Standard for abscess drainage

Standard for Performance of Adult Dual - or
Single-Energy X-Ray Absorptiometry
(DXA/pDXA/SXA)

Approved: June 1999
Modified by Brian C. Lentle, MD, and the CAR Task Force for Standards and Guidelines for Radiological
Practice. Principal Drafter- William T. Thorwarth, Jr., MD
I. INTRODUCTION
Bone densitometry whether by single (SXA) or dual x-ray absorptiometry (DXA) is a clinically proven and precise method of measuring bone mineral density (BMD). DXA is typically applied to the central skeleton (lumbar spine, proximal femur or, even the whole skeleton). Peripheral DXA (pDXA) and single X-ray absorptiometry (SXA) are absorptiometric techniques used on the peripheral skeleton, mainly the forearm, calcaneus, and phalanges. These examinations provide equally valuable tools in assessing osteoporosis and other disease states characterized by abnormal BMD as well as estimating risk of fracture. This standard outlines the principles of performing high-quality DXA. II. GOAL
The goal of DXA is to accurately and reproducibly measure a patient's bone mineral density, and compare that
measurement to reference population standards. This comparison contributes to the referring physician's
diagnosis of osteoporosis in asymptomatic people, assessment of the patients risk of sustaining fracture, and
the possible need for appropriate therapy and fracture-prevention programs for the patient. It is also useful in
evaluating the effectiveness of prior or current therapy.
III. INDICATIONS AND CONTRAINDICATIONS
BMD measurement is indicated whenever a clinical decision to intervene will be directly influenced by the result
of the test. Indications for densitometry include, but are not limited to:
A. Patients of any age with suspected insufficiency (fragility) fractures.
B. Women with estrogen deficiency (perimenopausal, postmenopausal, or following oophorectomy).
C. Additional risk factors for osteoporosis, such as:
1. A family history of hip fracture or osteoporosis; 2. Low body mass; 3. A personal History of bulimia or anorexia; 4. History of amenorrhea (> 1 year before age of 42 years); 5. A current gastrointestinal malabsorption disorder; 6. Smoking history (> 1 pack per day x 5 years or more); 7. Alcoholism 8. Loss of height, thoracic kyphosis. D. Patients with radiographic findings suggesting osteoporosis, such as radiographic osteopenia with or without
vertebral deformity.
E. Patients with metabolic and other disorders that could alter BMD, such as:
1. Primary hyperparathyroidism 2. Primary hypogonadism 3. Hyperthyroidism 4. Cushing's disease 5. Chronic renal failure 6. In follow-up of organ transplant recipients 7. Prolonged immobilization 8. Conditions associated with secondary osteoporosis, such as osteomalacia, vitamin D deficiency, endometriosis, acromegaly, and multiple myeloma F. Patients beginning or receiving long-term therapy with corticosteroids (glucocorticoids), thyroid
replacement, or other medications (such as phenytoin or heparin therapy) which may adversely affect bone
density.
G. Follow-up at appropriate intervals of patients receiving therapy for altered bone mineral density.
Contra-indications or relative contra indications to DXA include: 1. Recent barium (for spine measurements) or radionuclide studies should be considered in scheduling; 2. Severe arthritic or fracture deformity or other degenerative changes at the site to be measured; 3. Radio-opaque implants in the measurement area, most commonly at the hip; 4. A patient's inability to maintain correct position and/or remain motionless for the duration or the measurement; and 5. Extreme obesity or extremely low body mass may compromise measurements and the capacity to produce accurate and precise measurements. While this limitation should be considered in the interpretation of bone densitometry results, this does not preclude the examination being undertaken in such patients. All imaging facilities should have policies and procedures to reasonably attempt to identify pregnant patients prior to the performance of any diagnostic examination involving ionizing radiation. If the patient is known to be pregnant, the potential risks to the fetus and clinical benefits or the procedure should be considered before proceeding with the study. IV. QUALIFICATIONS AND RESPONSIBILITIES OF PERSONNEL
A. Physician
That Physicians involved in the performance, supervision and interpretation of bone densitometry should be
Diagnostic Radiologists and must have a Fellowship or Certification in Diagnostic Radiology with the Royal
College of Physicians and Surgeons of Canada and/or the Collège des médecins du Québec. Also acceptable
are foreign Specialist qualifications if the Radiologist so qualified holds an appointment in Radiology with a
Canadian University.
