A Dual-Energy X-ray Absorptiometry (DEXA) scan is a non-invasive imaging test that uses a small dose of X-ray to measure the composition of the body. Its primary purpose is to measure bone mineral density (BMD) to assess bone strength and determine the risk of conditions like osteoporosis. The same technology is also used to analyze body composition, providing detailed measurements of fat mass, lean mass, and bone mineral content. The preparations necessary for the scan depend entirely on which of these two primary measurements the procedure is targeting.
Preparation for Bone Density Testing
For a DEXA scan focused solely on measuring bone density, preparation requirements are minimal, and fasting is generally not needed. Patients can typically eat and drink normally before their appointment. The dual-energy X-rays used in the scan are designed to pass through soft tissues, so food in the stomach does not interfere with the measurement of minerals in the bone. The one significant restriction is the temporary avoidance of calcium supplements, multivitamins containing calcium, and antacids. These supplements should be paused, often 24 hours, before the scan. Undissolved calcium from a recent supplement might be mistakenly registered as higher-than-actual bone density, leading to inaccurate results. Normal foods containing calcium are permissible because the mineral is naturally integrated into the food matrix.
Specific Instructions for Body Composition Scans
The preparation rules are stricter when the DEXA scan is used to measure body composition, such as fat percentage and lean muscle mass. The goal is to ensure all measurements are consistent over time. This consistency requires minimizing temporary fluctuations in body water and gut content. A fasting period is often required, typically ranging from four to twelve hours before the scan, though an overnight fast is frequently recommended. This ensures the gastrointestinal tract is relatively empty, preventing the weight of undigested food and fluid from skewing the total lean mass reading. Furthermore, strenuous exercise should be avoided for at least 12 to 24 hours prior to the test. Intense workouts cause temporary fluid shifts in the muscles, which can artificially inflate the measurement of lean tissue.
Medication and Clothing Requirements
For both bone density and body composition scans, specific rules apply to medications, supplements, and clothing. Patients should generally continue taking all regular prescription medications. The only common exception is the temporary pause of calcium supplements, which must be held for 24 to 48 hours before the procedure. If a patient has recently undergone imaging involving contrast dye (such as a CT or MRI scan), they may need to wait at least seven days before having a DEXA scan, as the contrast material can interfere with results. Clothing must be loose-fitting, comfortable, and completely free of any metal components. Metal items, including zippers, snaps, buckles, and jewelry, can absorb or scatter the low-dose X-ray beams, creating artifacts that compromise accuracy. Patients are asked to remove all metal items before the scan, sometimes changing into a hospital gown.
Interpreting the Scan Results
Once the DEXA procedure is complete, the data is analyzed to provide specific metrics. For bone density scans, results are reported using T-scores and Z-scores. The T-score compares the patient’s bone density to that of a healthy young adult of the same sex. A score of -1.0 or higher indicates normal density. A T-score between -1.0 and -2.5 is classified as osteopenia, while a score of -2.5 or lower suggests osteoporosis. The Z-score compares the patient’s bone density to the average density of people who are the same age, sex, and ethnicity. This score helps determine if the bone loss is unusual for their age group, potentially signaling an underlying medical condition or medication effect. For body composition scans, the report details whole-body and regional measurements of fat mass, lean mass, and the calculated body fat percentage. All results should be discussed directly with a healthcare provider, who can use this detailed information to guide treatment.