Hip Fracture Incidence among Elderly Asian-American Populations

American Journal of Epidemiology Copyright 1997 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved Vol. 146, No. 6 Printed in U.S.A. Hip Fracture Incidence among Elderly Asian-American Populations Diane S. Lauderdale, 1 Steven J. Jacobsen, 2 Sylvia E. Furner, 3 Paul S. Levy, 3 Jacob A. Brody, 3 and Jack Goldberg 34 This study estimated national age- and sex-specific nontraumatic hip fracture incidence rates for elderly Chinese Americans, Japanese Americans, and Korean Americans. Based on a 50 percent sample of 1992 Medicare enrollees with the race/ethnicity code "Asian" and "other," cohorts of persons with distinctive Chinese (n = 24,366), Japanese (n = 28,762), and Korean (n = 5,470) names were followed passively for 2 years for a hospitalization with a diagnostic code indicating hip fracture. Cohorts of whites and blacks were followed for comparison. Year of immigration was deduced from the year of issuance of the Social Security number. Age-adjusted hip fracture incidence was lower for all three Asian-American groups than for whites. For females, the standardized fracture ratio relative to whites was 30.1 for Chinese, 73.2 for Japanese, and 52.8 for Koreans; for males, the standardized fracture ratio was 41.9 for Chinese, 58.1 for Japanese, and 90.7 for Koreans. Persons whose Social Security numbers were issued after the Immigration Act of 1965 had an adjusted relative risk of 1.37 (95% confidence interval 1.05-1.78) compared with those in the US before that year, after adjustment for age, sex, and ethnic group. Am J Epidemiol 1997; 146:502-9. Asian Americans; emigration and immigration; hip fracture; names Rates of hip fracture vary widely, both internationally and within the United States. In populations throughout the world, risk increases with age, and females consistently have higher rates than do males (1-4). Hip fracture in the United States elderly has been described nationally using Medicare claims records from the Health Care Financing Administration (HCFA), which insures nearly all of the population aged 65 years and older. Significant variation has been observed by age, sex, geographic location, and season (5-10). Medicare data reveal that blacks experience consistently lower rates of hip fracture than do whites at all ages. To our knowledge, no national population-based studies have addressed the risk of hip fracture in the elderly Asian-American population, either in aggregate or by specific ancestry. Regional studies have Received for publication October 30,1996, and in final form June 20, 1997. Abbreviations: HCFA, Health Care Financing Administration; ICD-9-CM, International Classification of Diseases, Ninth Revision, Clinical Modification. 1 Department of Health Studies, University of Chicago, Chicago, IL. 2 Section of Clinical Epidemiology, Mayo Clinic, Rochester, MN. 3 Division of Epidemiology-Biostatistics, School of Public Health, University of Illinois at Chicago, Chicago, IL. 4 Vietnam Era Twin Registry, Center for Cooperative Studies in Health Services, Department of Veterans Affairs, Hines, IL. Reprint requests to Dr. Diane S. Lauderdale, Department of Health Studies, University of Chicago, 5841 S. Maryland Ave. MC 2007, Chicago, IL 60637. found low incidence of hip fracture among Japanese Americans in Hawaii and the heterogeneous Asian- American population of California (11, 12). Internationally, hip fracture incidence has been studied in Japan and Hong Kong (2, 12-15). In this study, we used augmented Medicare enrollment files to identify a cohort of Asian-American enrollees in 1992, and we followed them passively through their Medicare claims records to estimate hip fracture incidence rates. Hip fracture incidence among Chinese Americans, Japanese Americans, and Korean Americans is presented separately, and the effect of recent immigration on hip fracture risk is explored. MATERIALS AND METHODS Enrollment records Medicare enrollment records were used to identify the cohort for this study. Enrollment records include demographic data transferred from the agency that established individual entitlement, either the Social Security Administration or the Railroad Retirement Board. Data include name, date of birth, sex, a race/ ethnicity code, address, and month-by-month enrollment status, including health maintenance organization participation. We obtained from the HCFA two separate random samples of records from 1992 based on the race/ethnicity code: a 50 percent sample of persons with race/ethnicity codes Asian American and 502