As new imaging modalities and interventional techniques are developed additional clinical training, under supervision and with proper documentation, should be obtained before radiologists interpret or perform such examinations or procedures independently. Such additional training must meet with pertinent provincial/regional regulations. Continuing professional development must meet with the requirements of the Maintenance of Certification Program of the Royal College of Physicians and Surgeons of Canada. B. Technologist
1. The technologist should have the responsibility for patient comfort and safety, preparing and properly
positioning the patient, and of placement of regions of interest for assessment of bone mineral density
measurements, monitoring the patient during the measurements, and obtaining the measurements prescribed
by the supervising physician.
2. Documented formal training in the use of the DXA equipment including performance of all manufacturer-
specified quality assurance (QA) procedures is required.
3. The technologist must read, be familiar with, and have accessible, the manufacturer's operator manual for
the specific scanner model being used.
4. The technologist shall be certified in radiographic or nuclear medicine technology by the Canadian
Association of Medical Radiation Technology and comply with that Association's requirements for continuing
education.
5. If plain radiographic images are performed to correlate with DXA studies, the technologist's qualifications
must be appropriate.
V. SPECIFICATIONS OF THE EXAMINATION
A. The written request for DXA examination should contain appropriate clinical history and the reason for
examination. A history should be obtained from the patient regarding risk factors as listed in Section Ill. B, C, D, E, and F, including family history, prior fragility fractures, and prior bone trauma/fractures or surgery which could potentially affect the accuracy of measurements. B. Standard central DXA examination should consist of PA spine and proximal femur scans. In some cases
(degenerative disease, scoliosis, fractures, orthopedic hardware), other sites should be scanned (lateral,
forearm, or total body),
C. Images indicating the areas of bone mineral density measurement should be obtained with the DXA device;
generally radiographs are not necessary. If prior radiographs of these anatomic areas are available, these
should be reviewed to determine if specific sites should not be analyzed.
D. Positioning and soft tissue equivalent devices issued by the manufacturer must be used consistently and
properly. Comfort devices, such as pillows under the head or knee, must not interfere with proper positioning
and must never appear in the scan field.
E. Anatomic areas of known prior fracture or prior surgery should be excluded from measurement.
F. If significant discordance is present between two areas measured with no evident explanation from patient
history, DXA images or plain radiographic correlation, additional DXA acquisitions (e.g., lateral lumbar spine,
opposite proximal femur/forearm), or other bone density measurement techniques (e.g., QCT) should be
considered.
G. Measured values must be compared with young-adult control population values yielding a T-score. It is
suggested that Canadian population reference standards be used when the CAMOS (Canadian Multi-Centre
Osteoporosis Study) study is completed and such standards are available. Comparison of age-matched values
(Z scores) may be reported at the discretion of the physician. Fracture risk should be estimated.
H. Comparison should be made to any prior comparable DXA exams of the same site to assess any statistically
significant interval change. Comparable DXA scans include in order of decreasing validity:
1. Previous examinations using the same well-maintained device.
2. Previous examinations on another device made by the same manufacturer.
Previous examinations on a device from another manufacturer with results reported in standardized units. The use of standardized units introduces an unknown additional degree of uncertainty. Repeat examination should be done at the same time of the year as there are seasonal fluctuations in BMD. VI. DOCUMENTATION
A. A permanent record must be maintained, including:
1. Patient name, identification number, date, device serial number, and facility of examination.
2. Clinical notes of any unique history positioning, anatomy, and/or technique setting that would be important
for performing serial measurements,
3. Printouts of the images and regions of interest, if provided by the device, and the bone mineral
measurement values obtained.
B. Reports should include, for each site examined: bone mineral density, T-score, corresponding percentages
of mean, and fracture risk. A statement comparing the current study to prior available comparable studies
should be included. Reports should classify patients according to World Health Organization criteria. If serial
examinations are reviewed, a statement whether a change in BMD is significant should be included. If needed,
suggestions for conclusive radiographs and interval follow-up DXA scan should be provided.
C. Reporting should be done in accordance with the CAR Standard for Communication: Diagnostic Radiology.

VII. EQUIPMENT SPECIFICATIONS
Multiple equipment designs are available that can accurately and precisely measure bone density using dual-energy x-ray absorptiometry. The equipment should provide the following: A. Normal young adult and age-matched control population standards matched for sex applicable to the
equipment being used must be available. Some devices also provide standards matched for ethnicity, weight,
and body mass index.
