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Keywords:

  • osteoporosis;
  • epidemiology;
  • population studies;
  • health services and economics;
  • menopause

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Reference

Osteoporosis public health measures are hindered by the inability to easily identify subclinical disease. We have now estimated state-specific osteoporosis prevalences using a simple formula (OST Index) to analyze age and weight of 62,882 older women; the prevalences determined are similar to those based on BMD. This new method has potential use for guiding implementation of osteoporosis prevention/treatment programs.

Introduction: Although osteoporosis-related fractures are a major U.S. public health issue, population-based prevention programs have not yet been developed. One contributing factor has been lack of a suitable screening test to detect asymptomatic high-risk individuals.

Materials and Methods: We estimated state-specific prevalences of postmenopausal osteoporosis using the Osteoporosis Self-Assessment Tool Index (OST Index; [self-reported weight in kg - age] × 0.2) to analyze data from 62,882 women ≥50 yr of age who participated in the 2002 Behavioral Risk Factor Surveillance System (BRFSS). The OST Index, designed to assess an individual's risk of disease, has previously been shown to have modest positive and high negative predictive value for osteoporosis defined by BMD criteria. Based on this index, women from each state were distributed among high-, moderate-, and low-risk OST categories. Calculated percentages for each category were weighted to U.S. Census Bureau population projections for 2002. By adjusting results to reflect previously validated percentages of women with osteoporosis in each risk category, we estimated the prevalence of postmenopausal osteoporosis in each state.

Results: Our calculated weighted prevalence estimates agreed closely with those of the National Osteoporosis Foundation derived from actual femoral neck BMD measurements obtained in the third National Health and Nutrition Examination Survey (1988-1994) and projected to U.S. census state population predictions for 2002. Comparison of unweighted BRFSS-OST results and NHANES BMD data revealed similar percentages of osteoporosis among all women ≥50 yr of age (BRFSS, 18.5%; NHANES, 18.0%; p = 0.47) and also among white women (BRFSS, 19.0%; NHANES, 20.0%; p = 0.28). However, the percentages of osteoporosis among blacks and Hispanics did not correspond, at least partly because of the lack of race-specific reference standards for BMD measurements and OST index ranges.

Conclusions: Analysis of readily available BRFSS data with the OST index formula is a simple, no-cost technique that provides state prevalence estimates of postmenopausal osteoporosis that could be used to guide allocation of resources to statewide osteoporosis prevention programs.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Reference

With an estimated prevalence of 10 million cases in 2002(1) and projected doubling of the population ≥65 yr of age by 2030,(2) osteoporosis is a significant public health challenge. In addition to excess morbidity and mortality caused by osteoporosis-related fractures,(3) the economic burden placed on our health care system is severe; direct expenditures in the United States for fracture care in 2005 were >19 billion dollars.(4) Despite the magnitude of this problem and broad availability of effective antifracture therapy,(5) population-based interventions have not yet been developed.(6) One important reason for this deficiency has been the inability to perform population-based screening of asymptomatic at-risk persons to identify the percent of the population at high risk for fracture. In this paper, we propose a new and simple solution to this problem.

The major consequence of osteoporosis is fractures, and one of the best predictors of fracture risk is reduced BMD.(7) However, DXA, the gold standard for BMD measurement,(8) is too expensive and inefficient for application to large populations.(9,10) As a result, several risk assessment tools have been proposed to distinguish high-risk individuals who are likely to have low BMD and who would therefore be the best candidates for BMD testing.(11) Each of these risk indices consists of a scorecard of two or more clinical risk factors (e.g., history of fracture after age 50, female sex, advanced age, low body weight, maternal history of hip fracture).(12-17)

The simplest screening tool to identify candidates for BMD testing, and therefore the most appealing for use in large populations, is the Osteoporosis Self-Assessment Tool (OST). The OST index,(14) an integral value based only on age and weight, is calculated using this equation:

  • equation image

In performing this calculation, the value of the OST index is truncated to an integral value by dropping numbers to the right of the decimal point. A recent review on OST has been published elsewhere.(18)

