Dual-energy X-ray absorptiometry (DEXA or DXA) is a low-radiation imaging technology used primarily to determine bone health and strength. The technique passes two distinct X-ray energy beams through the body, which are absorbed differently by bone and soft tissue. By measuring this absorption, the scanner calculates Bone Mineral Density (BMD), the standard measure for diagnosing conditions like osteoporosis. DEXA is also used to analyze body composition, providing estimates for total body fat mass and lean mass. Its accuracy is high, but reliability varies depending on whether it measures bone density or body composition, and it is subject to several technical and physiological factors.
Technical Precision in Bone Density Measurement
DEXA is considered the gold standard for measuring BMD because of its high precision and reproducibility, especially when monitoring changes over time. The fundamental output is a Bone Mineral Density value, typically measured in grams per centimeter squared (\(\text{g/cm}^2\)). This value is converted into standardized scores for clinical interpretation, involving two primary metrics: the T-score and the Z-score.
The T-score compares a patient’s BMD to the peak bone mass of a healthy young adult of the same sex, and is the metric used to officially diagnose osteopenia (low bone mass) and osteoporosis. A T-score of -2.5 or lower defines osteoporosis, while scores between -1.0 and -2.5 indicate osteopenia. The Z-score compares the patient’s BMD to that of an average person of the same age, sex, and ethnicity. It is useful for younger individuals to identify unusual bone loss patterns that may suggest an underlying medical condition.
The high precision of the BMD measurement means that results are highly reproducible on the same machine when a scan is performed correctly. This reproducibility allows clinicians to reliably track small changes in bone density, sometimes as small as one percent, to determine if a treatment plan is effective. The lumbar spine is often the preferred site for monitoring treatment effectiveness, while the total hip is the best predictor for future hip fracture risk.
Limitations in Assessing Body Composition
While DEXA is widely used for body composition analysis, its accuracy for measuring fat and lean mass relies on a fundamental assumption. The technology operates on a two-compartment model, separating the body into fat mass and fat-free mass (lean mass). This model assumes that the hydration of the fat-free tissue remains constant at roughly 73% water.
Variations in a person’s hydration status can temporarily skew the results, particularly the lean mass reading. For example, intense exercise or severe dehydration before a scan can cause the DEXA to miscalculate the lean tissue mass, as the fluid shifts are interpreted as changes in tissue density. In a state of severe dehydration, the scan may overestimate the body fat percentage, while over-hydration can overestimate lean mass.
The accuracy of DEXA for body composition is superior to other field methods like bioelectrical impedance analysis (BIA), but it has limitations. In regions where bone is present, the software must estimate the soft tissue composition, which can introduce error, especially in the thoracic and pelvic areas. For individuals who are markedly obese, the accuracy of body composition measurements can be lower.
External Factors Influencing Scan Results
The accuracy of a DEXA scan is highly dependent on external factors during the acquisition and analysis phases, not just the machine’s technical specifications. Operator errors are common, including improper patient positioning, which can significantly alter the measured Bone Mineral Density (BMD). For example, incorrect rotation of the femur during hip scans can lead to inaccurate BMD measurements.
Patient-related artifacts can interfere with results by artificially inflating or deflating density readings. The presence of metal implants (like hip replacements) or external objects (like jewelry) creates density anomalies that must be corrected or excluded from the analysis. Although metal artifacts can elevate the measured BMD in the affected region, they have minimal effect on whole-body fat and lean mass measurements.
Machine-related issues, such as calibration drift over time, can also affect device precision. Regular quality assurance checks and phantom measurements are necessary to detect these subtle changes before they compromise clinical accuracy. Patient movement during the brief scan acquisition can create motion artifacts, often requiring the technologist to repeat the scan for a reliable result.
Understanding the Clinical Margin of Error
In a clinical setting, accuracy is judged by the ability to detect a true biological change in a patient’s bone density over time. This is quantified using the concept of the Least Significant Change (LSC). The LSC is a statistically derived value that represents the minimum difference required between two consecutive DEXA measurements to be certain the change is real and not just random measurement error or noise.
For a change in BMD to be considered meaningful, it must exceed the LSC value, which is calculated with a 95% confidence interval. If bone density changes by only one or two percent, but the LSC for that machine and site is three percent, the change is considered statistically insignificant and should not trigger a change in treatment. The LSC is around \(0.03\text{ g/cm}^2\) for the lumbar spine, but this value can be higher for individuals with higher body mass index.
To maintain the highest accuracy for monitoring changes, follow-up scans should be performed using the same DEXA machine and, if possible, by the same technologist. If a patient switches machines, the two devices should undergo a cross-calibration to account for differences in their baseline measurements. Adherence to standardized protocol ensures that longitudinal data is reliable for informed clinical decisions.