How to read DXA scan Dr. Basmah Alwahabi DXA : Dual-energy X-ray absorptiometry Dual-energy X-ray absorptiometry is a means of measuring bone mineral density (BMD). Two X-ray beams with different energy levels are aimed at the patient's bones. When soft tissue absorption is subtracted out, the BMD can be determined from the absorption of each beam by bone. Dual-energy X-ray absorptiometry is the most widely used and most thoroughly studied bone density measurement technology. Indications for DXA scan are: 1) To diagnose osteoporosis during screening. 2) To predict fracture risk in high risk patients. 3) To monitor therapy. Bone Mineral Content (BMC) in Grams and Area In cm2 “Areal” BMD is calculated in g/cm2 “T-score” compares the patient’s BMD with the young-normal mean BMD and expresses the difference as a standard deviation (SD) score. The following equation is used to calculate T-score: T score= Patient’s BMD – Young adult Mean BMD ---------------------------------------------------1 SD of Young adult Mean BMD “Z-score” compares the patient’s BMD with the age matched normal mean BMD and expresses the difference as a standard deviation (SD) score. The following equation is used to calculate T-score: T score= Patient’s BMD – age matched Mean BMD ---------------------------------------------------1 SD of Young adult Mean BMD Reference Database used for T-Scores are the following:  Caucasian (non-race adjusted) female normative database for women of all ethnic groups.  Caucasian (non-race adjusted) male normative database for men of all ethnic groups.  The NHANES III database should be used for T-score derivation at the hip regions. Which Skeletal Sites Should Be Measured? For every Patient: 1) Spine at L1-L4 2) Hip: Total Hip or femoral Neck Some Patients: Forearm (33% radius, 1/3 radius), of the non-dominant arm 1) If hip or spine cannot be measured (e.g. hip surgery, spinal deformity) 2) Hyperparathyroidism 3) Very obese - Use lowest T-score of these skeletal sites T or Z score? T-scores  Is the WHO diagnostic classification in postmenopausal women and men age 50 and older.  WHO classification with T-score cannot be applied to healthy premenopausal women, men under age 50, and children. Diagnosis based on DXA scan Normal Osteopenia Osteoporosis Severe Osteoporosis  T-score More than -0.1 0.1 to -2.5 Less than -2.5 T-score less than -3.5 or T-score less than -2.5 and Low trauma fracture Z-scores  For use in reporting BMD in healthy premenopausal women, men under age 50, and children. Diagnosis based on DXA scan Z-score more than -2.0 Z-score within the expected range for age Z-score less than -2.0 below the expected range for age  Possible problems that may influence results (false decrease or increase BMD): 1. Vitamin D deficiency: vitamin D deficiency causes defective mineralization of bone. This will give false low BMD reading. 2. Artifacts :this applies to anything seen in the scanners field of vision. This can occur due to digesting calcium pills (often seen in the stomach or intestines on the scan), back arthritis, vertebroplasty as well as with zippers on dresses. This will give false high BMD reading. 3. Patient weight: On very obese SPs, the fat pad of the belly can overlie the head of the femur, artificially increasing the BMD. 4. Anatomy :It is extremely important that the areas in the scan are the same as what were done previously. Small shifts can greatly change the measured mineralization. 5. Different machine :Results that are performed on different manufacture's machines or different machines from the same manufacturer cannot be compared accurately due to the significant variations in the technology. 6. Different location of the same machine :Machines that were moved into a different location can alter the results and make the comparisons inaccurate. 7. Different technician :Different people performing the scan on the same machine can affect the results. 8. Different positioning :Shift in the patient's position on the examination table can considerably change the results . What is the value of DXA scan in Osteoporosis diagnosis? In spite of being the gold standard test in osteoporosis diagnosis and screening, the information about fracture risk obtained from a DXA is only ~20% of the patient's total fracture risk . Remember that aim of screening is to find patients with the highest risk of fracture and not those with the lowest bone density. This is a fact because we have evidence that low bone density is not equivalent to increase fracture risk, so certain clinical risk factors must be used for proper assessment of fracture risk and this should be combined with BMD measurement. The NORA study (National Osteoporosis Risk Assessment) study found that 82% of patients with hip fracture had BMD of more than -2.5. In another study 54% of the 243 women with incident hip fracture were not osteoporotic at start of follow-up. This is because risk of fracture depends on bone strength which is a combination of bone density and bone quality. BMD scan measures bone density alone. There is no scan test for measuring bone quality which depends on a number of factors e.g. architecture, bone turnover and accumulation of damage. How frequent do you need to repeat DXA scan? In high risk patients with no fracture and normal DXA scan, repeat every 2-5 years. In a patient who is starting osteoporosis medication, repeat DXA scan in 1-2 years. In high risk patients with no fracture and osteopenia by DXA scan use the FRAX to predict the risk of absolute fracture in the following ten years and treat according to the recommendations. Follow with DXA every 1-2 years. How to compare DXA results on follow up? During follow up DXA scan, compare bone density with the previous one. If the change in BD exceeds the LSC or the least significant change(The smallest change in BMD that would exceed the error of the machine, hence would be considered clinically significant in determining the success or failure of therapy). In general LSC for most centers is about 4%. Dr Basmah Alwahhabi, MD, MRCP(UK), FRCP(UK), SCE(UK) Endocrine Consultant Prince Sultan Medical and Military Hospital References: Risk Factors for Hip Fracture in Older Home Care Clients. Wainwright SA, et al: J Clin Endocrinol Metab. 2005;90:2787-93. The effect of age and bone mineral density on the absolute, excess, and relative risk of fracture in postmenopausal women aged 50-99: results from the National Osteoporosis Risk Assessment (NORA). Siris ES, Brenneman SK, Barrett-Connor E, Miller PD, Sajjan S, Berger ML, Chen YT. Osteoporos Int. 2006;17(4):565-74. Epub 2006 Jan 4.