What is a bone density test?
Dual-energy X-ray absorptiometry (dual-energy X-ray absorptiometry) is a non-invasive medical imaging technique primarily used to measure the mineral content of bones and assess the risk of osteoporosis. The principle is to calculate the bone density value by penetrating the bone through two X-rays of different energies. Compared to conventional X-rays, it has the advantages of lower radiation dose (about 1/10th of chest X-rays) and high accuracy (less than 1% error rate).
Applicable groups include:
- Women over 65 years old and men over 70 years old
- Those with a history of fractures or a family history of osteoporosis
- Those who have been taking drugs that affect bone metabolism for a long time, such as steroids and antidepressants.
- Postmenopausal women or men with low testosterone
Comparison with other test methods:
| Check the type | sharp | limit |
|---|---|---|
| Gold standard, fast (5-10 minutes) | Only the axial bone will be measured. | |
| Quantitative Ultrasound (QUS) | Radiation-free and portable | Low accuracy |
| Quantitative (Q) | Volumetric bone density can be measured | High radiation and high cost |
Explain the inspection process in detail
Preparation before the exam
It is recommended to avoid taking calcium tablets or drugs containing barium 24 hours before surgery, and to wear loose clothing without metal accessories (zippers, buttons, etc.) on the day of the examination. Some medical institutions in Hong Kong (such as sanatoriums and hospitals in Hong Kong) require you to complete a questionnaire to see if you have recently had a contrast screening. If you may be pregnant, let your healthcare provider know in advance.
Review the process
During the actual scan, the patient will be required to lie flat on the examination table and the device will scan the lumbar spine (L1-L4) and femoral neck for approximately 5-10 minutes. In the process, they need to stay still and breathe naturally. Newer devices, such as the GE Lunar Prodigy, are equipped with a curved probe arm to reduce claustrophobia in patients. According to statistics from the Hong Kong Radiological Society, 90% of patients reported “almost no discomfort”.
Precautions after the test
No special recovery measures are required after the examination, and normal activities can be carried out immediately. The report is usually interpreted by a radiologist or endocrinologist within 3 to 5 working days. The Hong Kong Health Bureau recommends that the original data CD-ROM be kept with the report for future follow-up and comparison.dexa scan
How to interpret an inspection report?
Key metrics in the report include:
- T-value: Standard deviation (SD) units compared to peak bone mineral density in healthy adults aged 30 years
- Z-value: Comparison with the same age and sex
World Health Organization (WHO) diagnostic criteria:
| Range of T-values | Diagnosis results |
|---|---|
| ≥-1.0 | normal |
| -1.0~-2.5 | Osteopenia (osteopenia) |
| ≤-2.5 | Osteoporosis (osteoporosis) |
Attention should be paid to the influencing factors: degenerative spinal lesions can incorrectly increase bone density values, and a study by the Chinese University of Hong Kong pointed out that measuring the femoral neck of a person with a BMI of >30 may underestimate the actual risk.
Risks and limitations of testing
The radiation dose per dose is about 1 to 10 μSv (data from the Hong Kong Radiological Bureau), which is lower than the daily ambient background radiation (about 5 to 8 μSv per day). However, pregnant women should avoid the examination unless there is an urgent clinical need, and the child should use a special pediatric mode and have a shorter scanning area.
Key limitations include:
- Inability to assess bone microstructure
- Accuracy decreases in people with extreme obesity (130 kg >).
- Peripheral bones (e.g. wrist) should be examined separately
Prevention and treatment of osteoporosis
Dietary advice
The Hong Kong Dietitians Association recommends daily consumption.
- 鈣質:1000-1200mg(1杯牛奶含約300mg)
- Vitamin D: 800-1000 IU (15 minutes of sun exposure can synthesize about 400 IU)
Exercise Recommendations
The Hong Kong Physiotherapy Association emphasizes that combining “weight-bearing exercises” such as brisk walking and tai chi for at least 150 minutes per week with resistance training (rubber bands or appliances) 2-3 times a week can increase bone density by 1-3% per year.
Drug therapy
Comparison of common drugs:
| Types of drugs | Represents medicine | Precautions |
|---|---|---|
| Bisphosphonates | Allendronate | Take on an empty stomach and leave for 30 minutes |
| preach | Raloxifene | May make hot flashes worse |
Frequently Asked Questions (FAQs)
How often should I get tested?
The Hong Kong Osteoporosis Foundation recommends that high-risk groups be followed up every 1-2 years, and normal people should be followed every 3-5 years. When starting treatment, the first review should be done 1-2 years after administration.
How much does the test cost?
The cost of the private sector in Hong Kong is about HK$800 to HK$2,000 (for example, Union Hospital has a basic package of HK$1,500), and referrals to public hospitals require a waiting time of HK$6 to HK$12.
What should I do if my test results are abnormal?
Referral to endocrinology or rheumatology and immunology should be used for FRAX fracture risk assessment, and spinal x-rays should be used to rule out hidden fractures if necessary. According to data from the Hong Kong Hospital Authority, early drug treatment can reduce the risk of hip fractures by 50%.