Osteoporosis: Diagnosis and Managements _ _____________________________________________________
T-SCORE A T-score is the quantitative measurement of bone mineral density obtained by an examination, such as DXA, of the hip or other acceptable skeletal region. The score is the number of standard deviations from the mean (average) bone density for a young healthy adult. The exact age range used varies among authorities, but it is usually from 20 to 30 years of age. Z-SCORE Similarly, a Z-score is the number of standard deviations from the mean bone density for age-matched, sex-matched, and ethnicity-matched patients. For example, a woman 75 years of age with a Z-score of -1.0 is one standard deviation below the BMD of average women 75 years of age, but her T-score may be -3.0 because she is 3 standard deviations below the BMD of an average woman 30 years of age. Alternatively, an elderly patient’s T-score may be low, but average for her age by Z-score. For a young adult woman, the T-score and Z-score should be the same. For each standard deviation decrease in BMD, there is a doubling of fracture risk [18]. A patient with a T-score of -1.0 is twice as likely to sustain a fracture as someone with a T-score of zero; a patient with a T-score of -2.0 indicates a fourfold increase in risk of fracture, and so on. The WHO working group determined that patients with T-scores of at least -2.5, or 2.5 standard deviations below the young healthy mean, would meet the diagnostic criteria for osteoporosis. Those with T-scores from -1.0 to -2.5 would fall into the range for osteopenia ( Table 1 ). Statistically, a cutoff of one standard deviation below the mean would categorize roughly 24% of all women with osteopenia and around 1% with osteoporosis. (Note that these statistics assume a normal distribution of data.) The WHO criteria are easy to use for study inclusion criteria as well as epidemiologic data; however, individual patient decisions should not be based solely on a T- or Z-score. Just as total cholesterol is not the only risk indicator for coronary events, single quantitative measurements, like a T- or Z-score, must be combined with individual patient characteristics to make clinical decisions. Bone mineral density may account for 70% of bone strength; however, bone quality, the rate of bone turnover, and other architectural properties of bone (as well as genetics) play an important role in the development of osteoporosis and bone fragility [5; 21].
Although the WHO definition includes measurement of bone density at several possible sites, such as the spine, heel, or wrist, BMD measured at the hip, femoral neck, and lumbar spine is preferred by most authorities. There are slight variations in the degree of fracture risk with BMD measurements at the different sites (e.g., T-score at the hip correlates to greater fracture risk than the same T-score taken at the spine). If measurements are made at different sites, fracture risk is determined according to the lowest values obtained. It must be emphasized that the WHO BMD T-score diagnostic classification should be used with caution in men and children because established criteria are primarily based on an adult female population. The diagnosis of osteoporosis in these groups should not be made based on densitometric criteria alone; the International Society for Clinical Densitometry (ISCD) has recommended instead that ethnicity- or race-adjusted Z-scores be used [20]. EPIDEMIOLOGY As noted, an estimated 10 million individuals in the United States already have osteoporosis, and another 44 million have low bone density [1; 2]. According to data from the BHOF, 8.2 million American women 50 years of age and older have osteoporosis and 27.3 million are at risk of developing the disease [22]. The diagnosis of osteoporosis is important as a predictor of fracture. Osteoporosis results in more than 2 million osteoporotic fractures every year. This number is expected to double or triple by 2040 [23]. To fully understand the epidemiology of osteoporosis, one must examine the effects of race, gender, and age. ETHNICITY/GENDER Women are the most commonly affected population in the United States due to a lower peak bone mass and an accelerated bone loss in the postmenopausal period [3]. Osteoporosis is under-recognized and undertreated in African American women and is increasing most rapidly among Hispanic women [3; 5]. White and Asian women are at highest risk for osteoporotic fracture; African American and Hispanic women have a lower but significant risk [3; 5]. The National Osteoporosis Risk Assessment (NORA) study found that the fracture rates in postmenopausal Hispanic, African American, and Asian women were 91%, 54%, and 41%, respectively, of the fracture rates in white women [24].
WHO CRITERIA FOR DIAGNOSIS OF OSTEOPOROSIS BY T-SCORE
T-Score
Diagnosis
Equal to or above -1 Between -1 and -2.5 Equal to or below -2.5
Normal range
Osteopenia
Osteoporosis
Equal to or below -2.5 + fracture
Severe osteoporosis
Source: [19; 20]
Table 1
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