How Is a Bone Density Test Done and What to Expect

A bone density test is a quick, painless scan that measures how much mineral your bones contain, and the most common version takes about 10 to 20 minutes while you lie still on a padded table. The machine most often used is called a DXA (dual-energy X-ray absorptiometry) scanner, which sends two low-dose X-ray beams through your hip and spine to gauge bone strength. The experience is closer to lying on an exam table for a few minutes than to anything resembling an MRI or CT scan, and the radiation exposure is extremely low.

What Actually Happens During the Scan

You walk into the imaging room and lie face-up on a flat, open table. There is no tunnel or enclosed space. A technologist positions your body so the scanner can capture images of the sites it needs, which are usually the lower spine and the top of the thighbone near the hip. For the spine portion, your legs are placed on a padded box to flatten the curve of your lower back and give the scanner a clearer view. For the hip portion, your foot is placed in a brace that rotates your leg slightly inward.

Once you are positioned, a mechanical arm passes slowly over your body. You do not feel anything from the X-ray beams. The arm moves in a smooth, quiet sweep, and most people find the hardest part is simply holding still. There is no injection, no contrast dye, and no need for anesthesia. When the arm finishes scanning one area, the technologist repositions it for the next site, and the process repeats. The whole appointment, including check-in and positioning, rarely takes more than half an hour.

How to Prepare

Preparation is minimal compared to most medical imaging. You should wear loose, comfortable clothing and avoid anything with metal, such as buckles, buttons, or zippers, since metal can interfere with the X-ray image.1Centers for Disease Control and Prevention. Facts About Bone Density (DEXA Scan) – Section: What to expect Elastic-waist pants and a cotton T-shirt work well. If you forget and show up in jeans, many facilities will give you a gown to change into, but avoiding the wardrobe swap makes the visit faster.

If you take calcium supplements, your doctor will likely ask you to stop taking them for at least 24 hours before the test. Calcium tablets sitting in your digestive tract can show up on the scan and artificially inflate the reading in your spine. You can eat normally before the appointment and do not need to fast. If you have had a barium study or received contrast dye for a CT scan recently, mention it when you schedule, because residual barium or contrast material can also skew results. Most facilities will ask you to wait a week or two before doing the bone density scan.

How the DXA Machine Works

The DXA scanner uses a C-shaped arm positioned so that the X-ray source sits below you and the detector sits above. It sends photons at two distinct energy levels: one tuned to pass through soft tissue and one tuned to be absorbed by dense cortical bone.2NCBI Bookshelf. Dual-Energy X-Ray Absorptiometry – Section: Introduction By comparing how much energy gets through at each level, the machine calculates how dense the bone is at every point in the scanned area. The result is a detailed map of mineral content in your hip and spine, measured in grams per square centimeter.

This dual-energy approach is what makes DXA the standard for bone density testing. It can distinguish bone from the muscle, fat, and organs surrounding it with high precision, which is why doctors trust it more than simpler screening tools. The images it produces look nothing like a regular X-ray photograph. Instead, the output is a color-coded or grayscale density map, along with numerical scores that compare your bones to reference populations.

Is the Radiation Dangerous

The radiation dose from a DXA scan is very low, comparable to the amount you receive from a standard X-ray and far less than what a CT scan delivers.3Centers for Disease Control and Prevention. Facts About Bone Density (DEXA Scan) – Section: The basics To put it in perspective, you absorb more ionizing radiation during a cross-country flight than during a DXA scan. The natural background radiation you encounter every day from cosmic rays, soil, and building materials dwarfs the dose from this test. For that reason, the scan is considered safe to repeat at regular intervals over many years, which is exactly how doctors use it to track bone health over time.

That said, you should tell your technologist if there is any chance you are pregnant. As with all X-ray-based imaging, routine scans are avoided during pregnancy as a precaution, even though the dose is tiny.

Reading Your Results

Your report will include two main numbers: a T-score and a Z-score. Both compare your bone density to a reference group, but they answer different questions.

The T-score compares your bone density to that of a healthy young adult at peak bone mass. A T-score of 0 means your bones are exactly average for a young adult. Positive numbers mean denser than average, and negative numbers mean less dense. Doctors use specific cutoffs to classify the results:

The Z-score, on the other hand, compares you to other people of the same age, sex, and body size. This score is more useful for younger adults and for spotting unusual bone loss that does not fit the expected pattern of aging. A Z-score of –2.5 or lower raises suspicion that something beyond normal aging is driving bone loss, such as a thyroid disorder, celiac disease, long-term steroid use, or another underlying condition.4PubMed Central. Bone mineral density: testing for osteoporosis – Section: Bone mineral density If your Z-score is that low, your doctor will likely order additional blood work or other tests to look for a secondary cause rather than simply prescribing osteoporosis medication.

One thing worth knowing: your T-score can differ between the spine and the hip, and sometimes the difference is significant. It is common to have osteopenia in the spine but normal density in the hip, or vice versa. Your doctor will typically base treatment decisions on the lowest T-score from either site.

Who Should Get Tested

The U.S. Preventive Services Task Force recommends bone density screening for all women aged 65 and older. For postmenopausal women younger than 65 who have elevated risk factors, such as low body weight, a history of fracture, smoking, or a family history of osteoporosis, screening is also recommended based on a formal risk assessment.5JAMA Network. Screening for Osteoporosis to Prevent Fractures: US Preventive Services Task Force Recommendation Statement – Section: Recommendations These recommendations carry a “B” grade, meaning there is moderate certainty that the benefit of screening outweighs the harms.

