DEXA Scan

DEXA Scan: what is it?

DEXA was built to measure bone density and was later repurposed for body composition, which is why a scan report gives you a bone mineral figure alongside fat and lean tissue. It is the only widely available method that tells you where fat and lean mass sit rather than just how much there is: left arm versus right arm, trunk versus legs, android versus gynoid. That regional detail is genuinely useful to a coach. What DEXA is not is an oracle, and treating a printed percentage as ground truth is the most common mistake around it.

What the scan actually reports

DEXA uses a three-compartment model. Two X-ray beams of different energies are attenuated differently by fat, lean soft tissue and bone, and software solves for the three from that difference. The scan takes roughly ten minutes with the client lying still on a table, fully clothed apart from metal.

Precision, accuracy and the machine problem

DEXA is precise, meaning it repeats itself well on the same machine with the same protocol. It is less reliably accurate in absolute terms, because different manufacturers, different hardware generations and different software versions apply different calibrations and can produce meaningfully different body fat percentages for the same person on the same day.

The practical rule follows directly. Never compare a scan from one clinic to a scan from another, and never compare a scan to a caliper or bioimpedance reading. A client's DEXA history is only a history if every scan came off the same machine with the same software.

How to prepare a client for a scan

Lean soft tissue includes body water, so anything that shifts hydration shifts the lean reading. A client who arrives after a heavy training session, a high-carbohydrate day or a large meal is not being measured under the same conditions as last time, and the difference can be read as muscle gain or loss that never happened.

• Scan in the morning, fasted, after an empty bladder.
• No training in the twelve hours before, and no unusually high or low carbohydrate day before.
• Same time of day and same day of the week as previous scans.
• Same clinic, same machine, and ask them to confirm the software version has not changed.
• Remove all metal: jewellery, underwire, zips, piercings.

Radiation, cost and when it is worth it

A DEXA body composition scan uses a small dose of ionising radiation. It is low, comparable to everyday background exposure over a short period, but it is not zero, which is why scanning every four weeks is not a sensible plan and why pregnancy is a contraindication. That is a medical decision belonging to the clinic, not to you.

Given the cost and the dose, DEXA earns its place once or twice a year: a genuine baseline at the start of a long body composition project, and a follow-up at the end. Between those two points, girths, weight trends and photos do the job for free and can be repeated weekly.