The World Health Organization (WHO) has defined osteoporosis as a BMD measured by DXA at the hip of ≥2.5 SDs below the mean for non-Hispanic white women, 20-29 yr of age,(19) and this is the reference standard to which OST indices have been compared in studies on American and European women.(20-23) Using the WHO definition, the OST index has been divided into three risk categories. Those at high risk have OST Indices of ≤−4 (the cut-point), those at moderate risk have indices ranging from −3 to 1, and those at low risk have the highest scores of ≥2.(20-23) The prevalence of osteoporosis is ∼60% for those in the high-risk group, ∼21% in moderate-risk women, and 4% or less for those in the low-risk category, and these were the prevalences applied to the risk categories generated in our study. As described in the Materials and Methods section, Asian women have lower cut-points.(14)

In this study, by applying the OST index to age and weight data from the Behavioral Risk Factor Surveillance System (BRFSS; see following paragraph), we estimated the state-specific prevalence of osteoporosis in women ≥50 yr of age. We compared these estimates to those published by the National Osteoporosis Foundation that were based on results of BMD measurements from the third National Health and Nutrition Examination Survey.(1)

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Reference

Program description

We estimated state and national prevalence of postmenopausal osteoporosis by calculating OST indices for women, ≥50 yr of age, who self-reported age and weight (as well as race) in the 2002 BRFSS.(24) The BRFSS is a random-digit-dialed telephone survey of noninstitutionalized adults, ≥18 yr of age. It is conducted annually by health departments in U.S. states, the District of Columbia, and territories, in collaboration with the Centers for Disease Control and Prevention (CDC). The results are in the public domain, accessible at http://www.cdc.gov/brfss/index.htm. In 2002, the median BRFSS survey response rate among U.S. states and territories was 44.5% (ranging from 25.2% in New Jersey to 79.3% in Puerto Rico).(25) We restricted our study to women, ≥50 yr of age, because most of the studies correlating OST indices with BMD have been performed in this high-risk group.(18)

We compared our weighted prevalence estimates to similar predictions reported by the National Osteoporosis Foundation (NOF).(1) The NOF estimates were derived from femoral neck BMD measurements performed on 3311 older women who had participated in the Third National Health and Nutrition Examination Survey (NHANES III, 1988-1994).(26) NHANES III is a national probability sample of the civilian noninstitutionalized population of the United States.(27) Of NHANES III eligible selected participants, 63% responded positively to having BMD measurements.(28) BMD respondents were more likely than nonrespondents to be non-Hispanic whites, to be physically active, and to have used hormone replacement therapy. However, these differences were reportedly minor and did not produce any major response biases affecting the BMD data.(28)

Because the NOF prevalence estimates were derived from NHANES III BMD studies, we compared (1) distributions of age and race and (2) frequency of postmenopausal osteoporosis across age and weight categories in the BRFSS and NHANES III BMD samples. Proportions of osteoporotic women in the BRFSS sample were calculated from OST indices; in the NHANES sample, they were determined from BMD measurements.

NHANES BMD data were analyzed using the same WHO diagnostic criteria for femoral neck osteoporosis as have been applied to OST-BMD correlation studies in white women (i.e., BMD measurement ≥2.5 SD below the mean for young non-Hispanic white women). Non-age-adjusted femoral neck measurements were used to determine the percentage of women by race (which was self-reported) with osteoporosis.

Data analysis

OST indices were calculated for all women of ≥50 yr of age who had self-reported age and weight data in the 2002 BRFSS. For this purpose, a categorical variable was created for the OST index formula with a coding scheme for low-, moderate-, and high-risk OST categories, as defined above. Weighted frequencies for the three OST risk groups were calculated by adjusting the sum of the unweighted BRFSS frequencies to reflect the state-specific populations projected for 2002 by the U.S. Census Bureau. State-specific weighted prevalence figures for postmenopausal osteoporosis were derived by adding together the number of women predicted to have osteoporosis within the weighted frequencies of each OST risk category (i.e., 60%, 21%, and 4% of women in high-, moderate-, and low-risk OST categories, respectively; see Introduction). These percentages apply to white but not to Asian women because of the latter's generally smaller body habitus. In that population, OSTA (OST for Asians) risk categories have necessarily been defined by lower cut-points (i.e., <−4 [high risk], −4 to −1 [moderate risk], and >−1 [low risk]),(14,29,30) and BMD measurements have been compared with a young-adult reference population of Asian women (rather than non-Hispanic white women).