For men, the guidelines are less clear-cut. There is no universal screening recommendation for men at any age from the Task Force, though many physicians will order a DXA scan for men over 70 or for younger men who have risk factors like long-term corticosteroid use, heavy alcohol intake, or conditions that affect hormone levels. Men tend to develop osteoporosis later than women and less frequently, but when they do, the fracture consequences can be just as serious.

If you are taking medication known to weaken bones, such as glucocorticoids (prednisone and similar drugs), aromatase inhibitors for breast cancer, or androgen deprivation therapy for prostate cancer, your doctor will often order a baseline DXA scan soon after you start treatment and follow-up scans at regular intervals to monitor how your bones respond.

Peripheral Screening Devices and Why They Are Less Reliable

You may have seen bone density screenings offered at pharmacies, health fairs, or community events. These portable tests typically measure your heel, wrist, or finger rather than your hip and spine. They use either a small X-ray device or ultrasound, and they are fast and convenient. The trouble is that they are not very accurate at identifying who actually has osteoporosis.

When researchers compared heel ultrasound and finger DXA devices against central DXA as the reference standard, the peripheral tools missed a large proportion of people with osteoporosis. Heel ultrasound detected only about a third of cases that central DXA identified, and finger DXA caught even fewer, roughly one in four. The overall disagreement between peripheral devices and central DXA was around 21 to 23 percent.6PubMed Central. Comparison of heel ultrasound and finger DXA to central DXA in the detection of osteoporosis. Implications for patient management In practical terms, this means that a peripheral screening result telling you your bones are fine does not rule out osteoporosis in your hip or spine, which are the sites most vulnerable to the fractures that cause real disability.

That does not mean peripheral screenings are worthless. They can serve as a rough filter: if a portable device flags a problem, it is a reasonable prompt to schedule a proper central DXA scan. But a normal peripheral result should not be treated as a clean bill of health for your bones, especially if you have risk factors. If you are due for screening based on your age or medical history, get the full scan.

How Often to Repeat the Test

If your first DXA scan comes back normal, most guidelines suggest waiting at least a few years before repeating it. For women with normal bone density at 65, some research suggests the interval can be as long as 10 to 15 years before a recheck adds useful information, since bone loss in people with normal starting density tends to be slow. For those with mild osteopenia, a shorter interval of two to five years is more common, since there is more reason to track how quickly density is changing.

If you are diagnosed with osteoporosis and start treatment, your doctor will typically repeat the DXA scan after about two years to see whether the medication is working. Bone density changes slowly, so testing more frequently than every two years in most situations does not give the scanner enough time to detect a meaningful difference from the measurement noise. The DXA machine has a precision error of about one to two percent, meaning a change smaller than that could just be normal test-to-test variation rather than real bone gain or loss.

One practical note: try to get your follow-up scans on the same machine, or at least the same brand of machine, that you used the first time. Different DXA manufacturers calibrate slightly differently, and switching machines between scans can introduce discrepancies that look like bone changes but are really just differences in how the equipment measures. If you move or switch medical providers, mention what type of machine was used previously so your doctor can interpret the comparison correctly.

What the Test Cannot Tell You

A DXA scan measures bone mineral density, which is one important predictor of fracture risk but not the only one. Bone quality, meaning the internal architecture and the tiny connections between the hard outer shell and the spongy interior, also matters. Two people with the same T-score can have very different fracture risks depending on factors like bone geometry, microarchitecture, and turnover rate. DXA does not capture any of those characteristics.

This is why your doctor will often combine your DXA results with a clinical risk calculator, such as the FRAX tool, which factors in your age, sex, body weight, fracture history, smoking status, alcohol use, steroid use, and whether a parent fractured a hip. The calculator estimates your ten-year probability of a major osteoporotic fracture. In some cases, a person with a T-score in the osteopenia range but several clinical risk factors may benefit from treatment, while someone with a T-score just barely at –2.5 and no other risk factors may reasonably hold off.

DXA also has limitations in certain body types. In people with significant spinal arthritis or compression fractures that have already occurred, the spine measurement can read falsely high because the degenerative changes add extra mineral to the scan area. Severely overweight individuals can pose positioning challenges, and the extra soft tissue can reduce measurement accuracy. In these situations, your radiologist may rely more heavily on the hip measurement or may recommend alternative imaging.

Conditions That Can Interfere with Accuracy

Beyond the clothing and supplement advice already covered, several medical situations can throw off a DXA reading. Spinal hardware from previous surgery, such as metal rods or screws, will render the spine measurement unreliable on that segment, so the technologist will exclude it and focus on unaffected vertebrae or the hip. Kidney stones that happen to sit in the path of the beam can artificially elevate the reading, as can aortic calcifications in older adults, where calcium deposits in the large blood vessel overlying the spine get counted as bone.

If you have had a joint replacement in one hip, the scan will be done on the other hip. If both hips have been replaced, the forearm becomes the fallback measurement site. While the forearm is less commonly used and its T-score thresholds were developed primarily from hip and spine data, it still provides useful tracking information when the preferred sites are unavailable.

These are things your imaging team will handle, but knowing about them in advance helps you have a more informed conversation with your doctor when the results arrive. If a reading seems surprisingly high or low relative to your overall health profile, one of these confounders could be the explanation.