January
2006 | Volume 73
Number 1 | Pages 29-32
Any
system that sets an arbitrary cut point inevitably misclassifies some
patients
Most
patients who have fragility fractures have T scores above —2.5
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| Q: |
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What
is osteopenia, and what should be done about it? |
I
am on a crusade to eliminate the term "osteopenia"
NELSON B. WATTS,
MD
Professor of Medicine, University of Cincinnati College of Medicine; Director,
University of Cincinnati Bone Health and Osteoporosis Center
A: Consider the following
patients:
A 35-year-old woman who is premenopausal and has no risk factors
for osteoporosis. After being sedentary for most of her life, she began
training for a marathon and sustained stress fractures in her left foot.
Her
orthopedist recommended a dual-energy x-ray absorptiometry (DXA) bone density
study of the spine and hip. The diagnosis was osteopenia,
lowest T score –1.1.
A 52-year-old woman who just started menopause and has no other risk
factors for osteoporosis. Her primary care physician recommended
a DXA study “to be on top of things.” The diagnosis was osteopenia,
lowest
T score –1.3.
A 57-year-old woman who went through natural menopause in her late 40s.
She has no other risk factors for osteoporosis. She had an ultrasound test
of the heel at a local health fair. The diagnosis was osteopenia, lowest
T score –1.7.
A 66-year-old woman who went through natural menopause in her early
50s and has no other risk factors for osteoporosis. Her primary care physician
recommended a DXA study based on the recommendations of the US Preventive
Services Task Force1 that healthy women without risk factors
be screened for osteoporosis at age 65. The diagnosis was osteopenia, lowest
T score –1.8.
A 76-year-old woman who went through natural menopause in her early
50s. She has lost 3.5 inches in height. Her primary care physician recommended
a DXA study. The diagnosis was osteopenia, lowest T score –2.3.
OSTEOPENIA IS NOT
A USEFUL DIAGNOSIS
What do these women have in common? They have all had bone density
tests. Although the diagnosis is the same in all five, their risk of fracture
and need for pharmacological intervention differ considerably.
Because the term “osteopenia” is not useful as a diagnosis and
can actually be harmful, I am on a personal crusade to eliminate it from
the bone density lexicon. (I am happy to let the radiologists use it to
describe “washed out” bones seen on standard radiographs, but
I realize that someone with washed-out bones has probably lost 30% of
her or his young adult bone mass and therefore has osteoporosis.)
What is a T score?
The unit of measure for bone mineral density (BMD) by DXA is grams
per square centimeter. If there were only a single device for measuring
bone density, and if only a single skeletal site were measured absolute
BMD would be used clinically. However, few (if any) clinicians would be
able to remember ideal or threshold cut-point values for multiple skeletal
sites (spine, femoral neck, total hip) for a single machine, much less
deal with machines from different manufacturers that are calibrated differently.
Thus, the unit for reporting bone density, at least for postmenopausal
women, is the T score, in which the patient’s BMD is compared
with the young normal mean value and the difference expressed as a standard
deviation score. For example, 0 is equal to the young adult mean value,
+1 is 1 standard deviation above the young adult mean, –1 is 1 standard
deviation below. In theory, the T score provides a way of using a single
set of numbers for all devices and all skeletal sites.
T scores apply only
to postmenopausal women
In 1994, a working group of the World Health Organization (WHO) put
forth an operational definition of postmenopausal osteoporosis (TABLE
1).2 The purpose was to have a common framework that
would allow collection of epidemiological data in different countries
to convince government and public health authorities that osteoporosis
is a serious health problem.
The working group made it clear that their classification was to be
applied only to postmenopausal women. The International Society for Clinical
Densitometry (ISCD) recommends that T scores not be used in premenopausal
women or in younger men, and certainly not in children.3 Instead,
Z scores should be used. The Z score compares a patient with age-matched,
sex-matched, and race-matched norms and expresses the difference as a
standard deviation score.
The working group also made it clear that their classification was
applicable only to measurements made at the spine, hip, and forearm. The
ISCD specifically states that patients should be classified based on the
lowest T score of the posteroanterior spine, femoral neck, trochanter,
or total hip4 and that the WHO classification should not be
used with peripheral measurements.5
The WHO working group selected a cutpoint T score of –2.5 to define
osteoporosis because “such a cutoff value identifies approximately
30% of postmenopausal women as having osteoporosis using measurements
made at the spine, hip, or forearm. This is about the
same as the lifetime risk of fracture at these sites. When measurements
are made at the hip alone, then the prevalence is about one in five white
women, comparable to the lifetime risk of a single osteoporotic fracture,
such as a hip fracture."2
No threshold BMD
for fracture risk
The relationship between BMD and fracture risk is continuous; there
is no magic fracture threshold.