Hip Fracture among Asian-American Elderly 503 other and a 10 percent sample of persons with race/ ethnicity codes white and black. Only persons who were age 65 years by the end of 1992 and who resided in one of the 50 states or District of Columbia were included. The Health Insurance Claim number was used to link records and to obtain date of death through 1993 and the enrollee's own Social Security number. Identification of Asian-American, white, and black cohorts The identification of persons with Asian ancestry in the Medicare enrollment files can be accomplished in one of two ways: either with the Asian-American race/ethnicity code or by surname. Elsewhere, we showed that the Asian-American code in the HCFA enrollment files is incomplete and biased, in that persons born in some Asian countries (e.g., Vietnam) are more likely to have the Asian code than are persons born in other countries (e.g., Japan) (16). In this study, we identify persons of Asian ancestry by surname from among those with race codes Asian American and other. We use three of the surname lists developed at the Census Bureau in the 1980s by Passel et al. (17): those for Chinese (150 names), Japanese (274 names), and Korean (77 names) surnames. The surname lists for Chinese, Japanese, and Koreans were matched to the 50 percent sample of persons with race code Asian and other. While some persons born in Asian countries have the race code white (16), the lists were not matched to the records for whites for two reasons. First, since the prevalence of Asian ethnicity is very low among records for whites, the likelihood of false positives is far greater in this file. Second, by omitting persons with the race code white, the chances of wrongly identifying non-asian females who have married Asian-surnamed males are reduced. Asian females who have married non-asian males are not generally identified by surname methodology, but this proportion is likely to be small for females born before 1928. For comparison, we used a 10 percent sample of persons with the race codes white and black. Cohorts of whites and blacks were formed by selecting persons resident in one of the 50 states or District of Columbia by race/ethnicity codes for whites and blacks and excluding those with Spanish surnames (18). While the Social Security number application (the source of the race/ethnicity code) has, since 1980, qualified the choices white and black with the phrase "not Hispanic," this was not done before 1980. Year of migration To explore possible differences in hip fracture risk between persons who immigrated later in life and those who were either born in the United States or who immigrated early in life, we use the year of issuance of the enrollee's own Social Security number. For immigrants, the date of the Social Security number application often approximates the date of immigration (19). The year of issuance can be deduced from the sequencing of the fourth and fifth digits of the Social Security number within each initial three-digit geographic number (20). The history of migration from Asia to the United States is a complex one and differs by country of origin. The 1965 Immigration Act established uniform quotas by country of origin, resulting in dramatically increased immigration from Asia. Additional categories of persons are exempt from these numerical limitations, such as refugees and those seeking asylum. With the exception of Japanese Americans, most elderly Asian Americans were born abroad. Prior to 1965, there were significant numbers of persons of Japanese or Chinese ancestry, but only a few with a Korean origin. Asian-surnamed persons with Social Security numbers issued since 1965 are presumed to have immigrated as adults. While this will misclassify some persons who were in the United States before 1965 but did not apply for Social Security numbers until after 1965 (when their age was at least 38 years), few persons would be misclassified because they received Social Security numbers before entering the United States. Identification of hip fractures All institutional inpatient claim records with a primary or an additional diagnostic code for fracture of the hip (International Classification of Diseases (ICD- 9-CM) codes 820.0-820.9) for the calendar years 1992 and 1993 were obtained from the HCFA. From these files, fractures were excluded when the other diagnostic codes indicated that the fracture could have been attributed to neoplastic processes (ICD-9-CM codes 170.7, 171.3, 198.5, 196.5-6, 213.7, and 215.3) (0.51 percent of fracture records), when multiple fractures suggested trauma (ICD-9-CM codes 800-819, 821-829, 860-869, 870-897, and 925-929) (7.03 percent of fracture records), or when diagnostic codes indicated late effects of fracture (ICD-9-CM codes 905.3 and v54.0-v54.9) (0.33 percent of fracture records) (8). Remaining hip fracture inpatient claims were linked to the enrollment records, with the date of the first claim record taken as the date of incident fracture. Statistical analysis Each member of the cohort was followed for hip fracture from the first month of 1992 in which the

