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

Transcription

1 Standard for Performance of Adult Dual - or Single-Energy X-Ray Absorptiometry (DXA/pDXA/SXA) Last update: Modified by Brian C. Lentle, MD, after principle drafter William T. Thorwarth, Jr., MD Each standard, representing a policy statement by the Canadian Association of Radiologists, has undergone a thorough consensus process. The standards recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques as described in each document. I. INTRODUCTION Bone densitometry whether by single (SXA) or dual x-ray absorptiometry (DXA) is a clinically proven and accurate 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 monitoring patient response to therapy for these conditions. 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 patient's 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 oopherectomy). 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 42years); 5. A current gastrointestinal malabsorption disorder; 6. Smoking history (> 1 pack per day x 5 years or more); 7. 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 which could alter BMD, such as: 1. Primary hyperparathyroidism 2. Primary hypogonadism 3. Hyperthyroidism 4. Cushing's disease 5. Chronic renal failure

2 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. H. A peripheral DXA normal examination (T >-1), since this finding at a single site does not preclude an abnormal BMD at other sites. Thus in patients at high risk for osteoporosis, additional testing is recommended. 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 of the measurement; and 5. Both extreme obesity or extremely low BMD may compromise measurements and the capacity to attain accurate and precise measurements. 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 of the procedure should be considered before proceeding with the study. IV. QUALIFICATIONS AND RESPONSIBILITIES OF PERSONNEL A. Physician 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 equivalent foreign Radiologist qualifications if the Radiologist is certified by a recognized certifying body and holds a valid provincial license. It is recommended that physicians responsible for densitometry programmes seek certification by the International Society for Clinical Densitometry or any equivalent competency training in BMD. Also acceptable are foreign Specialist qualifications if the Radiologist so qualified holds an appointment in Radiology with a Canadian University WITH THE SAME CONSTRAINTS. B. Technologist The technologist shall be certified in radiographic or nuclear medicine technology by the Canadian Association of Medical Radiation Technology or be certified by an equivalent licensing body recognized by the CAMRT and comply with that Association=s requirements for continuing education. Under the overall supervision of the radiologists, the technologist will have the responsibility for patient comfort and safety, for examination preparation and performance, and for image technical evaluation and quality and applicable quality assurance. The training of technologists engaged in specialty activities shall meet with applicable and valid national and provincial specialty qualifications.

3 Continued education of technologists is encouraged by the C.A.M.R.T. and should meet pertinent provincial regulations. 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. Documented formal training in the use of the DXA equipment including performance of all manufacturer-specified quality assurance (QA) procedures is required. The technologist must read, be familiar with, and have accessible, the manufacturer's operator manual for the specific scanner model being used. If plain radiographic images are performed to correlate with DXA studies, the technologist's qualifications must be appropriate. The standard for technologist competance shall be completion of the International Society for Clinical Densitometry certification course in bone mineral densitometry. 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 III. 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 a 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. 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. 3. Previous examinations on a device from another manufacturer with results reported in standardized units. Repeat examinations should ideally be done at the same time of the year as there may be seasonal fluctuations in BMD.

4 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 settings 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 followup 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 which can accurately and precisely measure bone density using dualenergy x-ray absorptiometry. The equipment should provide the following: 1. 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. 2. A phantom or other standard must be provided in order to evaluate the accuracy, precision and linearity of response of BMD measurement. 3. A permanent recording of labeled images of the anatomic site measured and measurement results for patient records. 4. Precision error of 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 of therapy or progression of disease. The importance of DXA quality control cannot be overstated. A. Quality control procedures should be performed and permanently recorded by a trained technologist. These procedures are generally required at least 3 days a week and always before the first patient measurement of the day. They should be interpreted immediately upon completion according to the guidelines provided by the manufacturer to ensure proper system performance. 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 qualitycontrol 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. IX. QUALITY CONTROL AND IMPROVEMENT, SAFETY, INFECTION CONTROL, AND PATIENT EDUCATION CONCERNS