B. A phantom or other standard must be provided in order to evaluate the accuracy, precision and linearity of
response of BMD measurement.
C. A permanent recording of labeled images of the anatomic site measured and measurement results for
patient records.
D. Precision error or the coefficient of variations for measurements of the phantom or standard should not
exceed the specifications or recommendations of the manufacturers and should be less than 1%, In vitro
(phantom) precision should not be equated with in vivo (patient) short-term precision, as the role of the
technologist in positioning and scan analysis is critical.
VIII. EQUIPMENT QUALITY CONTROL
DXA equipment quality control is extremely important for long-term monitoring of the effectiveness or therapy
or progression of disease. The importance of DXA quality control cannot be overstated. Quality control
procedures should be performed and permanently recorded by a trained technologist. As there is variation in
the frequency of recommended quality control amongst the equipment currently available, compliance with the
manufacturers' guidelines is required.
A. If a problem is detected according to manufacturer guidelines, notify the service representative and do not
scan patients until the equipment has been cleared for use.
B. Each imaging facility should have documented policies and operations for monitoring and evaluating the
effective management, safety, and operation of imaging equipment. The quality-control program should be
designed to minimize patient, personnel, and public radiation risks and to maximize the quality of the
diagnostic information.
C. At least annually, equipment performance should be monitored and a quantitative dose determination
should be conducted by a qualified medical physicist.
BIBLIOGRAPHY
1. Baran DT, Faulkner KG, Genant HK, et al. Diagnosis and management of osteoporosis: guidelines for the
utilization of bone densitometry, Calcif Tissue Int 1997; 61:433-440
2. Blake GM, Gluer CC, Fogelman I. Bone densitometry: current status and future prospects. Brit J Radiol 1997; S 177-186 3. Cooper C, Atkinson EJ, Jacobsen SJ, et al. Population based study of survival after osteoporotic fracture. Am J Epidemiol. 1993; 137:1001-1005 4. Cummings SR, Black DM, Nevitt MC, et al. Bone density at various sites for prediction of fractures. 5. Eddy DM Johnston CC, Cummings SR et al. Osteoporosis: review of the evidence for prevention, diagnosis and treatment and cost-effectiveness analysis. Osteoporosis Int 1998; 8: Sl-S88 6. Fogelman 1, Ryan P. Measurement of bone mass. Bone 1992; 13(suppl 1): S23-S28 7. Franck H, Munz M, Scherrer M. Bone mineral density of opposing hips using dual energy X-ray absorptiometry in single beam and fan-beam design. Calcif Tissue Int 1997; 61-.445447. 8. Genant HK, Grampp S, Gluer CC, et al. Universal standardization for dual x-ray absorptiometry: patient and phantom cross-calibration results. J Bone Miner Res 1994; 9:503-1513 9. Genant HK. Letter to the editor: development or formulas for standardized DXA measurements. J Bone Miner Res 1995; 9:997-998 10. Genant HK, Engelke K, Fuerst T, et al. Noninvasive assessment of bone mineral and structure: state of the art. J Bone Miner Res 1996; 11:707-730 11. Genant HK, Guglielmi G, Jergas M. Bone densitometry and osteoporosis. New York: Springer-Verlag,1997 12. He Y-F, Ross PD, Davis JW, et al. When should bone mass measurements be repeated? Calcif Tissue Int 1994; 55:2-248 13. Hodgson SF, Johnston CC Jr. Introduction to guidelines: AACE clinical practice guidelines for the prevention and treatment of postmenopausal osteoporosis. Endocr Pract 1996; 2:155-171 14. Jaovisidha S, Sartoris DJ, Martin EM, et al. Influence or spondylopathy on bone densitometry using dual energy X-ray absorptiometry. Calcif Tissue Int 1997; 60:424-429 15. Jergas M, Genant HK. Spinal and femoral DXA for the assessment of spinal osteoporosis. Calcif Tissue Int 1997; 61:351-357. 