To enable comparisons between the BRFSS and NHANES survey areas, weighted state-specific BRFSS data were modified to exclude Guam, Puerto Rico, and the Virgin Islands. The proportion of women with postmenopausal osteoporosis (by age and race) in the BRFSS and NHANES BMD samples were compared using two-sided t-tests. p < 0.05 was considered significant. All analyses of BRFSS data were performed using SAS Version 9.1.2 (SAS Institute, Cary, NC, USA).(31)

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Reference

Characteristics of BRFSS 2002 and NHANES III (1988-1994) cohorts

Table 1 shows the distribution of age and race of older women in the BRFSS (n = 62,882) and the NHANES III samples (n = 3311), together with the corresponding U.S. Census Bureau population estimates for 2002.(32) Table 1 presents unweighted data because, to our knowledge, the weighted data used by the NOF for their 2002 state-specific osteoporosis prevalence estimates (see below) are not publicly available. In general, the BRFSS race and age distributions were more consistent with those of the U.S. census.

Table Table 1.. Age and Race Distribution of Women ≥50 yr of Age in the BRFSS and NHANES III Samples Compared With U.S. Census Estimates for 2002
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Table 2 shows the proportion of women with osteoporosis, across age and race categories, calculated from OST categories in the BRFSS sample and from BMD measurements in the NHANES sample. There were no statistically significant differences between the calculated proportions either within the total sample (BRFSS, 18.5%; NHANES, 18.0%; p = 0.47) or for the sample of white women (BRFSS, 19%; NHANES, 20%; p = 0.28). However, there were significant differences between the proportions calculated for both blacks and Hispanics. There was no separate category for Asian women in the NHANES BMD data. However, we calculated the percentage of Asian women with osteoporosis in the BRFSS sample using two different sets of OST index ranges. With OST cut-points validated in whites, the proportion of Asian women with osteoporosis was 26.9% (95% CI, 23.4, 30.4). However, when using cut-points validated in Asians (OSTA),(14,29,30) the proportion fell to 15.8% (95% CI, 12.9, 18.7; p < 0.0001).

Table Table 2.. Frequencies of Postmenopausal Osteoporosis According to Age and Ethnic Group: A Comparison of BRFSS and NHANES III Samples
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Percentile ranking of states in OST risk categories

The 2002 BRFSS survey collected self-reported age and weight data from a sample of women in every state. Sample sizes varied widely among states, ranging from 476 for Alaska to 3625 for Pennsylvania. From these data, we calculated state-specific weighted percentages of subjects constituting the three OST risk categories (Table 3).

Table Table 3.. State-Specific Prevalence of Osteoporosis in Women ≥50 yr of Age, Using BRFSS, 2002
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Hawaii, the New England states, Arizona, and Florida ranked among the top quarter of states for both high-risk (9.9-14%) and moderate-risk (53.5-58.7%) OST index scores, suggesting that these states have the highest population prevalences of postmenopausal osteoporosis. Conversely, Texas, Louisiana, and Alaska shared the lowest percentile rankings for high- (2.7-7.1%) and moderate-risk (43.5-47.1%) OST index scores, indicating that older women in these states have relatively low risk for this disease.

Because we generated the values in Table 3 using OST Index cut-points for non-Asian women, and because Hawaii has a large Asian population, we thought it likely that the results for Hawaii were falsely elevated. Of 926 Asian women surveyed nationwide, 658 were in Hawaii (nearly one half of the 1542 Hawaiian women in the sample). Therefore, we performed a sensitivity analysis to determine the effect of subtraction of Asians from the national sample. Removal of Asian women caused the weighted percentage of Hawaiian women to drop from 14.0% to 7.7% in the high-risk category (from a ranking of 1st to 43rd) and from 57.3% to 47.9% in the moderate-risk group (from a ranking of 2nd to 42nd). California had the second highest number of Asian women (34 of 992 surveyed). However, omission of Asians from the California analysis had minimal effect on its ranking (data not shown).

The distribution of weighted frequencies of high-risk OST scores by state is shown in Fig. 1. Quintile 1 has the highest percentage of these scores, whereas quintile 5 has the lowest (Fig. 1). With the exceptions of Florida, Arizona, and New Mexico, there is a concentration of quintiles 1, 2, and 3 in the northern states.

