How to Prepare for a DEXA Scan: The Complete Pre-Test Checklist

I have watched people ruin their DEXA scans before they even lie down on the table. They show up after a marathon. They show up dehydrated from a night out. They show up wearing jeans with metal buttons, carrying phones in their pockets, and wondering why the technician is asking them to change into a hospital gown. A DEXA scan is the most accurate body composition test available, but it is only accurate if you prepare for it. The scan captures a snapshot of your body at a single moment in time, and that moment needs to represent your normal state, not your dehydrated, overtrained, or overfed state. At our Portland studio, we sent every client a preparation checklist forty-eight hours before their appointment. Not twenty-four hours. Forty-eight. Because some of the things that affect DEXA — alcohol, intense exercise, major meals — have effects that persist longer than a day. If you want your scan to be comparable to your last scan, and comparable to the research literature, you need to control the variables that DEXA cannot control for you.

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Body Fat Percentage Estimator

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Here is the complete checklist I used for every client: **48 Hours Before** - No alcohol. Alcohol dehydrates, shifts fluid distribution, and affects liver glycogen stores for up to forty-eight hours. - Maintain normal hydration. Do not chug extra water, and do not restrict. Drink your usual amount. - No major dietary changes. Do not start a new diet, do not carb-load, and do not fast. Eat normally. **24 Hours Before** - No intense exercise. Cardio and resistance training shift fluid into muscles, deplete glycogen, and alter hydration status for twelve to twenty-four hours. - No NSAIDs or supplements that affect water retention. Creatine loading, in particular, can increase total body water and shift DEXA lean mass readings. - Maintain normal sodium intake. Do not eat an unusually salty or unusually bland meal. **12 Hours Before** - No food. A fasted state is ideal for consistency, but a light meal four hours before is acceptable if fasting is not practical. - No caffeine. Caffeine is a diuretic and can shift hydration status. - Normal water intake. Sip water if thirsty, but do not drink excessively. **Day Of** - Wear comfortable, metal-free clothing. No zippers, buttons, underwire bras, or jewelry. The technician will provide a gown if needed. - Empty your bladder and bowels before the scan. Urine and fecal mass are counted as lean mass on DEXA, and you want to minimize variability. - No phone, keys, or metal objects in pockets. These appear as dense artifacts on the X-ray image. - Arrive in a calm, rested state. Stress elevates cortisol, which can shift fluid distribution.

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The clothing issue is more important than most people realize. DEXA uses X-rays, and metal absorbs X-rays completely, creating dense shadows on the image that the software interprets as bone or lean mass. A zipper on your jeans can add half a pound of apparent lean mass to your trunk. A phone in your pocket can shift your arm lean mass reading by 10%. I once had a client forget about a titanium hip replacement — which is fine, DEXA handles implants well — but she also had a hair clip with a metal spring that created a bizarre artifact on her skull region. The technician had to rescan her. Hydration is the biggest variable most people miss. DEXA assumes a normal hydration state. If you are dehydrated from a hard workout or a night of drinking, your lean mass will read lower because there is less water in your muscles. If you are overhydrated from chugging water before the scan, your lean mass will read higher. The effect is smaller than with BIA — typically plus or minus 0.5% to 1% — but it is real, and it can make the difference between "maintained muscle" and "lost muscle" if you are comparing scans.

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Bioelectrical Impedance Guide

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Here is how different prep states affect DEXA results:

Prep Violation	Effect on Body Fat %	Effect on Lean Mass	Duration
Dehydrated	+0.5% to +1%	-1 to -2 lbs	12-24 hours
Overhydrated	-0.5% to -1%	+1 to +2 lbs	2-6 hours
Post-exercise	+0.3% to +0.8%	-0.5 to -1.5 lbs	12-24 hours
Full bladder	-0.2% to -0.5%	+0.5 to +1 lb	Until emptied
Metal in clothing	Variable	+0.5 to +2 lbs (artifact)	Immediate
Alcohol (previous night)	+0.3% to +0.6%	-0.5 to -1 lb	24-48 hours

The goal of preparation is not perfection. It is consistency. You cannot control every variable, and you should not try. What you can do is create a standardized state that is repeatable scan after scan. If you test fasted, rested, hydrated, and metal-free every time, your scans will be comparable. If you test once after a marathon and once after a rest day, the difference you see is mostly preparation, not body composition change.

Should I fast before a DEXA scan?

Ideally yes, for 8 to 12 hours. A fasted state eliminates variability from recent meals and digestion. If fasting is not practical, eat a light meal 4 hours before and avoid heavy, high-fat foods that slow gastric emptying.

Can I work out the day before a DEXA scan?

Light activity is fine, but avoid intense exercise for 24 hours before. Hard training shifts fluid into muscles, depletes glycogen, and alters hydration status in ways that can skew lean mass readings by 1 to 2 pounds.

Will drinking water affect my DEXA results?

Moderately. DEXA is less hydration-sensitive than BIA, but overhydration can lower body fat percentage by 0.5% to 1% by increasing apparent lean mass. Drink normally, do not chug extra water before the scan.

Can I wear jewelry during a DEXA scan?

No. All metal objects — jewelry, watches, piercings, phones, keys — must be removed. Metal creates dense artifacts on the X-ray image that the software misinterprets as bone or lean mass, corrupting your results.

How long should I wait between DEXA scans for meaningful comparison?

At least 10 to 12 weeks for most people. Body composition changes slowly, and scanning more frequently wastes money while increasing radiation exposure. Athletes in active prep may scan every 4 to 6 weeks, but monthly scanning is unnecessary for most.

I have seen too many clients panic over scan-to-scan changes that were really just preparation differences. A two-pound lean mass drop between scans means nothing if the first scan was after a rest day and the second was after a hard workout. The preparation is part of the measurement. Treat it that way. Your DEXA scan is only as good as your preparation. Do the work before you lie down. — Emily Clarke