16. Johnston CC Jr, Slemenda CW, Melton U III. Clinical use of bone densitometry. N Engl J Med 1991; 324: 1105 - 1109 17. Kanis JA, Delmas P, Burckhardt P, et al. Position paper: guidelines for diagnosis and management of osteoporosis. Osteoporosis Int 199; 7:390-406 18. Kanis JA, Melton U III, Christiansen C, et al. The diagnosis of osteoporosis. J Bone Miner Res 1994;9.1137-1141 19. Lai D, Rencken M, Drinkwater B et al. Site of bone density measurement may affect therapy decision. Calcif Tissue Int 1993; 53: 225-228 20. Lentle BC. Bone densitometry: does the emperor have clothes? Canad Med Ass J 1998; 159: 1261-1264 21. LoCascia V, Bonnucci E, Imbimbo B, et al. Bone loss in response to long-term glucocorticoid therapy. J Bone Miner Res 1990; 8:39-51 22. Lyles KW, Gold DT, Shipp KM, et al. Association of osteoporotic vertebral compression fractures with impaired functional status. Am J Med 1993; 94:595-601 23. Massie A, Reid DM, Porter RW. Screening for osteoporosis: comparison between dual energy x-ray absorptiometry and broadband ultrasound attenuation in 1000 Menopausal women. Osteoporosis Int 1993; 3:107-110 24. Melton L, Atkinson E, O=Fallon W, et al. Long-term fracture prediction by bone mineral assessed at different sites. J Bone Miner Res 1993; 6:1227-1233 25. Melton U, III Osteoporosis: a worldwide problem. In: Proceedings of the third international symposium on osteoporosis, Washington, DC: Osteoporosis Foundation/National Institutes of Health, 1994:23 26. Melton U, Kan SH, Wahner HW, et al. Lifetime fracture risk: an approach to hip fracture risk assessment based on bone mineral density and age. J Clin Epidemiol 1988; 41:985-994 27. Meunier PJ. Osteoporosis: diagnosis and management. London: Martin Dunitz 1997 28. Miller PD, Bonnick SL, Rosen CJ. Consensus of an international panel on the clinical utility of bone mass measurements in the detection of low bone mass in the adult population. Calcif Tissue Int 1996: 58;207-214 29. Mundy GR. Bone remodeling and its disorders, 2nd ed. London: Martin Dunitz, 1997 30. National Osteoporosis Foundation Advisory Board. Physicians resource manual on osteoporosis. Washington, DC: National Osteoporosis Foundation, 1994:7 31. Pocock NA, Noakes KA, Griffiths M, et al. A comparison of longitudinal measurements in the spine and proximal femur using lunar and hologic instruments. J Bone Miner res 1997; 12:2113-211832 Pouilles JM, Ribot C, Termollieres F, et al. Risk factors of vertebral osteoporosis, Results of a study of 2279 women referred to a menopause clinic. Rev Rheum Mal Osteo Articularies 1991; 58:96-101 33. Pouilles JM, Tremollieres R, Ribot C, et al. Spine and femur densitometry at the menopause: are both sites necessary in the assessment of the risk of osteoporosis? Calcif Tissue Int 1993; 52:344-347 34. Rand T, Seidl G, Kainberger F, et al. Impact of spinal degenerative changes on the evaluation of bone mineral density with dual energy X-ray absorptiometry (DXA). Calcif Tissue Int 1997; 60;430-433 35. Reid JR, Evans MC, Wattie DJ, et al. Bone mineral density of the proximal femur and lumbar spine in glucocorticoid-treated asthmatic patients. Osteoporosis Int 1992; 2:103405 36. Rizzoli R, Slogman D, Bonjour JP. The role of dual energy X-ray absorptiometry of lumbar spine and proximal femur in the diagnosis and follow-up of osteoporosis. Am J Med 1995; 98(2A):33-36S 37. Rosen CJ. Osteoporosis: diagnostic and therapeutic principles. New Jersey: Humana Press 1997 38. Ross PD, Davis JW, Epstein RS, et al. Pre-existing fractures and bone mass predict vertebral fracture incidence in women. Ann Intern Med 1991; 114:919-923 39. Ross PD, Genant HK, Davis JW, et al. Predicting vertebral fracture incidence from prevalent fractures and bone density among non-black, osteoporotic women. Osteoporosis Int 1993; 3:120-126 40. Rubin SM, Cummings SR. Results of bone densitometry affect women's decisions about taking measures to prevent fractures. Ann Intern Med 1992; 116:990-995 41. Staron RB, Greenspan R, Miller TT et al. Computerized bone densitometric analysis: Operator-dependent errors. Radiology 1999; 211:467-470. 42. Sturtridge W, Lentle B, Hanley DA. Prevention and management of osteoporosis: consensus statements from the Scientific Advisory Board of the Osteoporosis Society of Canada. The use of bone density measurement in the diagnosis and management of osteoporosis. Can Med Assoc J 1996; 55:924929 43. Verheij LF, Blokland JAK, Papapoulos SE, et al. Optimization of follow-up measurements of bone mass. J Nucl Med 1992; 33:1406-1410 44. Wahner HW, Fogelman I. Clinical bone density. London: Martin Dunitz 1994 45. Wahner HW, Fogelman I. The evaluation of osteoporosis: dual energy x-ray absorptiometry in clinical practice. London: Martin Dunitz, 199845. The WHO Study Group. Assessment of fracture risk and its applications to screening for postmenopausal osteoporosis. Switzerland: World Health Organization, 1994 APPENDIX- GLOSSARY
(Reprinted, in part, from Rosen CJ, Osteoporosis: diagnostic and therapeutic principles. Humana Press,
Totowa, NJ, 1996:287-290)
Alendronate: A third generation bisphosphonate with an amino-terminal substitution of the bisphisphonate
sleton. Its brand name is Fosomax,
Anterior wedge: A type of fracture where the interior portion of the vertebral spine is collapsed in a wedge-
shaped appearance.
