Radiology Section - Whole Body Dual X-Ray Absorptiometry (DEXA) to Determine Body Composition

IMPORTANT REMINDER

Regence Medical Policies are developed to provide guidance for members and providers regarding coverage in accordance with contract terms. Benefit determinations are based in all cases on the applicable contract language. To the extent there may be any conflict between the Medical Policy and contract language, the contract language takes precedence.

PLEASE NOTE: Contracts exclude from coverage, among other things, services or procedures that are considered investigational or cosmetic. Providers may bill members for services or procedures that are considered investigational or cosmetic. Providers are encouraged to inform members before rendering such services that the members are likely to be financially responsible for the cost of these services.

Description

Measurements of body composition have been used to study how lean body mass and body fat change during health and disease and have provided a research tool to study the metabolic effects of aging, obesity, and various wasting conditions such as occurs with AIDS or post-bariatric surgery. A variety of techniques have been researched, including most commonly, anthropomorphic measures, bioelectrical impedance, and dual X-ray absorptiometry (DEXA) scans. All of these techniques are based in part on assumptions regarding the distribution of different body compartments and their density, and all rely on formulas to convert the measured parameter into an estimate of body composition. Therefore, all techniques will introduce variation based on how the underlying assumptions and formulas apply to different populations of subjects, e.g., different age groups, ethnicities, or underlying conditions. Anthropomorphic, bioimpedance, and DEXA techniques are briefly reviewed as follows:

Anthropomorphic Techniques

Anthropomorphic techniques for the estimation of body composition include measurements of skin-fold thickness at various sites, bone dimensions, and limb circumference. These measurements are used in various equations to predict body density and body fat. Due to its ease of use, measurements of skin-fold thickness are one of the most commonly used techniques. The technique is based on the assumption that the subcutaneous adipose layer reflects total body fat, but this association may vary with age and gender.

Bioelectrical Impedance

Bioelectrical impedance is based on the relationship between the volume of the conductor (i.e., the human body), the conductor’s length (i.e., height), the components of the conducter (i.e., fat and fat-free mass), and its impedance. Estimates of body composition are based on the assumption that the overall conductivity of the human body is closely related to lean tissue. The impedance value is then combined with anthropomorphic data to give body compartment measures. The technique involves attaching surface electrodes to various locations on the arm and foot. Alternatively, the patient can stand on pad electrodes.

Underwater Weighing

Underwater weighing (UWW) has generally been considered the reference standard for body composition studies. This technique requires the use of a specially constructed tank in which the subject is seated on a suspended chair. The subject is then submerged in the water while exhaling. While valued as a research tool, UWW is not suitable for routine clinical use. UWW is based on the assumption that the body can be divided into two compartments with constant densities, namely, adipose tissue with a density of 0.9gm/cm3 and lean body mass (muscle and bone) with a density of 1.1g/cm3. One limitation of the underlying assumption is the variability in density between muscle and bone; for example, bone has a higher density than muscle, and bone mineral density varies with age and other conditions. In addition, the density of body fat may vary, depending on the relative components of its constituents, e.g., glycerides, sterols, and glycolipids.

DEXA

While the above techniques assume two body compartments, dual energy X-ray absorptiometry can estimate three body compartments consisting of fat mass, lean body mass, and bone mass. DEXA systems use a source that generates X-rays at two energies. The differential attenuation of the two energies is used to estimate the bone mineral content and the soft tissue composition. When two X-ray energies are used, only two tissue compartments can be measured; therefore, soft tissue measurements (i.e., fat and lean body mass) can only be measured in areas where no bone is present. DEXA also has the ability to determine body composition in defined regions, such as the arms, legs, and trunk. DEXA measurements are based in part on the assumption that the hydration of fat-free mass remains constant at 73%. Hydration, however, can vary from 67%–85%, and can be variable in certain disease states. Other assumptions used to derive body composition estimates are considered proprietary by DEXA manufacturers (e.g., Lunar, Hologic, and Norland).

Note: DEXA for screening for vertebral fracture is addressed separately in Regence Medical Policy, Radiology, No. 48

Policy/Criteria

Whole body dual x-ray absorptiometry (DEXA) to determine body composition is considered investigational for all indications.

Scientific Background

Several different clinical roles for whole body DEXA scans to assess body composition have been suggested. Each clinical application requires different data for analysis.

DEXA as Reference Standard for Body Composition Assessment

In general, reference standards for diagnostic tests, often used primarily in research settings, serve to evaluate and verify the use of simpler and more convenient alternative tests that measure the same diagnostic parameter. For body composition studies, underwater weighing has been historically considered the reference standard. The emergence of DEXA as a potential new reference standard reflects its ease of use and the fact that it provides a 3-compartment model of body density, i.e., lean body mass, bone mass, and fat mass, compared to the 2-compartment model of underwater weighing. More recently, a 4-compartment model has been suggested as the reference standard, consisting of measurements of bone/mineral, protein, water, and fat. Studies to evaluate different techniques of measuring the same parameter typically consist of correlation studies that compare values between the two techniques. However, correlation studies do not provide information on which diagnostic technique more closely represents the true value. For example, a lack of correlation between DEXA and underwater weighing may reflect the lack of accuracy of underwater weighing, as opposed to any deficiency in the DEXA technique. Furthermore, two diagnostic techniques may be highly correlated but produce different values of body composition. For example, compared to underwater weighing, DEXA may identify different groups of patients as abnormal and normal.

