The osteoporosis selfassessment tool

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Koh and colleagues (11) developed the original Osteoporosis Self-Assessment Tool for Asians (OSTA) based on a study of 860 non-Caucasian, postmenopausal women from eight Asian countries. Risk factors were captured from a self-administered questionnaire and bone density was measured by DXA in the proximal femur. Proximal femur T-scores were based on the manufacturer's reference data for Asian women. Statistical analysis was performed to determine which risk factors were independent predictors of BMD. The risk factors that were captured are listed in Table 8-9. These independent predictors were combined in a multivariable model from which risk factors were dropped one at a time until only statistically significant variables remained in the model. An index was developed from the variables in the final model to identify those women with a high probability of having a femoral neck T-score of -2.5 or less.

The final model included 11 variables that were significantly and independently associated with femoral neck bone density: age, weight, current estrogen use, current thyroid hormone use, any fracture after age 45, spine fracture after age 45, Chinese or Thai ethnicity, and being from Malaysia, Hong Kong, or Taiwan. Each of these variables was assigned a value based on the regression coefficient for that variable in the statistical model. The index values for all 11 variables were then added for each woman.

The sensitivity and specificity for an OSTA cutpoint of -1 were 95% and 47%, respectively. The AUC was 0.85. When thyroid hormone use and the three countries were dropped from the model, the AUC was still 0.83. Dropping Chinese and Thai ethnicity and current estrogen use lowered the AUC to only 0.80. Finally, dropping a history of any fracture or spine fracture after age 45 still resulted in an AUC of 0.79. This left only age and weight in the final index. Using an OSTA cutpoint of -1 with only age and weight in the model, the sensitivity was 91% and the specificity was 45% for a femoral neck T-score -2.5 or less in the development cohort. The two-variable index was validated in a cohort of 1123 Japanese women. A cutpoint of -1 resulted in a sensitivity of 98% and a specificity of 29% in this group.

The OSTA score is calculated by subtracting age in years from weight in kilograms, multiplying the result by 0.2 and truncating to an integer. For example, a 64-year-old

Table 8-10

Performance of the OSTA Index in the Development and Validation Cohorts by Risk Category

Table 8-10

Performance of the OSTA Index in the Development and Validation Cohorts by Risk Category

Index Score

Asian Development Cohort

Japanese Validation Cohort

n (%)

n (%) with T < -2.5

n (%) n (%) with T < -2.5

< -4

62 (8%)

38 (61%)

281 (25%) 123 (44%)

-1 to -4

417 (52%)

62 (15%)

562 (50%) 56 (10%)

> -1

318 (40%)

10 (3%)

280 (25%) 4 (1%)

Adapted with permission of the publisher from Koh LKH, Sedrine WB, Torralba TP, et al. A simple tool to identify Asian women at increased risk of osteoporosis. Osteoporos Int 2001;12:699-705. ©SpringerVerlag

Adapted with permission of the publisher from Koh LKH, Sedrine WB, Torralba TP, et al. A simple tool to identify Asian women at increased risk of osteoporosis. Osteoporos Int 2001;12:699-705. ©SpringerVerlag

Asian woman who weighs 56 kg would have an OSTA score of -1.7 Using age and body weight to calculate the OSTA score, the authors identified three categories of risk. The low-risk category included women with an OSTA score greater than -1. The women in the intermediate risk category had scores of -1 to -4 and the women in the high risk category had scores less than -4. Table 8-10 gives the total percentage of women in each group and the percentage with femoral neck T-scores of -2.5 or less for both the development and validation cohorts.

The OSTA index is easily transformed into a nomogram as shown in Fig. 8-1A. Women in the high and medium-risk category should be referred for bone density testing. It must be kept in mind that the cut point shown here is based on the original study of Asian women and is specific for predicting a T-score of -2.5 at the femoral neck. As will be seen, different cutpoints appear to be necessary if this index is to be used in other populations as well as for predicting different T-scores at other skeletal sites.

In 2002, Koh (12) reported the performance characteristics of the OSTA index in an additional group of294 normal Singapore women with an average age of 59. The OSTA index calculated for each woman ranged from -10 to 7. The original OSTA cutpoints of -1 and -4 were again used to establish high, moderate and low-risk categories. In this group, the index had a sensitivity of 90%, a specificity of 58% and an AUC of 0.82 for predicting a femoral neck T-score or -2.5 or poorer.

Koh (13) also attempted to utilize the OSTA index in 98 Asian men with an average age of 61 years. The calculated OSTA index values for these men ranged from -7 to 8. Using male Asian reference values to establish a T-score of -2.5 or poorer at the femoral neck, the original OSTA cutpoint of -1 resulted in a sensitivity of only 50% and a specificity of 78%.

Hochberg et al. (14) utilized the OSTA index to predict an osteoporotic femoral neck T-score in 140 Asian women who participated in the Fracture Intervention Trial in the United States. Utilizing an OSTA index cutpoint of 0s or less for the prediction of an osteoporotic femoral neck T-score based on reference data for Asian women, the sensitivity was 96% and the specificity was 37%. The positive likelihood ratio (LR+) was 1.52 and the negative likelihood ratio (LR-) was 0.12.

7 The OSTA score is calculated as follows: (56 - 64) x 0.2 = -1.6. This value is truncated to an integer resulting in an OSTA score of -1.

s Note that the original OSTA index cut point value for women was -1 or below, not 0 or below.

Osta Score

Fig. 8-1. (A) The OSTA nomogram for Asian women. (B) The OST nomogram for Caucasian women. The cells are shifted to the right, reflecting the effects of weight. Women in the high- and medium-risk categories should undergo BMD testing because of a sufficiently high probability of osteoporosis at the femoral neck. These nomograms are found on the accompanying CD-ROM. ©2001, Merck & Co., Inc., Whitehouse Station, NJ, USA. All rights reserved. Reproduced here with permission.

Fig. 8-1. (A) The OSTA nomogram for Asian women. (B) The OST nomogram for Caucasian women. The cells are shifted to the right, reflecting the effects of weight. Women in the high- and medium-risk categories should undergo BMD testing because of a sufficiently high probability of osteoporosis at the femoral neck. These nomograms are found on the accompanying CD-ROM. ©2001, Merck & Co., Inc., Whitehouse Station, NJ, USA. All rights reserved. Reproduced here with permission.

The OSTA index has also been utilized in Caucasian women. In this context, the name is shortened to the Osteoporosis Self-Assessment Tool (OST). Ben Sedrine and Reginster (15) reported the use of the OST index in 4035 Caucasian Belgian women with an average age of 61 and an overall prevalence of osteoporosis of 19%. Using the original OSTA index cutpoints of -1 and -4 to establish high-, moderate-, and low-risk

Table 8-11

Performance Characteristics of 154 lb (70 kg) Weight Criterion for Osteopenia and Osteoporosis at the PA Lumbar Spine and Femoral Neck

Table 8-11

Performance Characteristics of 154 lb (70 kg) Weight Criterion for Osteopenia and Osteoporosis at the PA Lumbar Spine and Femoral Neck

Site

Sensitivity

Specificity

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