Bone mineral density (BMD): The mineral content of bone divided by its volume when measured by QCT
and divided by the projected area when measured by DXA. The former measurements should be reported in
mg mm-3 . Measurements made by DXA. (or other methods) are reported in g cm-2, which is representative
of areal bone density. BMD is reported for most areas of the body as spine BMD, hip BMD, total body BMD,
wrist BMD, and so on,
Bone remodeling: The physiologic process whereby bone is resorbed and then reformed. This process
provides a constant calcium source to the body and keeps the skeleton elastic enough to serve its structural
functions. In general, there is no net change in bone mass with physiologic remodeling (resorption formation),
in contrast to modeling where scalloping of bone and addition of new bone is often a characteristic of the
growing skeleton.
Dual-photon absorptiometry: An older method for measuring bone density using a radioactive source
(Gd153). It produces two photons of differing energies used to determine bone density; this application has
been surpassed by more efficient and less costly DXA machines in which X-rays of two energies can be used to
measure bone density.
DXA: Dual X-ray absorptiometry (also referred to as dual energy x-ray absorptiometry, DEXA); it uses a
conventional X-ray tube to measure density. It is a precise and accurate tool for measuring BMD.
HRT: Hormone replacement therapy, usually implying estrogen, with or without progesterone medication used
in post-menopausal women.
Kyphosis: An abnormal condition of the vertebral column characterized by increased convexity in the
curvature of the thoracic spine as viewed from the side. Kyphosis is often associated with osteoporotic thoracic
compression fractures although uncommonly it can be caused by tuberculosis or Tickets. In lay terms this may
be described as a ADowager hump or Astoop
Osteoblast: The bone cell that is responsible for bone formation. This cell type is derived from rnesenchyrnal
stem cells, which can then differentiate into adipocytes or stromal cells. Stromal cells eventually can become
osteoblast through several differentiation steps. The osteoblast can produce collagen products and participates
in the mineralization process as well as orchestrating osteoclastic bone resorption.
Osteoclast: The bone cell responsible for bone resorption. This cell type is derived from a
monocytemacrophage precursor, and under the influence or 1,25 dihydroxyvitamin D- certain colony-
stimulating factors, and interleukins can differentiate into a mature osteoclast able to secrete protons and
resorb bone.
Osteogenesis imperfecta (0I): A genetic disorder involving detective development of the connective tissue.
It is inherited as an autosomal dominant trait end is characterized by abnormally brittle and fragile bones that
are easily fractured by the slightest trauma. It can be present in one of several different phenotypes (a pure
form, a mixed form, or a late onset type) and is associated with translucent skin, hyperextensibility of
ligaments, hypoplasia of teeth, epistaxis, easy bruisability, blue sclerae, and hearing loss. Various mutations in
the genetic marker for type I collagen are responsible for the abnormalities associated with this condition.
Osteomalacia: Strictly defined as an abnormal condition of lamellar bone characterized by a loss of
calcification of the matrix, resulting in softening of the bone, accompanied by weakness, fracture, pain,
anorexia, and weight loss. In contrast to osteoporosis (reduction in bone mass) the bone mineral density is
usually normal or only slightly reduced. The disorder is due to a defect in mineralization, leading to
accumulation of unmineralized osteoid tissue. Although vitamin D deficiency (acquired or inherited) is the most
frequent cause of osteomalacia, other conditions are associated with osteomalacia including various genetic
disorders. Osteomalacia can co-exist with osteoporosis, especially in elderly people with dietary vitamin D
deficiency.