Marshall and colleagues6 performed a meta-analysis of 11 prospective
cohort studies involving 90,000 person-years of observation and more than
2,000 fractures. The correlation between BMD at baseline and fractures
occurring during prospective observation was remarkably consistent: for
each standard deviation decrease in BMD, the risk of fracture increased
approximately twofold. This was true whether the correlation was between
forearm BMD and hip fracture risk or hip BMD and spine fracture risk.
Thus, any system that sets an arbitrary cut point will inevitably
misclassify some patients. Nevertheless, it is helpful at times for clinicians
to think categorically (ie, “normal” vs “osteoporosis”).
Although it was not intended to be applied to individual patients,
the WHO classification works well to define “normal” (T score
–1.0 and above) and “osteoporosis” (T score –2.5 and
below). Several large studies have shown a high risk of fracture in patients
who have T scores of –2.5 and below and also a significant reduction
in fracture risk with treatment in such patients, making this threshold
a reasonable, evidence-based criterion for the diagnosis of osteoporosis
and for starting treatment in patients without other risk factors for
fracture.
Four problems with ‘osteopenia’
Although it is sometimes useful to think categorically when dealing with
a continuous variable, the category of “osteopenia” creates
problems in at least four ways.
- The WHO classification
was specifically meant to apply to postmenopausal white women; applying
a medical label such as osteopenia to a healthy young person can create
considerable anxiety that may last one’s whole life.
- It was meant to
be applied only with DXA and only to specific skeletal sites; using
the WHO classification with peripheral measurements (eg, heel, finger,
tibia) or other techniques (eg, quantitative computed tomography, quantitative
ultrasonography) is not appropriate.
- Many postmenopausal
women who are in the upper part of this borderline range are perfectly
normal. After all, “normal” for most biological variables
is defined as the mean plus or minus 2 standard deviations.
- Postmenopausal
women who are in the lower part of this range are almost as likely to
have fractures as patients on the lower side of the arbitrary cut point
(or maybe more likely, depending on other risk factors for fracture).
I try to avoid using
the term osteopenia in reporting DXA results and in patient care. For patients
with T scores between –1.0 and –2.5, I use the term “low
bone density,” which does not sound like a medical problem in and of
itself, and (I hope) is a nonjudgmental term that should force the clinician
to think about the clinical context.
Postmenopausal women in the upper part of this range (–1.0 to –1.5)
should usually be reassured and monitored, perhaps every 5 years or so.
Patients in the middle part of this range (–1.5 to –2.0) rarely
need pharmacologic treatment but should be monitored every 3 to 5 years.
Patients in the lower part of the range (–2.0 to –2.5) should
be monitored at least every year or two and may even be candidates for pharmacologic
intervention, depending on how low their BMD is and if they have other risk
factors for fracture.7
FRACTURES DEFINE
SEVERE OSTEOPOROSIS
Often ignored is the WHO category of “severe” or “established”
osteoporosis, meant to apply to patients who have already had a fracture.
Although a fragility fracture, particularly a vertebral fracture, is a strong
predictor of future fractures, this is true whether the T score is –2.6
or –2.4. In fact, there is an apparent paradox: most patients who have
fragility fractures have T scores above –2.5!8,9 Patients
without fractures but with T scores of –2.5 or below are clearly at
high risk of fracture; however, that is a small percentage of the population.
Although the fracture rate is lower in the group with “low bone mass,”
there are so many more people with low bone mass than with osteoporosis
(using the WHO classification) that the absolute number of fractures is
greater
in the group with low bone mass. It is possible that some clinicians would
not diagnose osteoporosis in a patient with a fragility fracture but with
a T score above –2.5! I am not in favor of adding a category of “severe
osteopenia” for patients with borderline low bone mass and fragility
fractures; in my view, these patients have osteoporosis. In fact, a patient
with low bone mass and fragility fracture is much more likely to have a
fracture in the future than a patient with WHO-defined osteoporosis but
without a fragility fracture.10
IN SUMMARY
- The term “osteopenia”
should be eliminated in the context of BMD testing and replaced with
“low bone mass.”
- Patients with low
bone mass and fragility fractures have osteoporosis and should be treated.
- Apparently healthy
patients in the upper range of low bone mass should be reassured and
monitored periodically; those in the lower range deserve consideration
of pharmacologic intervention.
- T scores should
not be used in premenopausal women or in young men or children.
The WHO criteria should
not be applied to sites other than the posteroanterior spine, proximal
femur, or forearm.
WHAT ABOUT OUR FIVE PATIENTS?
