DXA equipment can measure the BMD of any skeletal region, including the most clinically relevant sites such as the hip. 

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... significant correlations 96 . CTX is an independent predictor of hip fracture risk 97 . In a study of elderly women, Garnero et al. found that a combination of bone density measurements (dual energy X-ray absorptiometry or calcaneal ultrasound broadband attenuation) plus CTX analysis increased the sensitivity in predicting hip fractures 97, 98 . Furthermore, it was found that combining CTX measurements with fracture history predicted hip fractures as well as hip DXA. As outlined above, several studies have demonstrated that common bone markers correlate with current bone density, rate of bone loss after menopause, and fracture risk. While the use of bone markers may potentially increase the sensitivity of fracture prediction when used in addition to BMD, no studies on pharmacological treatment on the basis of bone marker measurements have been performed; therefore, the clinical usefulness for treatment selection remains unclear 4, 98, 99 . Furthermore, the results of bone marker studies may not be sufficiently uniform to allow prediction of BMD or fracture risk for individual patients. Biochemical markers are not recommended for diagnosing osteoporosis, nor is it clear whether or how these markers can be used clinically for fracture prediction 4, 99-101 . A study of urinary NTX in a clinical setting found that the test rarely influenced osteoporosis management by clinicians 102 . Further, a study of postmenopausal women on HRT or placebo concluded that neither BAP, OC, DPYR/PYR, nor CTX/NTX offered useful information for predicting BMD or BMD changes 101 . The diagnosis of osteoporosis is primarily based on bone density measurements. Over the last several decades, different techniques for bone density measurements have been developed. Each of these techniques has its advantages and drawbacks: Some are well investigated and proven, while others are fairly new and have yet to prove their worth in research and clinical practice. The multitude of available methods is both a blessing and a curse. The field of clinical osteoporosis diagnosis and treatment is still fairly new, and the different available methods that use different reference values and measuring sites adds to the difficulty for clinicians entering this field. For hospital management, it can be difficult to decide what equipment is the most appropriate and most needed. A brief outline of current methods is therefore in order. Dual Energy X-ray Absorptiometry (DXA, sometimes called DEXA) is regarded as the “gold standard” in osteoporosis research and practice (Figure 12 ) 103-106 . The WHO osteoporosis criteria (see section Osteoporosis) and most treatment guidelines are based on DXA results. Most bone and osteoporosis research employing BMD measurements has also been performed using DXA. DXA is a radiation-based technique that has replaced other less accurate radioabsorptiometric methods. DXA employs two different energy levels of radiation to separate calcium from soft tissue by means of computer analysis, thus eliminating the need to manually correct for soft tissue thickness. Simultaneously, DXA enables accurate and precise body composition analysis (Figure 13), providing information about tissue fat content and fat-free or lean content (the latter roughly corresponds to muscle mass) 107 . First generation DXA equipment employs a narrow pencil beam of radiation; while the scans are time consuming, they have no magnification error and the radiation dose is very low—about 2 microSievert (μSv) per scan. In contrast, natural background radiation is about 2400 μSv per year 108 . This means that no radiation shielding is needed for patients or technicians during DXA testing. Newer DXA systems often employ a “fan-beam,” resulting in much faster scanning time and higher resolution. The newest computer software can even detect vertebral compression fractures from the scan. Drawbacks of the fan-beam DXA equipment include problems with magnification errors, which do not affect BMD, but do affect BMC, bone area, and soft tissue measurements. These errors can, however, be corrected 109-112 . Another issue is radiation exposure. Although fan-beam DXA systems use higher levels of radiation than do pencil-beam systems, the patient doses are still very low; however, technician doses could be a concern 113-115 . As noted above, one advantage of DXA is very low radiation exposure, making it safe for both patients and hospital staff. This also means that DXA can be used in regular hospital rooms, since no special radiation shielding is needed. Another advantage of DXA is that the measurements are very precise. The coefficient of variation is often reported to be <1% between scans; accuracy remains a larger problem, but is usually within 10% 66-68, 70 . Current fan-beam DXA scans take just a few minutes, making it possible to scan many patients per day at a relatively low cost. DXA can measure BMC and BMD of any desired skeletal site, albeit with varying precision (Figure 14). DXA can thus measure the BMD of the most clinically relevant sites, such as hips and vertebrae. It has been shown that site-specific measurements correspond better to bone strength at the site than do non-site-specific measurements 106, 116 . DXA can also predict fracture risk very well, even better than blood pressure readings predict stroke, and better than serum cholesterol predicts cardiovascular disease 20, 117 . Metaanalyses indicate that fracture risk roughly doubles for each standard deviation reduction in BMD 4, 117 . Especially proximal femur measurements may predict future fractures well: A decrease in bone density of 1 standard deviation at the femoral neck corresponds to a 2.6-fold greater risk for hip fracture, a similar decrease at the spine corresponds to a 2.3-fold greater risk for vertebral fracture (Figure 15) 4, 117, 118 . DXA measures areal bone mineral density (aBMD) as opposed to true volumetric density (vBMD). That is, the bone mineral per square centimeter of bone area viewed by the scanner is recorded and expressed as g/cm 2 instead of being expressed as g/cm 3 . This means that bone size affects the results of a measurement: A large bone will have a higher aBMD than a small bone, even if the vBMD is the same (Figure 16). This can lead to some difficulty in interpreting aBMD results; however, this might be compensated by the fact that larger bones are suggested to be stronger 119, 120 . So far, attempts to switch to vBMD measurements have not been successful in terms of clinical relevance 119-121 . A disadvantage of DXA is that interpretation of lumbar spine BMD measurements in older subjects is difficult as compression fractures or degenerative changes can give falsely elevated values; thus, most current guidelines suggest that the BMD of the femoral neck should be used for diagnosing osteoporosis . It should also be noted that different DXA machines may give different results 109, 122-124 . The reasons for this include different dual energy methods, different calibration, different detectors, different edge detection software, different regions of interest, and different reference populations. Therefore follow-up measurements should be made on the same DXA equipment. In multicenter studies, cross-calibrations of the DXA machines are required. Standard whole body DXA scanners are large and expensive, costing around 1 million Swedish crowns (~100,000 euros), and require specially trained technicians. Facilities with limited patient volume and limited funding may hesitate when considering the cost of the equipment. Therefore a market for small, inexpensive, and portable peripheral scanners has emerged. Heel DXA scanners, which measure calcaneal BMD, have been developed to meet this demand. There is currently a lack of reference data, as well as limited knowledge of how the peripheral data relate to more central measurements, to the WHO osteoporosis criteria, and to treatment cut-off values. As previously discussed, there is better correlation for fracture risk if site-specific DXA measurements of hip and spine are performed 117 , and site-specific measurements correlate better to bone strength 106, 116 . Quantitative ultrasound (QUS) bone density scanners have also been developed to meet the demand for inexpensive, radiation-free portable devices (Figure 17). ...