Osteopenia: An early definition was a reduction in bone mass noted on radiographs. Now osteopenia has
been defined in terms or bone mineral density by the WHO (see below).
Osteopetrosis: An inherited disorder characterized by a generalized increase in bone density but increased
bone fragility, almost always related to a defect in bone resorption. In its most severe form, it is inherited as
an autosomal recessive disease with almost complete obliteration of the marrow cavity, resulting in anemia
and marked deformities. The defect in this disorder occurs at the level or the osteoclast.
Osteoporosis: Osteoporosis has been defined as a chronic progressive disease characterized by low bone
mass and microarchitectural deterioration of bone tissue, which leads to bone fragility and a consequent
increase in fracture risk. The WHO has defined osteoporosis for epidemiological purposes in terms of bone
mineral density (BMD) as a BMD more than 2.5 S.D. below young normal CT-score <-2.5 (see below).
Peak bone mass: The time when bone acquisition is complete and bone mass is at its optimal point,
occurring in normal persons in the second or third decade.
Pyrophosphates (including disphosphonates): Naturally occurring compounds with a P-O-P structure. This
class of compounds serve as substrates for pyrophosphatases also found in nature and especially in the
skeleton. Pyrophosphates have a strong chemical affinity for calcium.
Quantitative computed tomography (QCT): Quantitative computed tomography measurements of true
bone density (mineral/volume) are usually performed in the spine or wrist at which site it may be qualified as
peripheral QCT (pQCT).
Quantitative ultrasonometry (QUS): Quantitative measurement of bone properties obtained by transmitted
ultrasound energy, often at the calcaneus. The findings may be reported in tenons of broadband ultrasound
attenuation (BUA), speed of sound (SOS), and a non-standardized mathematical combination of two called
Astiffness or the quantitative ultrasound index (QUI). Increasing evidence suggests that QUS may be used in
predicting fracture risk.
Radiographic absorptiometry (RA): A technique involving digitalization and computed analysis of
radiographs including a standardized wedge used to measure bone density. Accuracy and precision are
reported to be excellent, but outcome studies are lacking, More recently a machine has been marketed which
automates this analysis obviating the need to send films to a center for analysis.
RLFP (remaining lifetime fracture probability): This is a value based on meta-analyses of available data.
It attempts to relate age, life span, and BMD to predict potential future fracture risk. Measurement of
individual RLFP for a particular patient can be determined at http:\\www.medsurf.com.
Single photon absorptiometry (SPA): A technique largely superseded by DXA (q.v.). A single-energy
radiation source is used to determine bone at the distal radius and ulna. In such machines the radiation source
was either iodine-125 or americium-241.
T-scores: Units of standard deviation from the mean for BMD compared with the presumed peak bone mass
in given individual. A T-score value (-5 to +5) is reported on most if not all densitometers at the time of bone
density acquisition. (See the definitions of osteopenia and osteoporosis above).
WHO classifications of osteopenia and osteoporosis: Osteopenia and osteoporosis have been defined for
epidemiological purposes in menopausal women by a Working Group or the World Health Organization in terms
of bone density (i.e. before fracturing necessarily occurs) as follows (9):
Normal: A value for BMD or bone mineral content (BMC) within 1 SD (1 T score) of the young adult reference
mean.
Low bone mass (osteopenia): A value for BMD or BMC more than 1 SD (<1,0T) below the Young adult
mean but less than 2.5 SD (<2.ST) below this value.
Osteoporoa: A value for BMD or BMC 2.5 SD or more (<2.5T) below the young adult mean.
Severe (established)osteoporosis: A value for BMD or BMC 2.5 SD or more below the young adult mean in
the presence of one more fragility fractures.
Z-scores: Units of standard deviation form the mean represented by age, sex and height-matched controls. Z-
scores tend to be higher than T-scores in a given individual and may underestimate the true extent of
osteoporosis and fracture risk, since aging itself is associated with a significant reduction in BMD. It is possible
to have a low T-score and still have a normal Z-score if the person being measured is elderly. Furthermore, a
normal Z-score does not protect the individual from a future hip fracture.

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