504 Lauderdale et al. person was enrolled in Medicare; for most persons, this was January. Follow-up continued until the month of fracture or censoring due to death or study termination in December 1993. Persons enrolled in health maintenance organizations were excluded from the cohort because of potentially incomplete reporting of hospitalizations by health maintenance organizations paid on a per capita basis. Incidence rates and standard errors were calculated on the basis of ratio estimation from stratified random samples (21). Ratio estimation was used because both the numerators (hip fractures) and the denominators (Asian-surnamed Medicare enrollees) were subject to sampling variation. Age- and sex-specific and sex-specific indirectly age-adjusted rates were calculated for the three Asian-American groups and for whites. Age-adjusted rates were calculated by indirect adjustment with the white Medicare population as the standard. For assessment of whether occurrence of hip fracture was associated with immigration after 1964, proportional hazard regression was used (22). A dichotomous variable representing Social Security number issuance after 1964 was entered into a model with sex, 5-year age groups, and ethnic groups. The models were checked to ensure that there was no violation of the proportional hazards assumption. All analyses were performed using SAS 6.07 (SAS Institute Inc., Cary, North Carolina). RESULTS Cohort identification The 1992 enrollment file composed of 50 percent of the persons with race codes Asian American and other included 293,236 persons age 65 years or older. Of these, 246,879 had the race code other and 46,357 had the race code Asian. The numbers of persons identified by matching the file to the surname lists for Chinese, Japanese, and Korean are presented in table 1. The geographic distribution by state for each of the three Asian-American groups is presented in table 2. For comparison, the distribution of the corresponding populations aged 65 years and older in the 1990 Census for those stating a primary ancestry of Chinese, Japanese, or Korean is given. The correspondence in the geographic distribution is close for those with Japanese and Chinese ancestry and less so for Korean Americans. Hip fracture incidence After exclusion of those enrolled in health maintenance organizations, there were 346 hip fractures among persons with Chinese, Japanese, and Korean surnames in the 50 percent sample and 36,297 hip TABLE 1. Asian ethnic groups identified by surname in the 50% file of 1992 Medicare enrollees with race codes Asian and other Sex and age Female 65-74 75-84 85 Total Male 65-74 75-84 85 Total Chinese 7,614 3,700 1,232 12,546 7,656 3,388 776 11,820 Nationality Japanese 9,333 4,134 1,734 15,201 8,868 3,746 947 13,561 Korean 1,886 885 215 2,986 1,769 607 108 2,484 TABLE 2. Percentage distribution of Asian-surnamed Medicare enrollees in the race categories Asian and other, by state of residence, with the group aged 65 years and older in the 1990 Census giving the corresponding primary ancestry Ethnic group and state Chinese: US total California New York Hawaii Massachusetts Other states Japanese: US total Hawaii California Washington Other states Korean: US total California Hawaii New York Illinois Maryland Washington Other states Identified by surname (1992 Medicare)* No. 48,552 23,286 9,200 3,140 1,604 11,322 57,524 27,080 20,842 1,856 7,746 10,940 4,670 1,688 546 516 508 490 2,522 % 48.0 18.9 6.5 3.3 23.3 47.1 36.2 3.2 13.5 42.7 15.4 5.0 4.7 4.6 4.5 23.1 Self-Identified primary ancestry (1990 Census)t No. % 124,099 58,539 20,748 11,228 4,823 28,761 98,559 45,696 35,620 3,690 13,553 31,097 15,821 1,862 2,492 2,415 1,064 1,305 6,138 * The number of Medicare enrollees has been doubled for comparison to account for the 50% sampling, f Public use Microdata sample. 47.2 16.7 9.0 3.9 23.2 46.4 36.1 3.7 13.8 50.9 6.0 8.0 7.8 3.4 4.2 19.7 fractures among the 10 percent sample of persons with the race code white. Figures 1 and 2 present age-specific hip fracture incidence rates for Chinese Americans and Japanese Americans. Rates for whites and blacks are shown for comparison. Because the Korean-American sample is small and the number of fractures in some age-specific cells is fewer than five, age-specific rates are not presented for Korean Americans. The age-specific in-

