BMC Public Health 2004, 4 http://www.biomedcentral.com/1471-2458/4/16 susceptibility genes has only yielded weak linkages and inconsistent results [4-6]. This may be due, in part, to the limitations of current linkage strategies as well as our lack of understanding of the role of genetic polymorphisms in response to environmental factors. Several approaches (ie, linkage analysis, association stud- ies) can be used to identify disease causing or disease pre- disposing genes [7,8]. Most of them require, or their statistical power greatly benefits from, the use of "multi- plex" families (families with two or more affected mem- bers). While birth defects surveillance systems are adequate at identifying individual cases, they rarely collect family history. The objective of this study is to determine the utility of the National Health Interview Survey to iden- tify informative families for genetic studies of birth defects. Methods The National Health Interview Survey The National Health Interview Survey (NHIS) is a house- hold survey conducted by the U.S. Bureau of the Census annually since 1957. The NHIS focuses on the civilian, non-institutionalized population in the United States. Each year the NHIS randomly samples approximately 46,000 households with 116,000 members from 201 pri- mary sampling units nationally. In 1994 and 1995, a spe- cial two-year Disability Supplement was added to the NHIS to gather nationally representative data on the char- acteristics, service use, needs, circumstances and experi- ences of non-institutionalized people with disabilities in the United States. A subset of the approximately 90,000 households partici- pating in the 1994/1995 NHIS (about 28,000 house- holds) was asked about eligible birth defects including: spina bifida and hydrocephalus, other deformities of the central nervous system, congenital anomalies of the heart and circulatory system, cleft palate and cleft lip, other deformities of the digestive system, congenital dislocation of hip, other congenital anomalies of the musculoskeletal system, and others. Table 1 includes the ICD-9 codes of the conditions included in these categories. For the purpose of classifying the population by geo- graphic area, the states were grouped into four regions. These regions, which correspond to those used by the U.S. Bureau of the Census, are as follows: ▪ Northeast: Maine, Vermont, New Hampshire, Massachu- setts, Connecticut, Rhode Island, New York, New Jersey, and Pennsylvania. ▪ Midwest: Ohio, Illinois, Indiana, Michigan, Wisconsin, Minnesota, Iowa, Missouri, North Dakota, South Dakota, Kansas, and Nebraska. Table 1: Conditions reported in the four groups with birth defects extracted from the NHIS (1994/1995) Single Non-Famil- ial n* = 1,083 (%) Multiple Non-famil- Single Familial ial n = 115 (%) n = 113 (%) Multiple Familial n = 26 (%) Conditions Spina Bifida and Hydrocephalus Other deformities of the central nervous system Congenital anomalies of the heart and circulatory system Cleft palate and cleft lip Other deformities of the digestive system Undescended testicle Congenital dislocation of hip Other congenital anomalies of the musculoskeletal system Others ICD-9 Codes 741 (X71.9) 742.2, 4,5,8,9. 745 – 747 749, (X91.9) 750.2 – 9, 751 752.5 754.3 (X75.9) 754.0,1,756.4-9. (X20 – X29, X33-X35, X70, X73, X74, X76-X78, X93)** 744.4,748,752.0-4,6- 9,753.1-9,757,758.1- 9,759.0-6,8 (X30 – X32, X41, X79, X90)** *n: number of subjects. ** Includes the 4th digit .9 only. 43 (4.0) 10 (0.9) 225 (20.8) 37 (3.4) 25 (2.3) 4 (0.4) 11 (1.0) 573 (52.9) 155 (14.3) 5 (4.4) 2 (1.7) 18 (15.7) 5 (4.4) 0 (0.0) - 3 (2.6) 70 (60.9) 12 (10.4) 7 (6.2) 0 (0.0) 14 (12.4) 2 (1.8) 0 (0.0) - 1 (0.9) 79 (69.9) 10 (8.9) 0 (0.0) 0 (0.0) 7 (26.9) 0 (0.0) 1 (3.9) - 0 (0.0) 15 (57.7) 3 (11.5) Page 2 of 7 (page number not for citation purposes)