There is extensive literature comparing DEXA to other techniques for assessing body composition, most commonly underwater weighing, bioelectrical impedance, or skin-fold thickness in different populations of patients with differing age groups, ethnicities, and underlying disorders. (2-7, 23-25) In general, these studies have shown that DEXA is highly correlated to various methods of body composition assessment. Detailed review of this extensive literature is beyond the scope of this discussion; however, it is apparent that many authors would consider a DEXA body composition study the reference standard. For example, in various research studies, the results of DEXA body composition have been included as an intermediate outcome in studies of nutrition and various metabolic disorders. (8-14)

An updated search of the MEDLINE database found that dual-energy x-ray absorptiometry continues to be used as the reference standard for whole body composition analysis in research studies. Active research areas include comparison of established clinical measures of body composition (body mass index or BMI, anthropomorphic measurements, and bioelectrical impedance analysis) with this “gold standard” and improvement of equations for more accurate clinical assessment of lean and fat body mass. Although refinement of equations may lead to closer agreement with DEXA estimates of fat mass and fat free mass, for routine clinical use BMI is considered to provide satisfactory accuracy. (15)  Regardless of whether a DEXA scan is considered the reference standard, the key consideration regarding its routine clinical use is if the results of the scan can be used in the management of the patient to improve health outcomes.

DEXA as a Diagnostic Test to Detect Abnormal Body Composition

As a single diagnostic measure, it is important to establish diagnostic cutoff points for normal and abnormal values. This is problematic, since normal values will require the development of normative databases for the different components of body composition (bone, fat, and lean mass) for different populations of patients at different ages. In terms of measuring bone mineral density, normative databases have largely focused on postmenopausal white women, and these values cannot necessarily be extrapolated to either men or to different races. DEXA determinations of bone mineral density are primarily used for fracture risk assessment in postmenopausal women and to select candidates for various pharmacological therapies to reduce fracture risk. In addition to the uncertainties of establishing normal values for other components of body composition, it also is unclear how a single measure of body composition would be used in the medical management of the patient.

DEXA as a Technique to Monitor Changes in Body Composition

Changes in body composition over time may provide useful information. The ability to detect changes is related in part to the precision of the technique, defined as the degree to which repeated measurements of the same variable give the same value. For example, DEXA measurements of bone mass are thought to have a precision error of 1%–3%, and given the slow rate of change in bone mineral density in postmenopausal women treated for osteoporosis, it is likely that DEXA scans would only be able to detect a significant change in bone mineral density in the typical patients after two years of therapy. Of course, changes in body composition are anticipated to be larger and more rapid than changes in bone mineral density in postmenopausal women; therefore, precision errors in DEXA scans become less critical in interpreting results. Many studies have used DEXA to monitor changes in body composition, and the precision is similar to that estimated for DEXA measurements of bone mineral density. While measuring changes in body composition is widely used in athletes for training purposes, it is still unclear how monitoring changes in body composition could be used in the medical management of the patient.

DEXA measurements of body mass continue to be included as outcomes measures in various trials, frequently focusing on HIV-associated lipodystrophy. (16-19)  With regard to patient management, a few reports suggest that DEXA may have clinical utility for diagnosis of lipodystrophy in patients with HIV, for predicting metabolic insulin sensitivity in older men and women, and for predicting glomerular filtration rate in dialysis patients. (20–22) Research in these specific clinical applications of DEXA is at an early stage and studies have not shown if use of this test in clinical care improves health outcomes. 

Summary

DEXA body composition studies have emerged as a potential new reference standard for body studies, replacing underwater weighing. While DEXA scans have become a valued research tool, it is unclear how information regarding body composition could be used in the active medical management of the patient to improve health outcomes. A search of the literature did not identify any controlled studies in which DEXA body composition measurements were actively used in patient management compared to the use of other simpler techniques of body composition assessment, i.e., bioelectrical impedance or skin-fold thickness.