35 30 25 Hip Fracture among Asian-American Elderly 505 z 20 15 ZL 10 -White -Black -Chinese American -Japanese American 65-74 75-84 85+ FIGURE 1. Incidence rates per 1,000 person-years for female Medicare enrollees in the United States with Chinese, Japanese, and Korean surnames and for whites and blacks by age group, January 1992 to December 1993. 20 15 10 65-74 75-84 85+ -White -Black -Chinese American -Japanese American FIGURE 2. Incidence rates per 1,000 person-years for male Medicare enrollees in the United States with Chinese, Japanese, and Korean surnames and for whites and blacks by age group, January 1992 to December 1993. cidence rates for the Asian-American groups are generally lower than those for whites. For each age and sex group, the rates for Chinese Americans are consistently the lowest of those for Asian groups. For men, the Chinese-American incidence rate is lower than that for blacks (figure 2). Indirectly adjusted rates (standardized fracture ratio) are presented in table 3. For both males and females, the Chinese-American age-adjusted rates are lower than those for the Japanese Americans, while both are lower than those for whites. Immigration year To explore whether risk varied by year of immigration, the year of Social Security number issuance for each enrollee was deduced from the sequencing of the number relative to 1965. The percentages of each Asian-American cohort whose Social Security numbers were issued before 1965 are shown in figure 3. These percentages are roughly consistent with the immigration history of each group, with almost all of the elderly Japanese Americans having numbers issued prior to 1965 and a majority of elderly Korean

506 Lauderdale et al. TABLE 3. Number of fractures in each sex/ethnicity group and indirectly adjusted rates (standardized fracture ratio) of hip fracture among Medicare enrollees with Chinese, Japanese, and Korean surnames relative to whites without Spanish surnames, based on 1992-1993 Medicare claims data Ethnicity by surname Fractures SFB» Female 95% Cl«Sex Fractures SFR Male 95% Cl Chinese Japanese Korean White 97 135 22 28,347 30.1 73.2 52.8 t 24.6-36.7 61.8-86.7 33.1-79.7 33 45 13 7,950 41.9 58.1 90.7 t 28.7-59.1 42.4-77.4 48.3-155.1 * SFR, standardized fracture ratio; Cl, confidence interval, t Referent. Chinesesurnamed 1965 or After a Before 1965 0% 20% 40% 60% 80% 100% FIGURE 3. Percentages of Asian-American cohorts with Social Security numbers issued relative to 1965 in the Medicare Enrollment Database for 1992. Americans having numbers issued in 1965 or later. The Chinese Americans are more evenly divided between the two periods. There is a trend toward higher risk of hip fracture among the more recent immigrants in all three ethnic groups (table 4). Proportional hazards models were constructed to quantify the risk of hip fracture by age, sex, ethnic group, and immigration period. In the model with all these variables, immigration from 1965 to the present was a significant addition (x* ~ 8-79, 1 df). Asian-surnamed persons with Social Security numbers issued since the 1965 Immigration Act are at a 37 percent increased risk of hip fracture relative to those with Social Security numbers issued earlier (95 percent confidence interval 1.OS- US). The addition of the immigration variable increased differences in relative risk between the Asian groups. Chinese persons were at a lower risk than those who were Japanese, and Koreans were at intermediate risk. The interaction terms between immigration year and the ethnic groups were not a significant addition to the model. DISCUSSION This study found that three cohorts of elderly Asian Americans defined by surname have lower rates of hip fracture than do whites. Moreover, Asian Americans who immigrated since 1965 are at greater risk than are Asian Americans who were in the United States before that year, whether born in the United States or immigrants early in life. Both before and after adjustment for immigration timing, Japanese Americans were at greater risk of hip fracture than were Chinese Americans. These hip fracture incidence rates are generally consistent with low rates of hip fracture previously reported for Asian populations both in Asia and in the United States. Hip fracture rates in Hong Kong for 1988-1989 have been compared with those for US

