In order to understand the basics of Bone Mineral Density (BMD), it is helpful to simplify it for those of us without a medical or scientific background. Having said this, it is important to say that much can be lost in the simplification. For a thorough understanding, one must look at the quality of the research, the statistical underpinnings, and much more. BMD testing is performed for a variety of reasons. Here we seek to understand its use in relationship to osteoporosis.
Bone Densitometry, also known as Bone Mineral Density, is non-invasive testing that draws on three major technologies:
- Dual-Energy X-ray Absorption (DXA – also abbreviated as DEXA)
- Quantitative UltraSound (QUS)
- Quantitative Computerized Tomography (QCT)
In BMD testing, small amounts of radiation are used to determine the bone density. Wikipedia provides the following brief explanation of DXA: “Two X-ray beams with differing energy levels are aimed at the patient’s bones. When soft tissue absorption is subtracted out, the BMD can be determined from the absorption of each beam by bone.”1 Because DXA uses area and not volume to calculate the BMD, it is not an accurate measurement of true bone mineral density.
BMD is site specific. That is, an individual’s forearm measurement could be low while the lumbar spine could be high. Common measurement sites for BMD include the heel (calcaneus), the wrist/forearm (radius/ulnar), the lumbar spine and the hip (femoral neck). The DXA scan can also be used to measure the whole body.
Measurements of central body sites are considered more sensitive than measurements at the peripheral sites because changes occur more slowly in peripheral bones. Central areas such as the hip and lumbar, contain more spongy or trabecular bone. Peripheral sites may appear normal when central sites are already in decline.
The Gold Standard
The medical and research community often uses the term “gold standard” to indicate what is the best test, tool or application for assessing data parameters. In the case of BMD testing, the technological gold standard is the DXA scan. While the QUS and QCT also have positive data behind them in predicting hip fracture risk, the DXA scan is found to be have the combination of widely available in the United States, lower radiation that QCT and more accurate than the QUS.2 The World Health Organization’s recommendations were made for the DXA scan technology and central body site measurements. The guidelines were not established for use with other technologies or peripheral body sites.
The gold standard in measurement sites is the hip. While all of the listed sites-hip, lumbar, wrist, and heel, have some correlation to hip fracture risk, the strongest correlation is with hip bone density.3 Because hip fractures have such significant health consequences, reducing this fracture risk has been a primary goal. More recently, vertebral fractures have become of increasing concern due to their impact on quality of life.
The International Society for Clinical Densitometry (ISCD) recommends use of the lowest T-score between four sites (lumbar spine, femoral neck, trochanter and total hip). However, a recent study found no significant increase in predictive value when selecting the lowest score between the lumbar spine and the hip.4 Interpretation would conclude that either of these tests alone may serve as a good basis for traditional medical treatment plans.
Understanding the BMD Score
BMD is the ratio of the measured bone mineral content (BMC) in grams divided by the measured two-dimensional projected area in cm2 of the bone(s) being measured; thus the units of BMD are g/cm2. 5 This BMD score is then used to calculate two scores. One is the T-score and the other the Z-score.
T-Score: This calculation compares the patient’s BMD score against normal, 30 year olds and expresses the difference between the two as standard deviation points. If the patient’s score is better than the reference group, the score will be a positive number. If the patient’s score is worse, it will be a negative number. For example, a -2.0 means the measurement is 2.0 SD points below normal 30 year olds.
Z-Score: This calculation compares the patient’s BMD score against others of a similar age range, race and gender.
Treatment for osteoporosis is usually driven by the T-score:
- Normal – A BMD within 1 standard deviation (SD) point of a young adult reference mean.
- Osteopenia (low bone mass) – more than 1 but less than 2.5 SD below the young adult reference mean
- Osteoporosis – A value of 2.5 SD or more below the young adult reference mean.
- Severe or Established osteoporosis – more than 2.5 SD below the young adult mean with a fragility fracture
For more information on Osteoporosis as a diagnosis or precautions, see our articles: Osteoporosis and The Birth of Osteoporosis as Diagnoses.
Limitations of the BMD Test
- The technology and calculation methods of BMD are sometimes misunderstood by researchers. The test is often used to validate interventions and yield apparently good results, but when this research is reviewed by more qualified researchers, the analysis is shown to be inaccurate. In a literature review of research on nutrition and prevention of osteoporosis, it was found that the majority of research (done to date 2004) relating to nutrition had not been adjusted appropriately and therefore valid data connecting nutrition and bone health is minimally available.6
- While decreases in BMD are indications of increased fracture risk, increases in BMD do not necessarily reflect a decrease in fracture risk.
- BMD testing is not an appropriate test for measuring improvements in BMD in short‐term intervals. Once an intervention is made, it is recommended that at least one year transpires before re‐measuring.
- BMD is race and gender influenced. What is a low BMD for a male is different than that of a female. Differences between races can be marked. Reported T‐scores routinely account for gender, but are inconsistent (by manufacturer) for race. Z‐scores do routinely account for age, gender and race. However, since T‐score is the primary driver of treatment patterns, it is important that the physician knows whether race has been considered in calculating the T‐score.3
- A low T‐score does not identify the reason for the poor BMC.
