T-Score/Z-Score Mythology

1: J Clin Densitom. 2009 Jan-Mar;12(1):11-6.Click here to read Links

Dual-energy X-ray absorptiometry diagnostic discordance between Z-scores and T-scores in young adults.

Department of Rheumatology, Merlin Park University Hospital, Galway, Ireland. [email protected]

Diagnostic criteria for postmenopausal osteoporosis using central dual-energy X-ray absorptiometry (DXA) T-scores have been widely accepted. The validity of these criteria for other populations, including premenopausal women and young men, has not been established. The International Society for Clinical Densitometry (ISCD) recommends using DXA Z-scores, not T-scores, for diagnosis in premenopausal women and men aged 20-49 yr, though studies supporting this position have not been published. We examined diagnostic agreement between DXA-generated T-scores and Z-scores in a cohort of men and women aged 20-49 yr, using 1994 World Health Organization and 2005 ISCD DXA criteria. Four thousand two hundred and seventy-five unique subjects were available for analysis. The agreement between DXA T-scores and Z-scores was moderate (Cohen's kappa: 0.53-0.75). The use of Z-scores resulted in significantly fewer (McNemar's p<0.001) subjects diagnosed with "osteopenia," "low bone mass for age," or "osteoporosis." Thirty-nine percent of Hologic (Hologic, Inc., Bedford, MA) subjects and 30% of Lunar (GE Lunar, GE Madison, WI) subjects diagnosed with "osteoporosis" by T-score were reclassified as either "normal" or "osteopenia" when their Z-score was used. Substitution of DXA Z-scores for T-scores results in significant diagnostic disagreement and significantly fewer persons being diagnosed with low bone mineral density.


1: J Clin Densitom. 2007 Oct-Dec;10(4):351-8. Epub 2007 Jul 26.Click here to read Links

DXA-generated Z-scores and T-scores may differ substantially and significantly in young adults.

Center for Osteoporosis and Metabolic Bone Diseases, Cleveland Clinic, Cleveland, OH, USA. [email protected]

Central dual-energy X-ray absorptiometry (DXA) is the gold standard for non-invasive measurement of bone mineral density (BMD). Using this value and subject demographics, DXA software calculates T-scores and Z-scores. Professional society guidelines for the management of osteoporosis are based on T-scores and Z-scores, rather than on the actual BMD value. Although one expects T-scores and Z-scores to be very similar in young men and women for any given BMD measurement, little literature exists on this issue. Our clinical experience shows that some younger adult individuals (premenopausal women and men younger than 50 yr) have larger than expected difference between their DXA T-score and Z-score. This cross-sectional study evaluates the extent of this discordance between Z-scores and T-scores in a sample of 4275 men and women aged 20-49 yr. All subjects were scanned by central DXA using equipment manufactured by GE Lunar, GE, Madison, WI, or Hologic, Inc., Bedford, MA. Significant differences between Z-scores and T-scores were seen within individuals at the lumbar spine, total hip, femoral neck, and trochanter (p value<0.001) for both DXA systems. Although these differences were less than half a standard deviation (SD) in most instances, the magnitude of difference was substantial at times, being 1 or more SD in up to 11% of cases (range: -1.95 to +1.54 SD). The smallest differences were seen at the total hip and the largest differences were seen at the femoral neck for both technologies. This is in part because there is no single standard Z-score definition, resulting in different methods of calculation across, and even within, DXA manufacturers. Standardization of Z-score definition and method of calculation is indicated. DXA Z-scores should be interpreted with caution in men and women aged 20-50 yr.

1: Bone. 2008 Mar;42(3):467-75. Epub 2007 Nov 17.Click here to read Links

A reference standard for the description of osteoporosis.

WHO Collaborating Centre for Metabolic Bone Diseases, University of Sheffield Medical School, UK. [email protected]

In 1994, the World Health Organization published diagnostic criteria for osteoporosis. Since then, many new technologies have been developed for the measurement of bone mineral at multiple skeletal sites. The information provided by each assessment will describe the clinical characteristics, fracture risk and epidemiology of osteoporosis differently. Against this background, there is a need for a reference standard for describing osteoporosis. In the absence of a true gold standard, this paper proposes that the reference standard should be based on bone mineral density (BMD) measurement made at the femoral neck with dual-energy X-ray absorptiometry (DXA). This site has been the most extensively validated, and provides a gradient of fracture risk as high as or higher than that of many other techniques. The recommended reference range is the NHANES III reference database for femoral neck measurements in women aged 20-29 years. A similar cut-off value for femoral neck BMD that is used to define osteoporosis in women can be used for the diagnosis of osteoporosis in men - namely, a value for BMD 2.5 SD or more below the average for young adult women. The adoption of DXA as a reference standard provides a platform on which the performance characteristics of less well established and new methodologies can be compared.