Hip Fracture among Asian-American Elderly 507 TABLE 4. Relative risk of hip fracture for Chinese-, Japanese-, and Korean-surnamed Medicare enrollees, based on 1992-1993 Medicare claims data Variable Sex Male Female Age (per 5 years) Ethnicity Japanese Chinese Korean Immigration year Before 1965 1965 or after RRt 1.00* 2.64 2.10 0.73 0.87 1.25 Crude 95%Clt 2.07-3.35 1.97-2.24 0.58-0.91 0.61-1.25 1.00-1.55 RR 1.98 2.03 0.62 0.79 1.37 Adjusted* 95% Cl 1.55-2.53 1.90-2.17 0.48-0.81 0.53-1.19 1.05-1.78 * Adjusted relative risks were obtained from a Cox proportional hazards model that included all variables listed, t RR, relative risk; Cl, confidence interval. $ Referent. whites, with rate ratios close to 0.5 (2, 13). Similarly, low rates of hip fracture incidence have been found in Japan as a whole and in Niigata Prefecture, Tottori Prefecture, and Okinawa (12, 14, 15). From Hong Kong, Singapore, and Japan, there is evidence that rates for Asians have increased over the past 3 decades (23, 24). In California, the heterogeneous Asian- American population had rate ratios of hip fracture relative to whites of 0.61 for females and 0.54 for men, based on 1983-1984 hospital discharge data (11). These rates are consistent with the standardized fracture ratios in this study. The California Asian population includes many ancestral groups; one of the larger ones, Filipinos, is not included here. A second regional study in the United States was of Japanese Americans in Oahu, Hawaii, from 1979 to 1981 (12). The hip fracture rate ratio for Japanese Americans to whites in Oahu was 0.45 for males and 0.48 for females. Migration studies often find that the rate of disease in the migrant population approaches that of the host country as length of time in the host country increases. That does not appear to be the case here, suggesting either that environmental factors in the United States have a different effect for Asian immigrants or that other factors are responsible for the large difference in risk between Asians and whites, with environmental factors modifying that risk. One process through which immigration status could affect hip fracture risk would be bone mineral density. Two studies have compared bone mineral density among US- and Japanborn Japanese Americans in Hawaii (25, 26). Both found that those born in the United States had higher bone mineral density. This is consistent with the established role for bone mineral density in hip fracture risk (27). Differences in bone mineral density by birthplace might also pertain to the Chinese-American and Korean-American populations. Consistent with our findings, one study (28) concurrently measured bone mineral density among residents of Taiwan, Japan, and Korea. Among females aged 60-69 years (the oldest group measured), bone mineral density at the lumbar spine and femoral neck were lowest for the Japanese, intermediate for the Koreans, and highest for the Taiwanese. There are several potential limitations to the data in the current study. Hip fracture ascertainment may be incomplete, and rates may be underestimated. This could be due to Medicare enrollees being hospitalized in the Veterans Administration system, nursing home patients not being admitted to the hospital, persons dying before hospitalization, or missing or inappropriately coded claims records. Estimates of positive predictive value and sensitivity for fractures ascertained from Medicare data have been described (29). On the basis of an algorithm using inpatient, outpatient, and physician claims, probable hip fractures had a positive predictive value of 98 percent and a sensitivity of 97 percent. Of these, only 5 percent were identified from noninpatient records. The present study only uses inpatient diagnostic codes, so some fractures will have been missed (30). However, the positive predictive value of probable fractures identified only by procedure codes in physician claims may be much lower than that of fractures identified from inpatient diagnostic codes (31). Unless the relatively small proportion of hip fractures not identifiable from inpatient claims are systematically related to racial/ethnic groups, such omissions should not bias comparative findings. A second potential limitation could be due to differences between cohorts identified by surname and enrollees with like ethnicity but whose names were not on the surname lists. The use of surnames for Asian identification depends on persons with distinctive surnames being representative of all persons with a particular ancestry. Shin and Yu (32) demonstrated that persons with the surname "Kim" formed like proportions (20-23 percent) among a range of Korean and Korean-American directories defined by occupations, residence, and institutional affiliations. On these grounds, they suggested that persons with a single surname could form a sampling frame for Korean Americans. To identify samples of other Asian groups or a larger sample of Koreans, one must assemble longer lists of distinctive surnames. Different strategies have been used to select names (33, 34). Passel et