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Figure Fig. 1.. Weighted state-specific BRFSS high-risk OST frequencies in decreasing quintiles: (1) 10.1-11.5%; (2) 9.3-10.0%; (3) 8.6 to <9.3%; (4) 7.9 to <8.6%; (5) 2.7-7.8%. OST score for Hawaii was calculated after subtraction of Asian women from the national sample. OST score for District of Columbia fell within the fifth quintile (data not shown).

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State-specific and national osteoporosis prevalence: comparison of BRFSS- with NHANES-dependent methods

Table 3 also compares the state-specific osteoporosis prevalence estimates derived in the current analysis with corresponding estimates published by the NOF.(1) The NOF estimates of state-specific osteoporosis prevalence were determined by applying the NHANES III BMD results by age and race(26) (these results are the NHANES percentages reproduced in Table 2) to the U.S. Census Bureau age-, sex-, and race-specific state population predictions for 2002.(1)

Our figures were similar to those derived by the NOF from the NHANES BMD measurements. The national prevalence for postmenopausal osteoporosis was estimated to be 7.3 million women when using estimates derived from BRFSS data and 7.7 million based on NHANES III BMD data. The similarity between these weighted prevalences is consistent with the fact that there was no statistically significant difference between the sample proportions shown in Table 2.

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Reference

The major significance of this report is that the OST index, a simple risk assessment tool developed to estimate the osteoporosis risk of individuals, can also be used to assess the risk burden for postmenopausal osteoporosis in large populations. Table 3 shows close agreement between our estimates of weighted state-specific osteoporosis prevalence and those of the NOF (the only other nationwide state-specific prevalence estimates currently available). As described above, the NOF estimates presented in Table 3 were derived from the NHANES III BMD data shown in Tables 1 and 2. Although the similarity between the two sets of estimates is reassuring, the NOF estimates cannot be considered a gold standard. One reason for this is that the age and race distributions of the NHANES BMD data, on which the prevalence estimates are based, differ from the population distributions estimated by the U.S. Census Bureau (Table 1). Another reason is that BMD studies in minorities, in contrast to whites, are of questionable validity because of a lack of race-specific normative databases (see below).

Support for the reliability of our approach is provided in Table 2, which presents the percentage of women with osteoporosis in different age and race categories determined from BRFSS and NHANES data. There were no statistically significant differences in the percentages for the total sample of older women or for the subgroup of white women. In contrast, the differences in percentages for blacks and Hispanics were statistically significant, but no definitive conclusion can be drawn concerning the differences in minorities. This is because, as with the NHANES BMD data involving racial subgroups, the BRFSS-OST findings are questionable because OST index ranges validated in whites had to be used to analyze the data of minority women (no race-specific OST cut-points were available for blacks or Hispanics).

A recent systematic review determined that the OST index ranges developed in studies focused on white women detected femoral neck osteoporosis, as determined by DXA, with a median sensitivity of 92% (range, 88-96%), meaning that only 8% of cases are missed. The specificity was 39% (range, 30-71%).(18) The review concluded that, in white women, the performance of OST was moderate in ruling out femoral neck osteoporosis (negative likelihood ratio 0.19; 95% CI, 0.17-0.21), and there were similar findings (but with greater heterogeneity) for Asian women using OSTA cut-points.(18) This same review concluded that the use of the OST index is more reliable in women ≥65 yr of age and less so in early postmenopausal women.(18) This may have been a factor in the differences between BRFSS and NHANES results regarding age in Table 2. A potential problem with BRFSS data is that the weight values are self-reported; these are often inaccurate (usually in the downward direction).(33)

Our results suggest geographical differences in osteoporosis prevalence among older U.S. women. As shown in the map in Fig. 1, states with the highest osteoporosis prevalence estimates (quintiles 1 and 2) are located primarily in the northern parts of the country. In contrast, with the exceptions of Florida, Arizona, and New Mexico, there is a predominance of states in southern regions with lower prevalence estimates (quintiles 4 and 5). At first glance, this geographical pattern would seem related to generally better bone health in the southern United States because of more extensive sun exposure and superior vitamin D status.(34,35) In addition, differences in the racial makeup of state populations (e.g., relatively large proportions of blacks who typically have relatively high BMD(36)) might also contribute to lower estimates of osteoporosis prevalence in the southeastern states.(37)

However, the apparent effects of geography may be more complex. On the one hand, there is evidence that bone health in the South may be poorer than expected despite increased sun exposure. BMD data from NHANES III revealed that the mean value for total femur BMD was slightly lower in the South (although sufficient data for this analysis were available only for non-Hispanic whites).(28) Also, the incidence of hip fractures is higher among women in southern states(38) (although there have been suggestions that hip fracture rates depend on the region of residence early in life(39)). On the other hand, there is a higher prevalence of obesity in southern states,(40) and it has been widely reported that greater weight is associated with higher bone mass (recall that weight is one of two variables in the OST index formula). This would explain the generally higher OST indices across the South.