PMID: 18180210 [PubMed - indexed for MEDLINE]

1: Laeknabladid. 2001 Jan;87(1):15-20. Links

[Diagnosis of osteoporosis in the elderly; an overview.]

[Article in Icelandic]

Landspitali University Hospital, Fossvogi, 108 Reykjavík, Iceland. [email protected].

Measurement of bone mineral density (BMD) is the basis of the diagnosis of osteoporosis. WHO classification of osteoporosis is based on dual energy X-ray absorptiometry (DEXA) using the BMD of young women (20-30 years) as the reference value (T-score). For clinical purpose it is better to use age-matched control (Z-score) to evaluate future fracture risk in relation to other individuals of the same age. A further facility of the new bone densitometry technique is the option of vertebral morphometry, which makes it possible to assess previous vertebral fractures with similar precision as conventional X-ray. Such an assessment is of the greatest importance as patients with previous fractures and low BMD have several fold increased risk of further fractures and benefit most from medical therapy. There are errors of accuracy in all bone densitometry techniques and also in the interpretation of the data. Osteoarthritis in the lumbar spine, common in the elderly, creates false increment in BMD as measured by DEXA. For this reason the hip is the site of choice for BMD measurement in the elderly, especially as it predicts best femoral fractures, a major concern in the elderly. Quantitative computed tomography has the advantage of measuring separately cancellous and cortical bone. Ultrasound of bone (at present mostly in calcaneus) may provide new measures of bone fragility. Ultrasound has the advantage of no exposure to radiation and the equipment is portable. Although useful bone ultrasound cannot replace bone densitometry in the diagnosis and monitoring of therapy. Biochemical bone markers are not useful in the diagnosis of osteoporosis, but they can be useful in deciding on intervention and in monitoring the efficacy of treatment. Biochemistry is videly used in the differential diagnosis of secondary osteoporosis. History and physical examination are insufficient in diagnosing osteoporosis, but they are of utmost importance in finding individuals of high risk who might benefit most from undergoing bone densitometry. History and physical examination are also important in targeting other investigations to exclude secondary forms of osteoporosis. Although bone densitometry is usually necessary for the diagnosis of osteoporosis intervention by drugs should be based in addition on general assessment of the patient taking notice of other important independent risk factors for fractures.

1: J Intern Med Suppl. 1997;739:1-60. Links

Bone density measurement--a systematic review. A report from SBU, the Swedish Council on Technology Assessment in Health Care.

[No authors listed]