508 Lauderdale et al. al. (17), at the Census Bureau, constructed surname lists from the 1979 Alien Address File. They selected a name when at least 80 percent of the occurrences were from the target country and when the name appeared a specified minimum number of times in the file. Rosenwaike (35) explored the feasibility of using these lists to draw samples from the Medicare enrollment files. He selected only the most commonly occurring of the names from the Chinese (16 names), Japanese (13 surname beginnings), Korean (five names), and Vietnamese (11 names) lists. As an indicator of whether the samples were representative, he compared the geographic distribution by state of the four ethnic groups with the distribution of the population aged 65 years and older of the ethnic groups in the 1990 decennial census and found generally good agreement. He also compared the distributions of the Medicare and Census populations by age and sex. However, since the Medicare participation rate varies by age and sex (30) and is unknown for these groups, deviations in the distribution of the Medicare population identified by surname and the Census ancestry groups may reflect enrollment rates. This study confirms Rosenwaike's conclusion that Asian-American cohorts identified by surname have a geographic distribution that parallels that of the 1990 Census. A third limitation, one that may affect generalizability of these cohorts, is the unknown proportion of more recent immigrants who have not accrued a sufficient number of Federal Insurance Contribution Act credits to be eligible for Medicare. This proportion probably varies by ethnicity. Validation of ethnicity is problematic. While one study validated Asian surname lists against racial classification on California death certificates (36), Sorlie et al. (37) found that only 82 percent of persons enumerated as Asian/Pacific Islander in the Census Bureau's Current Population Survey had death certificates that agreed on race. Thus, Asian classification on a death certificate is not a good standard for validation. For Japanese, place of birth as listed on the death certificate is not useful either, as most were born in the United States. For Chinese, place of birth is also a poor standard. Not only were many elderly ethnic Chinese born in the United States, but some were born in other Asian countries, such as Indonesia, the Philippines, and Vietnam. Finally, the most serious potential limitation in this study is the relatively small cohorts identified, particularly for Koreans. However, these estimates appear to fill a gap in knowledge of the occurrence of hip fracture for Koreans, either in the United States or in Asia. While confidence intervals are broad for all of the age-specific rates, the rates for Asian-Americans are clearly lower than those for whites. Additional follow-up would provide more stable estimates of rates. The etiology of hip fracture is multifactorial. Low bone mineral density and long hip axis length (the length from the lateral aspect of the greater trochanter through the femoral neck to the inner pelvic brim) independently predict increased fracture risk (27, 38). While Asians have lower bone mineral density than do whites (25, 28, 39), they also have shorter hip axis lengths than do white females (40-42). Consistent with lower bone mineral density, Japanese do have higher rates of vertebral fracture than do Japanese Americans and whites (43). Factors that affect bone mineral density, such as estrogen replacement therapy, may differ by Asian-American group or by migration status. Risk factors for hip fracture that are not related to bone mineral density are less well understood. Factors, both physical and environmental, that affect risk of falls in the elderly have been one focus of study. While there are no comparative data on rates of falls in white and Asian populations (44), one indication that these rates may differ is the proportion of the elderly living alone. Fewer Asian-American elderly (10 percent) than whites (25 percent) or blacks (27 percent) (45) live alone. In summary, this study has estimated risk of hip fracture among national cohorts of elderly Chinese Americans, Japanese Americans, and Korean Americans. The risk for Asian Americans is lower than that for whites. The suggestion of variation in risk by particular Asian ancestry and by immigration status may help to direct further study into the genetic and environmental determinants of risk of hip fracture. 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