The 35-year-old premenopausal woman. Her stress fractures are almost
certainly due to repeated mechanical forces rather than a systemic
skeletal disease. A bone density study is not indicated for her. When
DXA studies are done in young women, Z scores rather than T scores should
be reported, to discourage inappropriate application of the WHO classification.
This woman’s lowest Z score is –1.2, which is below average
but still normal (within 2 standard deviations of the age-adjusted mean
value). This woman should have a DXA study repeated at age 65 or sooner
if there is
some new indication. She does not need pharmacologic therapy for bone
health.
The 52-year-old woman who has just started menopause. Bone density
testing is recommended for women 65 and older without
risk factors and for younger postmenopausal women who have risk factors,
and so is not indicated in this case. Her lowest T score of –1.3
is borderline low, indicating a low risk of fracture in the next 5 to
10 years. She does not need pharmacological therapy for bone health. She
should have a DXA scan
repeated at age 65 or sooner if there is some new indication.
The 57-year-old postmenopausal woman. She had an ultrasound test
of the heel at a local health fair, but the WHO criteria cannot be
used with peripheral measurements, so the diagnosis of osteopenia is not
appropriate with the data at hand. In many cases, T scores are lower in
the spine and hip than in the heel. With a T score of –1.7 in the
heel, she could
have a much lower score in the spine or hip. False-negative peripheral
tests can also be seen. She needs a central DXA scan to be done now
if there is concern about the implications of the abnormal ultrasound
test, or, because she has no risk factors for osteoporosis, she could
wait
until age 65.
The 66-year-old postmenopausal woman. Bone density testing is appropriate.
Her lowest T score of –1.8 is in the middle of the borderline
range. With no other risk factors for osteoporosis, her likelihood of
fracture in the next 5 to 10 years is fairly low, so pharmacologic therapy
is not indicated. Age-related bone loss occurs at a rate of about 0.5%
to 1.0% per year, and the minimum change measurable with DXA is about
3%, so she should have a repeat DXA in 3 to 5 years.
The 76-year-old woman. Her lowest T score is –2.3, and she
has lost 3.5 inches in height. Spine x-rays or vertebral fracture assessment
with DXA equipment should be done. If she does have vertebral fractures,
then she has osteoporosis and is a definite candidate for pharmacological
treatment. If she does not have vertebral fractures, treatment
may still be appropriate, as her age is an independent risk factor for
fracture.
REFERENCES
- US Preventive Services Task Force. Screening for osteoporosis
in postmenopausal women: recommendations and rationale. Ann Intern Med
2002; 137:526–528.
- Kanis JA, Melton LJ III, Christiansen C, Johnston CC, Khaltaev
N. The diagnosis of osteoporosis. J Bone Miner Res 1994; 9:1137–1141.
- Writing Group for the ISCD Position Development Conference. Diagnosis
of osteoporosis in men, premenopausal women, and children. J Clin Densitom
2004; 7:17–26.
- Hamdy RC, Petak SM, Lenchik L; International Society for Clinical
Densitometry Position Development Panel and Scientific Advisory Committee. Which central dual X-ray absorptiometry skeletal sites and regions of
interest should be used to determine the diagnosis of osteoporosis?
J Clin Densitom 2002; 5(suppl):S11–S18.
- Miller PD, Njeh CF, Jankowski LG, Lenchik L; International Society
for Clinical Densitometry Position Development Panel and Scientific
Advisory Committee. What are the standards by which bone mass measurement
at peripheral skeletal sites should be used in the diagnosis of osteoporosis?
J Clin Densitom 2002; 5(suppl):S39–S45.
- Marshall D, Johnell O, Wedel H. Meta-analysis of how well measures
of bone mineral density predict occurrence of osteoporotic fractures.
BMJ 1996; 312:1254–1259.
- Hodgson SF, Watts NB, Bilezikian JP; American Association of Clinical
Endocrinologists. American Association of Clinical Endocrinologists
2001 Medical Guidelines for Clinical Practice for the Prevention and
Management of Postmenopausal Osteoporosis. Endocr Pract 2001; 7:293–312.
- Wainwright SA, Marshall LM, Ensrud KE, et al; Study of
Osteoporotic Fractures Research Group. Hip fracture in women without
osteoporosis. J Clin Endocrinol Metab 2005; 90:2787–2793.
- Schuit SC, van der Klift M, Weel AE, et al. Fracture incidence
and association with bone mineral density in men and women: the Rotterdam
Study. Bone 2004; 34:195–202.
- Ross PD, Davis JW, Epstein RS, Wasnich RD. Pre-existing fractures
and bone mass predict vertebral fracture incidence in women. Ann Intern
Med 1991; 114:919–923.
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BRIEF
QUESTIONS
AND ANSWERS
ON CURRENT CLINICAL
CONTROVERSIES
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