Dual-energy X-ray absorptiometry, commonly known as a DEXA scan, is a non-invasive imaging technique that uses low-dose X-rays to assess the composition of the body. Its primary medical purpose is to measure bone mineral density (BMD) for diagnosing conditions like osteoporosis, which weakens bones and increases fracture risk. However, the scan also provides a detailed breakdown of body composition, quantifying fat mass and lean tissue mass. Evaluating the reliability of these measurements requires understanding the difference between the machine’s inherent capability to measure and how closely those measurements reflect a true, biological state.
Defining Precision and Accuracy in DEXA
To judge the quality of DEXA results, two technical metrics are applied: accuracy and precision. Accuracy is defined as how close the measured value is to the true value of the object being measured. For bone mineral density, the accuracy error of a DEXA instrument is typically better than 10%, which is sufficient for diagnosing conditions like osteoporosis.
Precision is the ability of the DEXA system to consistently reproduce the same result when measuring the same patient repeatedly. High precision is necessary for monitoring changes over time, such as tracking bone loss or the effectiveness of treatment. The precision of the measurement is influenced by the stability of the machine, patient movement, and the skill of the technologist.
Precision leads directly to the calculation of the Least Significant Change (LSC), which determines if a difference between two consecutive scans is a true biological change or random measurement noise. The LSC is calculated by multiplying the precision error of the machine and technologist by 2.77, corresponding to a 95% confidence level. For a change in bone density to be considered real, the difference between the two measurements must exceed the calculated LSC for that specific facility and skeletal site.
Precision error rates for BMD are low, often less than 1% for the spine and between 1% and 2% for the hip, indicating a high degree of reliability in repeated measurements. However, the LSC must be calculated for each specific machine and technologist. Using a general or manufacturer-provided LSC can lead to incorrect interpretations of a patient’s progress. While DEXA is accurate enough for diagnosis, its precision makes it a powerful tool for longitudinal tracking.
Patient and Procedural Factors Affecting Results
Even with a precise machine, various patient and procedural factors can introduce errors into the final results. One significant patient factor is hydration status, as the machine’s calibration assumes a standard level of hydration. Dehydration alters soft tissue density, which can cause the scan to underestimate bone mineral density, while over-hydration can cause an overestimation.
Foreign materials in the body can also skew measurements, as X-ray beams are designed to differentiate between bone, fat, and lean tissue. Metal implants, such as hip or knee replacements, can create artifacts that inflate body composition variables and cause errors in whole-body bone mineral content, sometimes by 1.5% to 3%. Recent diagnostic procedures involving contrast agents or nuclear medicine investigations can also temporarily interfere with the X-ray absorption measurements.
Procedural factors are a major source of potential error, often stemming from mistakes made by the technologist. Incorrect patient positioning is a frequent issue; even slight misalignment, such as improper rotation of the femur during a hip scan, can significantly alter the bone mineral density reading. Results also depend on the technologist’s skill in analyzing images, particularly when selecting the correct region of interest on the spine.
Comparing results obtained from different facilities or machines can be misleading due to a lack of standardization in calibration and algorithms across manufacturers. Body fat percentage results, for example, can vary by up to 5% between machines. To accurately track progress, consistency in pre-scan preparation, including diet and exercise, and using the same machine and technician whenever possible is recommended.
Interpreting Bone Density and Body Composition Scores
The numerical output from a DEXA scan is contextualized through specific scoring systems to make the data clinically useful. For bone mineral density, the T-score is the primary diagnostic value, comparing the patient’s BMD to the average peak bone density of a healthy young adult of the same sex. A T-score between -1.0 and -2.5 indicates low bone mass (osteopenia), while a score of -2.5 or lower diagnoses osteoporosis in postmenopausal women and men over 50.
The Z-score compares the patient’s bone density to that of an average person of the same age, sex, and body size. This score is more helpful for children, premenopausal women, and men under 50, as it indicates whether the bone density is typical for their demographic. This can help identify underlying medical conditions causing bone loss. Neither score is a standalone diagnosis; they are statistical measures that must be considered alongside a person’s clinical history and overall fracture risk.
When assessing body composition, the scan provides percentages for fat mass and lean mass by measuring the different attenuation properties of the two X-ray energies as they pass through the body. These percentages are not a direct measurement but result from a sophisticated computer model based on the X-ray attenuation data. Clinicians often use specific regional results, such as the Android/Gynoid ratio, to track fat distribution, which can be linked to various health risks.