References

1. BlueCross and BlueShield Association Medical Policy Reference Manual, Policy No. 6.01.40
2. Prior BM, Cureton KJ, Modlesky CM et al. In vivo validation of whole body composition estimates from dual-energy X-ray absorptiometry. J Appl Physiol 1997;83(2):623-30
3. Salamone LM, Fuerst T, Visser M et al. Measurement of fat mass using DEXA: a validation study in elderly adults. J Appl Physiol 2000;89(1):345-52
4. Kohrt WM. Preliminary evidence that DEXA provides an accurate assessment of body composition. J Appl Physiol 1998;84(1):372-7
5. Laskey MA. Dual-energy X-ray absorptiometry and body composition. Nutrition 1996; 12(1):45-51
6. Lane JT, Mack-Shipman LR, Anderson JC et al. Comparison of CT and dual-energy DEXA using a modified trunk compartment in the measurement of abdominal fat  Endocrine 2005;27(3):295-9
7. Buison AM, Ittenbach RF, Stallings VA et al. Methodological agreement between two-compartment body-composition methods in children Am J Hum Biol 2006;18(4):470-80
8. Smith SR, Lovejoy JC, Greenway F et al. Contributions of total body fat, abdominal subcutaneous adipose tissue compartments, and visceral adipose tissue to the metabolic complications of obesity. Metabolism 2001;50(4):425-35
9. Vandrely B, Chauveau P, Barthe N et al. Nutrition in hemodialysis patients previously on a supplemented very low protein diet. Kidney Int 2003;63(4):1491-8
10. Van Den Ham EH, Kooman JP, Christiaans ML et al. The influence of early steroid withdrawal on body composition and bone mineral density in renal transplantation patients. Transpl Int 2003;16(2):82-7
11. Smith DE, Hudson J, Martin A et al. Centralized assessment of dual-energy X-ray absorptiometry (DEXA) in multicenter studies of HIV-associated lipodystrophy. HIV Clin Trials 2003;4(1):45-9
12. Kamimura MA, Avesani CM, Cendoroglo M et al. Comparison of skinfold thicknesses and bioelectrical impedance analysis with dual-energy X-ray absorptiometry for the assessment of body fat in patients on long-term haemodialysis therapy. Nephrol Dial Transplant 2003;18(1):101-5
13. Arabmotlagh M, Rittmeister M, Hennigs T. Alendronate prevents femoral periprosthetic bone loss following total hip arthroplasty: prospective randomized double-blind study  J Orthop Res 2006;24(7):1336-41
14. Garcia Aparicio AM, Munoz Fernandez S, Gonzalez J et al. Abnormalities in the bone mineral metabolism in HIV-infected patients Clin Rheumatol 2006;25(4):537-9
15. U.S. Preventative Services Task Force. Screening for Obesity in Adults: Recommendations and Rationale. November 2003. Agency for Healthcare Research and Quality, Rockville, MD. (Verified 1/15/09)
16. Moran SA, Patten N, Young JR et al.  Changes in body composition in patients with severe lipodystrophy after leptin replacement therapy.  Metabolism  2004;53:513-9
17. Carr A, Law M.  An objective lipodystrophy severity grading scale derived from the lipodystrophy case definition score.  J Acquir Immune Defic Syndr  2003;33:571-6
18. Cavalcanti RB, Raboud J, Shen S et al. A randomized, placebo-controlled trial of rosiglitazone for HIV-related lipoatrophy. J Infect Dis 2007;195(12):1754-61
19. Podzamczer D, Ferrer E, Sanchez P et al. Less lipoatrophy and better lipid profile with abacavir as compared to stavudine: 96-week results of a randomized study. J Acquir Immune Defic Syndr 2007;44(2):139-47
20. Bonnet E, Delpierre C, Sommet A et al. Total body composition by DXA of 241 HIV-negative men and 162 HIV-infected men: proposal of reference values for defining lipodystrophy. J Clin Densitom 2005;8(3):287-92
21. Lee CC, Glickman SG, Dengel DR et al. Abdominal adiposity assessed by dual energy X-ray absorptiometry provides a sex-independent predictor of insulin sensitivity in older adults. J Gerontol A Biol Sci Med Sci 2005;60(7):872-7
22. Taylor TP, Wang W, Shrayyef MZ et al. Glomerular filtration rate can be accurately predicted using lean mass measured by dual-energy X-ray absorptiometry. Nephrol Dial Transplant 2006; 21(1):84-7
23. Elkan AC, Engvall IL, Tengstrand B et al. Malnutrition in women with rheumatoid arthritis is not revealed by clinical anthropometrical measurements or nutritional evaluation tools. Eur J Clin Nutr 2008;62(10):1239-47
24. Forrester JE, Sheehan HM, Joffe TH. A validation study of body composition by bioelectrical impedance analysis in human immunodeficiency virus (HIV)-positive and HIV-negative Hispanic men and women. J Am Diet Assoc 2008;108(3):534-8
25. Jebb SA, Siervo M, Murgatroyd PR et al. Validity of the leg-to-leg bioimpedance to estimate changes in body fat during weight loss and regain in overweight women: a comparison with multi-compartment models. Int J Obes (Lond) 2007;31(5):756-62

Cross References

Screening for Vertebral Fracture with Dual X-ray Absorptiometry (DEXA), Regence Medical Policy, Radiology, No. 48

Radiology Section Table of Contents