Because a reduction in bone density often correlates to an increased risk of fracture, bone density is usually measured in an attempt to establish the risk of fracture. The results from bone density measurement are intended to provide a potential basis for treating osteoporosis. When assessing the value of bone density measurement, the key issues concern the reliability of the various methods (i.e., how accurately they reflect bone density) and whether bone density treatment can actually prevent fracture. OSTEOPOROSIS: Humans begin to lose bone tissue as they become older. In most cases, this process is slow and gradual. Bone tissue begins to disappear when people are aged between 30 and 40 years and continues throughout life. However, bone loss varies greatly among individuals, and some elderly people show no sign of bone loss. Several factors influence both the loss of bone mass as people age and the formation of bone mass in the growing individual. The single most important factor associated with reduced bone mass is the loss of the female sex hormone (oestrogen). Tobacco smoking, lack of exercise, and low calcium levels in the diet also reduce bone density. Reduced bone density may lead to osteoporosis, which increases the risk of fracture, often affecting the vertebrae, hips and wrists. The most common direct cause of fracture, mainly among the elderly, involves falling or stumbling. Contributing factors here include diseases or medications that affect the sight, muscle strength, and balance. Osteoporosis is one of many risk factors for fractures resulting from falls. Fracture is a large and growing health problem. Each year, approximately 60,000 people in Sweden suffer some type of fracture. With an increasing percentage of elderly people in the Swedish population, it is estimated that every second woman over 50 years of age will experience fracture at some time during their remaining life. The risk in men is lower. It is essential to prevent, as far as possible, the onset of osteoporosis and other risk factors for fracture. Preventive approaches include, increased physical activity during youth when people develop their bone mass, sufficient intake of calcium in the diet among the young and old alike, and smoking cessation (or preventing people from starting to smoke). It is particularly important to treat osteoporosis effectively, or prevent osteoporosis from developing into a serious condition. This requires further research into strategies for treating osteoporosis. The various methods for measuring bone density represent an important component in such research. MEASURING BONE DENSITY: Bone density may be measured either to establish a diagnosis or to monitor changes, e.g. follow-up treatment for osteoporosis. Bone density can be estimated roughly by simply measuring height, weight, and age, but this approach has limited value for establishing the level of bone density in individuals. To a certain extent, x-ray examination can also be used to estimate the level of bone density. Special methods have been developed in recent decades for measuring bone density, and technologies for this purpose have become more widely available since the 1980s. The new methods for bone density measurement are based on either the energy/methodology used in ultrasound and magnetic resonance imaging (MRI), or on x-rays. Some methods are designed for measuring only the forearm, hip, lumbar spine, or heel bone (calcaneus), while others measure several sites in the body simultaneously. Most methods demonstrate good precision (i.e. repeated measurements yield the same results). However, to establish the bone density level reliably, methods must also be highly accurate (i.e. the values obtained by measurement must coincide with the individual's actual bone density). The accuracy of current technologies is substantially lower than their precision, so further research, technical development, and experience are required before the methods can be i

1: Wien Med Wochenschr. 2002;152(23-24):591-5. Links

Differences in distribution of T-scores and Z-scores among bone densitometry tests in postmenopausal women (a comparative study).

Division of Osteological Surgery, Derer's University Hospital and Polyclinic, Bratislava, Slovakia.

BACKGROUND: To determine the character of T-score and Z-score value distribution in individually selected methods of bone densitometry and to compare them using statistical analysis. METHODS: We examined 56 postmenopausal women with an age between 43 and 68 years with osteopenia or osteoporosis according to the WHO classification. The following measurements were made in each patient: T-score and Z-score for: 1) Stiffness index (S) of the left heel bone, USM (index). 2) Bone mineral density of the left heel bone (BMDh), DEXA (g of Ca hydroxyapatite per cm2). 3) Bone mineral density of trabecular bone of the L1 vertebra (BMDL1), QCT (mg of Ca hydroxyapatite per cm3). The densitometers used in the study were: Ultrasonometer to measure heel bone, Achilles plus LUNAR, USA; DEXA to measure heel bone, PIXI, LUNAR, USA; QCT to measure the L1 vertebra, CT, SOMATOM Plus, Siemens, Germany. Statistical analysis: Differences between measured values of T-scores (Z-scores) were evaluated by parametric or non-parametric methods of determining the 95% confidence intervals (C.I.) RESULTS: Differences between Z-score and T-score values for compared measurements were statistically significant; however, these differences were lower for Z-scores. CONCLUSIONS: Largest differences in 95% C.I., characterizing individual measurements of T-score values (in comparison with Z-scores), were found for those densitometers whose age range of the reference groups of young adults differed the most, and conversely, the smallest differences in T-score values were found when the differences between the age ranges of reference groups were smallest. The higher variation in T-score values in comparison to Z-scores is also caused by a non-standard selection of the reference groups of young adults for the QCT, PIXI and Achilles Plus densitometers used in the study. Age characteristics of the reference group for T-scores should be standardized for all types of densitometers.

1: J Bone Miner Res. 1996 Oct;11(10):1545-56. Links

Differences in bone mineral in young Asian and Caucasian Americans may reflect differences in bone size.

Department of Pediatrics, Stanford University School of Medicine, California, USA.