5 Policies and procedures related to quality, patient education, infection control and safety should be developed and implemented in accordance with the ACR Policy on Quality Control and Improvement, Safety, Infection Control and Patient Education Converns appearing elsewhere in this publication. 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: Blake GM, Gluer CC, Fogelman I. Bone densitometry: current status and future prospects. Brit J Radiol 1997; S Cooper C, Atkinson EJ, Jacobsen SJ, et al. Population-based study of survival after osteoporotic fracture. Am J Epidemiol. 1993; 137: Cummings SR, Black DM, Nevitt MC, et al. Bone density at various sites for prediction of fractures. Lancet. 1993; 341: 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: S1-S88 6. Fogelman I, 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: 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: Genant HK. Letter to the editor: development of formulas for standardized DXA measurements. J Bone Miner Res 1995; 9: 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: Genant HK, Guglielmi G, Jergas M. Bone densitometry and osteoporosis. New York: Springer- Verlag, He Y-F, Ross PD, Davis JW, et al. When should bone mass measurements be repeated? Calcif Tissue Int 1994; 55:2~ Hodgson SF, Johnston CC Jr. Introduction to guidelines: AACE clinical practice guidelines for the prevention and treatment of postmenopausal osteoporosis. Endocr Pract 1996; 2: Jaovisidha S, Sartoris DJ, Martin EM, et al. Influence of spondylopathy on bone densitometry using dual energy X-ray absorptiometry. Calcif Tissue Int 1997; 60: Jergas M, Genant HK. Spinal and femoral DXA for the assessment of spinal osteoporosis. Calcif Tissue Int 1997; 61: Johnston CC Jr, Slemenda CW, Melton U III. Clinical use of bone densitometry. N Engl J Med 1991; 324: Kanis JA, Delmas P, Burckhardt P, et al. Position paper: guidelines for diagnosis and management of osteoporosis. Osteoporosis Int 1997; 7: Kanis JA, Melton U III, Christiansen C, et al. The diagnosis of osteoporosis. J Bone Miner Res 1994; 9: Lai D, Rencken M, Drinkwater B et al. Site of bone density measurement may affect therapy decision. Calcif Tissue Int 1993; 53: Lentle BC. Bone densitometry: does the emperor have clothes? Canad Med Ass J 1998; 159: LoCascia V, Bonnucci E, Imbimbo B, et al. Bone loss in response to long-term glucocorticoid therapy. J Bone Miner Res 1990; 8: Lyles KW, Gold DT, Shipp KM, et al. Association of osteoporotic vertebral compression fractures with impaired functional status. Am J Med 1993; 94: Massie A, Reid DM, Porter RW. Screening for osteoporosis: comparison between dual energy x-ray absorptiometry and broadband ultrasound attenuation in 1000 perimenopausal women. Osteoporosis Int 1993; 3: 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; 8: 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: 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: Meunier PJ. Osteoporosis: diagnosis and management. London: Martin Dunitz 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: Mundy GR. Bone remodeling and its disorders, 2nd ed. London: Martin Dunitz, 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

6 and proximal femur using lunar and hologic instruments. J Bone Miner Res 1997; 12: 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: 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: 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: Reid IR, 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: Rizzoli R, Slosman 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 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: 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: Rubin SM, Cummings SR. Results of bone densitometry affect women's decisions about taking measures to prevent fractures. Ann Intern Med 1992; 116: 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: erheij LF, Blokland JAK, Papapoulos SE, et al. Optimization of follow-up measurements of bone mass. J Nud Med 1992; 33: Wahner HW, Fogelman I. Clinical bone density. London: Martin Dunitz Wahner HW, Fogelman I. The evaluation of osteoporosis: dual energy x-ray absorptiometry in clinical practice. London: Martin Dunitz, The WHO Study Group. Assessment of fracture risk and its applications to screening for postmenopausal osteoporosis. Switzerland: World Health Organization, Chesnut CH III. Osteoporosis, an underdiagnosed disease. J Am Med Ass 2991; 286: APPENDIX: GLOSSARY (Reprinted, in part, from Rosen CJ, Osteoporosis: diagnostic and therapeutic principles. Humana press, Totowa, NJ, 1996: ) Alendronate: A third generation bisphosphonate with an amino-terminal substitution of the bisphisphonate sleton. Its brand name is Fosomax7.. Anterior wedge: A type of fracture where the anterior portion of the vertebral spine is collapsed in a wedgeshaped appearance. Bone mineral density (BMD): The mineral content of bone divided by its volume when measured by QCT. Such measurements should be reported in mg.mm.-3 Measurements made by DXA (or other methods) are reported in g/cm2, 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.

7 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 rickets. 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 mesenchymal stem cells, which can then differentiate into adipocytes or stromal cells. Stromal cells eventually can become osteoblasts 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 of 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 defective development of the connective tissue. It is inherited as an autosomal dominant trait and 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 eldery 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 of 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 of 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 epidemiologtical purposes in terms of bone mineral density (BMD) as a BMD more than 2.5 S.D. below young normal (T-score <-2.5) (see below). Peak bone mass: The time when bone acquisition is complete and bone mass is at its optimal point, occuring in normal persons in the second or third decade. Pyrophosphates (including bisphosphonates: Naturally occurring compounds with a P-0-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 terms 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.

8 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 centre 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/ 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 individuals. 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 of 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 (<2.5T) below this value. Osteoporosis: 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 or more fragility fractures. Z-scores: Units of standard deviation from 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 fracfture 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.