Why then would hip fractures be more common in the South? One reason is that factors other than low BMD increase fracture risk, and lower socioeconomic status is a major contributor.(41) Another reason is that the relationship between body weight and BMD is less straightforward than previously thought because of emerging evidence that obesity (specifically, increasing fat mass), contrary to dogma, may actually be a risk factor for osteoporosis.(42-45) If these reports are confirmed, it could require reevaluating the correlations between OST indices and BMD for obese individuals as has been done already for ethnic groups such as Asians (see above).

Age, the critically important second variable in the OST index equation,(46) has a multifactorial association with the pathogenesis of osteoporosis. For example, increasing age is associated with estrogen deficiency, reduced cutaneous synthesis of vitamin D, decreased osteoblastic activity, and greater comorbidity, all of which contribute to the development of osteoporosis.(47) Therefore, one obvious explanation for the high frequency of postmenopausal osteoporosis in Florida is that the proportion of its population ≥65 yr of age is the largest in the country (nearly 18% in 2000 compared with a national average of 12.4%).(48) Alaska, at the opposite end of the age spectrum, had only ∼6% of residents in the ≥65-yr age group in 2000,(48) a demographic that presumably contributed to its distinction of having the lowest osteoporosis prevalence estimate in the United States (Table 3).

What are the effects of race and ethnicity? The agreement between BRFSS osteoporosis prevalence estimates and those derived from NHANES BMD data (Table 3) was likely favored by the white majority in the two study samples (Table 1). The reason, as mentioned above, is that both the NHANES III BMD measurements and the OST index ranges used in our calculations were based on white reference data. In contrast, the dramatic effect of using Asian-specific OST indices (OSTA) to estimate osteoporosis in an Asian population highlights the need for ethnic-specific OST index ranges. The current practice for measuring BMD by DXA in all women, regardless of race or ethnicity, is to use a young female white non-race-adjusted normative database as the reference standard.(49) The use of white BMD reference standards for Hispanics is probably less problematic than for blacks because Hispanic and white women have more similar BMD and fracture profiles.(36,50) However, it has been suggested that race-specific normative databases would be more appropriate.(51) To date, OST has been validated not only in white and Asian women, but also in a group of South American women.(52) One preliminary study has been conducted in black women.(17)

In future studies, the impact of geographical differences, race, and age, as well as multiple other factors (e.g., physical inactivity, smoking, alcohol consumption, arthritis, depression, health care access), on state-specific osteoporosis prevalence may be easily studied within the BRFSS dataset. Cross-tabulation of OST index findings with additional data obtained from the BRFSS core questionnaire and optional modules could provide information helpful for the development of targeted osteoporosis programs. Inclusion of self-reported age and weight in the BRFSS core questions is a guarantee that annual monitoring of changes in osteoporosis prevalence will be possible using OST index analyses.

The major value of combining OST with BRFSS is that it allows for the first time an easy and cost-free method for population surveillance of osteoporosis, a wide-spread chronic disease for which no practical surveillance method has been available. Surveillance data are crucial to guide the planning and implementation of public health programs for prevention and treatment of osteoporosis. Unfortunately, no federal funding is provided currently for collection of osteoporosis data in the BRFSS core questionnaire. Consequently, osteoporosis remains the one chronic disease of public health dimension that the BRFSS survey does not specifically address.

Despite this shortcoming, we showed that BRFSS core questions can still serve as a source of osteoporosis prevalence data by using the OST index to analyze self-reported age and weight data. The methodology involved is not only remarkably simple, but it is also free. Use of this technique to document state-specific osteoporosis prevalence would provide federal agencies and national organizations with the information necessary for allocation of funds to states and territories with the greatest need for bone health programs.(53)

Reference

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Reference