261
SUMMARY
•	 DXA	is	a	fast,	reliable,	low-radiation	method	for	assess-
ing BMD, which is the arbiter for osteoporosis treatment eli-
gibility and the best determinant of future fracture risk.
•	 Web-based	fracture	risk	calculators	can	utilise	BMD	data	to	pro-
vide further information for medical practitioners and patients.
•	 The	interval	for	serial BMD assessment for detecting development of 
osteoporosis should be planned according to the previous DXA findings.
degenerative bone/joint disease and aortic calcifi-
cation which leads to spuriously high BMD meas-
urements.12 However, some conditions do tend to 
preferentially affect BMD at the spine more than 
the hip (e.g. steroid use) or the hip over the spine 
(e.g. hyperparathyroidism).
How often should DXA be repeated?
The International Society of Clinical Densi-
tometry (ISCD) currently recommends no more 
frequent than biennial re-testing,13 formulated 
from the known precision of DXA equipment 
and that the normal rate of postmenopausal bone 
loss is 1–2% per year.
Prospective clinical study of postmenopausal 
women concluded that development of osteopo-
rosis is adequately detected by DXA frequency 
stratified by previous DXA findings for that 
individual14 (summarised in Table 5). In most 
instances, this is much less frequent than the 
minimum two-year interval recommended by 
ISCD. However, it is also suggested that patients 
with advanced osteopenia should have repeat 
testing in one year, which directly contradicts the 
ISCD’s recommendation. 
Are DXA results from different 
facilities interchangable?
No. Different facilities use different equipment 
made by different manufacturers which, in turn, 
utilise different reference databases. It is best, 
therefore, to use the same software for repeat 
scans wherever possible.
7. Kanis JA, Oden A, Johnell O, et al. The use of clinical risk 
factors enhances the performance of BMD in the prediction 
of osteoporotic fractures in men and women. Osteoporos Int. 
2007;18:1033–46.
8. Blake GM, Fogelman I. An update on dual-energy x-ray 
absorptiometry. Semin Nucl Med 2010;40:62–73.
9. Bolland MJ, Siu AT, Mason BH, et al. Evaluation of the FRAX 
and Garvan fracture risk calculators in older women. J Bone 
Miner Res. 2011;26:420–7.
10. Wilson K, Bloor R, Jennings D, editors. TEX Schedule Febru-
ary 2012 Volume 19. New Zealand: Pharmaceutical Manage-
ment Agency; 2012.
11. Chun KJ. Bone densitometry. Semin Nucl Med. 2011;41:220–8.
12. Reid IR, Evans MC, Ames R, et al. The influence of osteo-
phytes and aortic calcification on spinal mineral den-
sity in postmenopausal women. J Clin Endocrinol Metab. 
1991;72:1372–4.
13. Baim S, Binkley N, Bilezikian JP, et al. Official positions of the 
International Society for Clinical Densitometry and executive 
summary of the 2007 ISCD Position Development Confer-
ence. J Clin Densitom. 2008;11:75–91.
14. Gourlay ML, Fine JP, Preisser JS, et al. Bone-density testing 
interval and transition to osteoporosis in older women. New 
Engl J Med. 2012;366:225–33.
References
1. Patel R, Blake GM, Rymer J, et al. Long-term precision of DXA 
scanning assessed over seven years in forty post-menopausal 
women. Osteoporos Int. 200;11:68–75.
2. Blake GM, Fogelman I. How important are BMD accuracy er-
rors for the clinical interpretation of DXA scans? J Bone Miner 
Res. 2008;23:457–62.
3. Fogelman I, Blake GM. Different approaches to bone densi-
tometry. J Nucl Med. 2000;41:2015–25.
4. Marshall D, Johnell O, Wedel H. Meta-analysis of how well 
measures of bone mineral density predict occurrence of osteo-
porotic fractures. BMJ. 1996;312:1254–9.
5. Blake GM, Herd RJ, Fogelman I. A longitudinal study of 
supine lateral DXA of the lumbar spine: a comparison with 
posteroanterior spine, hip and total-body DXA. Osteoporos 
Int. 1996;6:462–70.
6. Faulkner KG. Bone densitometry: choosing the proper site to 
measure. J Clin Densitom. 1998;1:279–85.
COMPETING INTERESTS
None declared.
VIEWPOINT


http://espace.library.uq.edu.au/view/UQ:317214/UQ317214_OA.pdf

Frequently asked questions on measurement of bone mineral densitometry

Abstract

Similar works

Full text

Available Versions

Crossref

University of Queensland eSpace