- When looking for changes in BMD after interventions (such as medication or exercise), it is recommended that the actual BMD measurements be compared and not the T‐Scores.
- Different technologies and manufacturers produce variances. Results cannot be directly compared between manufacturers without adjusting for differences in methodology and regions of interest when performing the test.3
- Different reference databases used in the T‐ score calculations means variations in results. The hip is the only skeletal site with a standardized reference database used by all manufacturers.
Does low BMD mean increased Fracture Risk?
The short answer is yes. BMD as a means of understanding fracture risk is typically recommended for women over 65, men over 70 and at earlier ages for high-risk populations.3
While bone mineral content accounts for only 60-80% of bone strength and resiliency, fracture risk has been shown to double with every standard deviation decrease in BMD for both men and women.7 According to a study on the 10 year fracture probability, it was found that at a T-score of -2.5 the risk was slightly under 30%; a T-score of -2.0 at approximately 15%. Fractures included in this study were hip, distal forearm, proximal humerus, and symptomatic vertebral fracture.8
Studies have shown that non-BMD risk factors play a significant and independent role in the prediction of hip fracture. The EPIDOS study reported age, gait speed, and neuromuscular and visual impairments as independent predictors.9, 10 Other studies have found that combining BMD with clinical risk factors increases accuracy in the prediction of hip or osteoporotic fractures than using either BMD or clinical risk factors alone.11
Some clinicians point out that BMD is a poor test for predicting risk at the individual level. Lawrence Raisz of the University of Connecticut Health Center in Framington says DXA falls short of providing that kind of accuracy. Nonetheless, many researchers point out it’s a better predictor than is cholesterol level is for heart disease.12
The understanding of osteoporosis, fracture risk, BMD and treatment continues to undergo significant change. As individuals, it is important that we each do our own research in obtaining the best BMD measurements we can. For repeat tests it is particularly important to ask about changes in equipment, technology, methodology and measurement sites. As consumers and somatic educators, understanding the complexity of obtaining and using a BMD result appropriately can help us better evaluate and contribute to research.
Cynthia M. Allen is a Guild Certified Feldenkrais Practitionercm and Bones for Life® Teacher/Trainer. She has a private practice in Cincinnati, Ohio. You can find out more about her and her practice at www.futurelifenow.com Allen can be reached by email at CynthiaAllen@FutureLifeNow.com. She offers certification and continuing education programs through www.integrativelearningcenter.org.
- Wikipedia contributors, ʺDual energy X‐ray absorptiometry,ʺ Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Dual_energy_X‐ray_absorptiometry&oldid=151898582 (accessed November 2, 2007).
- Diagnosis of Osteoporosis with Bone Mineral Density Measurement. http://www.imaginis.com/osteoporosis/osteo_diagnose_print.asp (accessed November 18, 2007)
- Cummings SR, Black DM, et al. Bone density at various sites for prediction of hip fractures. The Study of Osteoporotic Fractures Research Group. Lancet 1992 Ap 10;341(7750):962.3
- Kanis JA, Johnell O, et al. The use of multiple sites for the diagnosis of osteoporosis. Osteoporos Int. 2006;17(4):527‐34. Epub 2006 Jan 10.
- Watts, Nelson B. Fundamentals and pitfalls of bone densitometry using dual‐energy X‐ray absorptiometry (DXA). Osteoporos Int. 2004 Nov;15(11):847‐54. Epub 2004 Aug 21.
- Prentice A, Diet, nutrition and the prevention of osteoporosis. Public Health Nutrition. 2004; 7(1A),227‐243
- Johnell O, Kanis JA, et al. Predictive value of BMD for hip and other fractures. J Bone Miner Res. 2005 Jul;20(7):1185‐94. Epub 2005 Mar 7.
- Abrahamsen B, Vestergaard P, et al. Ten‐year absolute risk of osteoporotic fractures according to BMD T score at menopause: the Danish Osteoporosis Prevention Study. J Bone Miner Res. 2006 May;21(5):796‐800.
- Dargent‐Molina P, Favier F, et al. Fall‐related factors and risk of hip fracture: the EPIDOS prospective study. Lancet. 1996 Jul 20;348(9021):145‐9.
- Dargent‐Molina P, Piault S, Bréart G; EPIDOS study group. A triage strategy based on clinical risk factors for selecting elderly women for treatment or bone densitometry: the EPIDOS prospective study. Osteoporos Int. 2005 Aug;16(8):898‐906. Epub 2004 Nov 27.
- Kanis JA, Oden A, et al. The use of clinical risk factors enhances the performance of BMD in the prediction of hip and osteoporotic fractures in men and women. Osteoporos Int. 2007 Aug;18(8):1033‐46. Epub 2007 Feb 24.
- Stokstad, E Bone Quality Fills Holes in Fracture Risk, Science 2005 Jun10(308):1580‐1