Bone mineral content (BMC) and areal bone mineral density (BMD) have been reported to be lower in Asian than in Caucasian adults. To determine if racial differences in bone mass are present in younger subjects and whether they reflect differences in estimated volumetric bone density or in bone size, we compared measurements of bone mineral in healthy young Asian- and Caucasian-American males and females. Bone mineral was measured at the lumbar spine (L2-L4), femoral neck (FN), and whole body (WB) by dual-energy X-ray absorptiometry (DXA) in 99 Asians (49 females, 50 males) and 103 Caucasians (54 females, 49 males) ages 9-26 years. Results were expressed as BMC, BMD, and apparent density (BMAD), an estimate of volumetric bone density that reduces the effect of bone size. Subjects were compared on the basis of chronological age as well as by Tanner stage to correct for potential differences in the timing of puberty. Habitual dietary intake and physical activity were also assessed and correlated with bone mineral. The Asian and Caucasian cohorts differed in body size, diet, and physical activity. Asian females were shorter than the Caucasian females at all stages of puberty and weighed less at pre-/early puberty (p < 0.05). Asian males were older than Caucasians at midpuberty (p < 0.01) and weighed less than the Caucasian males at pubertal maturity (p = 0.001). Asian youths also consumed less calcium and reported less weight-bearing activity. Racial differences were most apparent when comparing BMC data. Asian males had greater spine BMC at midpuberty and lower WB BMC at maturity (p < 0.05). Asian females had lower FN BMC through midpuberty and lower WB BMC in pre-/early puberty (p < 0.05). WB BMD and WB BMC/height values were significantly lower in mature Asian versus Caucasian males. No significant racial differences in BMAD were observed. Multivariate regression analysis indicated that the differences in BMD and BMAD between Asian and Caucasian subjects were largely attributable to differences in weight and pubertal stage, and, at the FN, in weight-bearing activity. Further, the explanatory variables were less strongly associated with BMAD than with BMD. In summary, no significant differences in BMD were found between Asian and Caucasian youths through midpuberty; however, WB BMD and WB BMC/height values were lower in Asian males at sexual maturity. We conclude that observed differences in bone mineral between Asians and Caucasians may be partially attributed to the smaller bone size of Asians.

1: J Bone Miner Res. 1994 Jul;9(7):1071-6. Links

Do variations in hip geometry explain differences in hip fracture risk between Japanese and white Americans?

Department of Medicine, Indiana University School of Medicine, Indianapolis.

Despite lower femoral neck bone mass, Japanese women have a substantially lower incidence of hip fracture than North American whites. Reasons for this discrepancy were sought in a study of 57 Japanese and 119 white American women aged 50-79. All women were in good health. Bone mineral content (BMC) in the femoral neck, femoral neck length (NL), femoral neck angle (theta), cross-sectional moment of inertia (CSMI), safety factor (SF), and fall index (FI) were calculated using dual x-ray absorptiometry. Height and weight were greater in Americans than in Japanese (1.62 versus 1.52 m; p < 0.0001 and 66.0 versus 49.4 kg; p < 0.0001, respectively). Mean BMC in the femoral neck and CSMI were greater in Americans than in Japanese (3.91 versus 3.02 g; p < 0.0001 and 0.99 versus 0.57 cm4; p < 0.0001, respectively). NL was longer in Americans (5.6 versus 4.4 cm; p < 0.0001) and theta was larger in Americans (130 versus 128 degrees; p < 0.01), whereas SF and FI were less in Americans than in Japanese (3.41 versus 5.12; p < 0.0001 and 1.00 versus 1.40; p < 0.0001, respectively). These results indicate that despite lower bone mass, Japanese women have lower risks of structural failure in the femoral neck, attributable primarily to shorter femoral necks and, to a lesser degree, a smaller femoral neck angle. Geometric characteristics of the femoral neck in Japanese women are associated with their lower hip fracture risk, and the measurement of proximal femoral geometry, combined with bone mass, may provide further clinical information about the risk of hip fracture.

1: Osteoporos Int. 2005 Apr;16(4):347-52. Epub 2004 Nov 23.Click here to read Links

The tale of the T-score: review and perspective.

GE Healthcare, 726 Heartland Trail, Madison, WI 53717, USA. [email protected]

The T-score is well known to anyone working in the field of bone densitometry. It is the primary output from a bone densitometry system and is most often used for diagnosis of osteoporosis and for making treatment decisions. Despite widespread acceptance of the T-score, most clinicians are unfamiliar with the historical evolution of the T-score as a clinical measure. Furthermore, evidence is mounting that the T-score is not the optimal diagnostic parameter for clinical decision making. Many additional risk factors have been reported which can be combined with bone density results to assess absolute fracture risk. This editorial provides an historical review of the T-score, followed by summary of the status of the T-score, and concludes with suggestions for the future use of the T-score in bone densitometry.


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