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DXA - a suitable validation method for BIA devices?

Bioimpedance analysis (BIA) is an established method for determining a person’s body composition. The analysis measures meaningful conclusions about the general, nutritional and hydration state, which can directly influence the treatment of many diseases. Since the BIA is an indirect process, the accuracy of a device's measurement is based on the right algorithms and reference methods, which are used to validate it.

Bioimpedance analysis uses a weak AC field to measure the electrical resistance of the body. From the body’s partial resistances, the device then calculates the amount of fat and skeletal muscle mass and body water. The individual compartments are thus not measured directly but calculated indirectly. The accuracy of the BIA results is determined by the mathematical equations, which serve as a basis for the calculation of these body compartments. They are the core element of every BIA device and are developed in complex procedures and validated against other reference methods.

BIA devices available on the market differ, as they make use of different types of validation and thus in the algorithms used.

This can lead to considerable differences in the measuring accuracy of two devices.

Despite the central role of validation, there is currently no standardized procedure. Likewise, there is disagreement among manufacturers about the selection of reference methods.

However, devices often mention they have been tested against the "gold standard". This term is widely used in the medical field and refers to an unsurpassed method that is considered the nominal standard or used as a reference. At the same time, its use is not free of criticism. Often experts do not agree on what the "gold standard" entails, so depending on the source, different methods may claim the title.

The Dual Energy X-ray Absorptiometry, DXA for short, is often applied for the validation of bioimpedance analyzers. Although, it is not the most accurate reference method.

A body’s composition can be examined in many ways. None of the currently available methods are considered to be the sole standard. Rather, certain methods are considered as a reference for individual body compartments. The selection of the reference methods determines the measurement accuracy of a BIA device and represents an important differentiator.

For example, many manufacturers use Dual Energy X-ray Absorptiometry, DXA for short, to calibrate their devices. It serves as a reference method for the determination of fat-free mass (FFM) and skeletal muscle mass (SMM).

The DXA is a common X-ray method for measuring bone density and is the standard in osteopenia and osteoporosis diagnostics. In addition, it is also used to analyze the body composition. For this purpose, X-rays of two energy levels are used, which are attenuated to different degrees as they pass through the body, depending on their energy and tissue density. With high and low energy beam attenuation, the system can distinguish between mineral-rich bony and soft tissues, and display them as two-dimensional pixels. Depending on the pixel density, the soft tissue can then be subdivided further into fat mass (FM) and lean soft tissue (LST).

A DXA scan yields a 2-dimensional image of the body. Calculating the body composition out of this can provide inaccurate results, because, for example, constant soft tissue thickness and water and protein fraction must be assumed.

However, the calculation of muscle mass, fat mass and body water requires a number of assumptions. These include a uniform soft tissue thickness, consistent tissue density and solid water and protein fraction of the lean mass.

In reality, human tissues are heterogeneous and subject to dynamic changes, which may offset the assumptions and even adversely affect the accuracy of the DXA. This is especially true for obese people and the elderly. In addition, the natural aging process, as well as the level of physical activity and disease, can cause the state of hydration to be significantly different.

The 4-compartment (4C) model is regarded as the most accurate method for detecting FM and FFM. It is based on different measuring techniques and divides the body into the four compartments: fat, water, bone minerals and protein. By combining different techniques, the 4C model requires fewer assumptions than the DXA and better reflects individual differences. In many cases, systematic differences between the measured results of the DXA and the 4C model could be demonstrated. Thus, DXA tends to underestimate fat mass and overestimate fat-free mass (Schoeller, 2005Tylavsky, 2003).

It also shows that DXA tends to be misjudged, especially in terms of overweight and obesity, which may limit the method’s informative value for individuals (LaForgia, 2012).

The skeletal muscle mass is another measurement parameter with high diagnostic and scientific relevance. It can be calculated approximately from the LST using DXA. However, whole-body magnetic resonance imaging is considered superior and is generally the preferred method of choice. In contrast to the DXA, it provides high-resolution cross-sectional images at any level, which allow a precise determination of the regional and total skeletal muscle mass.

In contrast to a DXA scan, in an MRI scan the body is "carved" into layers (slices). The volume of the subcutaneous fat (green) and visceral fat (yellow), as well as of the muscles (red), can be very accurately determined from the individual strata. (This picture illustrates the method; during the validation of the seca mBCA about 250 layers per subject were analyzed.)

If one compares the measurement results of the DXA and MRI, significant differences regarding the SMM become obvious. Studies showed that the DXA systematically overestimates the SMM (Bosy-Westphal, 2017). As with the FM, one of the possible sources of error is the assumption of a constant tissue hydration and composition. In addition to the state of hydration, the proportions of the LST may also change. As part of the aging process, the SMM decreases, while the proportion of connective tissue increases. This conceals the loss of SMM and leads to an incorrect appraisal of the results.

The comparison with other reference methods shows that the DXA is subject to systematic errors, which occur especially in the case of deviations from a normal body composition. For this reason, its use as a reference method for the validation of medical BIA devices is questionable.

If the validation method itself is subject to systematic errors, the accuracy of the calibrated BIA device is compromised, and the validity of its measurement results is limited.

The seca mBCA is a BIA device specially developed for the medical sector. It has undergone a lengthy and complex validation process. As reference methods, the most exact procedures were selected, such as the 4C model, MRI and the deuterium and sodium bromide dilution. Scientific studies have shown the seca mBCA and the individual methods match to a high degree. In addition, reference values ​​were defined in a large-scale study with more than 1,000 healthy volunteers of different age, sex and ethnicity, which allows a safe interpretation of the mBCA output parameters.

Due to its extensive validation, the seca mBCA not only meets the requirements of medical research but it is also a precise measuring tool for regular use in the outpatient and clinical environment.

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