Harris county birth certificate

A recent paper (Schreinemachers 2003) highlights both the opportunities and pitfalls implicit in the use of national public use datasets. Schreinemachers (2003) used the national linked birth and infant death certificate files for calendar years 1995-1997 to study the prevalence and risk factors for adverse perinatal outcomes in high and low wheat-producing counties of four northern Great Plains states. Although Schreinemachers did not clearly state the null hypothesis, she attempted to test the assertion that in utero exposure to agricultural herbicides is associated with birth defects, preterm delivery, and small-for-gestational age (SGA) infants. Vital records for residents of 147 rural, agricultural counties were classified into low-wheat and high-wheat areas based on agricultural production statistics, using the median split method. What Schreinemachers (2003) described as an ecologic analysis is perhaps better categorized as a cross-sectional study with a dichotomous exposure variable classified by study-subject residence in a high or low wheat-producing county. This means that neither the exposure (agricultural herbicides) nor the outcome (birth defects) was adequately measured. The study results and discussion emphasized the statistical analyses of associations with congenital anomalies. Schreinemachers' (2003) Table 3 shows no effect of residence in a high-wheat county on preterm or SGA birth, and a modest increase in male infant mortality due to congenital anomalies. Results comparing low or very low birth weight, or overall fetal or infant mortality outcomes between the two groups of counties were not provided. Most of the results focused on overall and subcategories of birth defects (termed "developmental outcomes" in the title of Table 3); although some of the odds ratios are statistically significant, the author made no adjustment for multiple comparisons. Epidemiologists studying birth defects typically avoid analyses in which "births with any anomaly" is the dependent variable (p. 1262) because of the heterogeneity of the conditions thereby grouped together. Schreinemachers (2003) also neglected to include a map of the study counties, leaving the reader to wonder whether other physiographic, demographic, or economic factors might also influence the study findings.

If the primary study hypothesis is that preconceptional or antenatal exposure to agricultural herbicides increases the risk for birth defects across large geographic areas, it should be noted that this study used, at best, proxy measures for both dependent and independent variables. Because Schreinemachers (2003) provided no direct measures of exposure to herbicides, the reader must presume that differences in agricultural activity across counties correlated directly to individual exposures. Schreinemachers (2003) could have provided a reference to a study demonstrating that all residents of agricultural areas have similar levels of biomarkers of exposure to herbicides.

The most troubling aspect of this paper (Schreinemachers 2003) is its reliance on vital statistics for data on the occurrence of congenital anomalies among the birth events analyzed. Although numerous state programs have been developed in the past 15 years, Schreinemachers selected a study area for which statewide, population-based birth defects surveillance programs did not exist in any of the four states during the study period (National Birth Defects Prevention Network 2002). Her arguments concerning data quality are unconvincing and ultimately raise concerns that the study findings will be misinterpreted and misconstrued, and also will lead to similar analyses with methodologic flaws based on assumptions about the completeness, accuracy, reliability, and validity of vital statistics reporting of birth defects. It is not surprising that results in this study mirror that of Garry et al. (1996), given the use of birth certificate reporting of congenital anomalies to measure the outcome variable in both studies.

Regarding data quality, Schreinemachers (2003) cited Watkins et al (1996), who examined the sensitivity and positive predictive value of birth defects reported on birth certificates in the metropolitan Atlanta, Georgia, area. They found that only 14% of all cases in the comprehensive multisource registry were reported on birth certificates, and that the overall sensitivity (proportion of all birth defects that were identified on birth certificates) was 28%, but ranged from 10% for rectal atresia/stenosis and 19% for Down syndrome to 40% for spina bifida, 47% for omphalocele/gastroschisis, and 86% for anencephaly. Rather than commenting directly on the implications of these results for the present analysis, Schreinemachers (2003) instead discussed the small proportion of birth certificates in the study sample that were unmarked for presence or absence of birth defects, a statistic that has no bearing on the issues of reliability and Validity of the key outcome measures. In a study of completeness of ascertainment in the New York Congenital Malformations Registry, Olsen et al. (1996) concluded that the yield from reabstraction of potential cases reported only on birth certificates was not sufficient compared to the time and expense involved. Many other studies have shown how poor vital statistics data are for identifying babies with birth defects, both in missed cases and in misdiagnosed and misclassified cases (e.g., Hexter etal. 1990; Hudome et al. 1994; Piper et al. 1993). For this reason, most birth defects epidemiologists exclude birth certificate-based cases from their analyses, and the National Birth Defects Prevention Network does not regard state databases based solely on vital statistics records as birth defects surveillance programs. Clinical data in vital statistics databases should not be used for multivariate epidemiologic analyses without independent assessment of its reliability and validity, lest scientifically erroneous conclusions be reached (Kirby 1997, 2001).

Schreinemachers' hypothesis (2003) is worthy of operationalization in a more sophisticated, albeit more expensive, study design. The study should be carried out in a region with comprehensive, multisource birth defects and disabilities surveillance records linked to vital records, with agricultural chemical applications cataloged and mapped through the use of a geographic information system. This may necessitate a large prospective study with a longitudinal component or the creation of the disease surveillance and environmental monitoring databases to support a retrospective study. In the meantime, researchers and clinicians should be wary of generalizing from the results of epidemiologic analyses based on birth defects reports obtained solely from vital statistics sources.

The authors declare they have no competing financial interests.

REFERENCES

Garry VF, Schreinemachers D, Harkins ME, Griffith J. 1996 Pesticide appliers, biocides, and birth defects in rural Minnesota, Environ Health Perspect 104:394-399.

Hexter AC, Harris JA, Roeper P, Croon LA, Krueger P, Gant D. 1990. Evaluation of the hospital discharge diagnoses index and the birth certificate as sources of information on birth defects. Public Health Rep 105:296-307.

Hudome SM, Kirby RS, Sennar JW, Cunniff C. 1994. Contribution of genetic disorders to neonatal mortality in a regional intensive care setting. Am J Perinatol 11:106-103.

Kirby RS. 1997. The quality of data reported on birth certificates [Letter]. Am J Public Health 87:301.

Kirby RS. 2001. Invited commentary: Using vital statistics databases for perinatal epidemiology: does the quality go in before the name goes on? Am J Epidemiol 154:889-890.

National Birth Defects Prevention Network. 2002. Birth defects surveillance data from selected states, 1995-1999. Teratology 66:S129-211.

Olsen CL, Polan AK, Cross PK. 1996. Case ascertainment for state-based birth defects registries: characteristics of unreported infants ascertained through their birth certificates and their impact on registry statistics in New York State. Paediatr Perinat Epidemiol 10:161-174.

Piper JM, Mitchel EF Jr, Snowden M, Hall C, Adams M, Taylor P. 1993. Validation of 1989 Tennessee birth certificates using maternal and newborn hospital records Am J Epidemiol 137:758-768.

Schreinemachers DM. 2003. Birth malformations and other adverse perinatal outcomes in four U.S wheat-producing states. Environ Health Perspect 111:1259-1264.

Watkins ML, Edmonds L, McClearn A. Mullins L, Mulinare J, Khoury M. 1996. The surveillance of birth defects: the usefulness of the revised US standard birth certificate. Am J Public Health 86:731-734.

Russell S. Kirby

Hamisu M. Salihu

Department of Maternal and Child Health

School of Public Health

University of Alabama at Birmingham

Birmingham, Alabama

E-mail: rkirby@uab.edu

Re: "birth malformations and other adverse perinatal outcomes in four U.S. wheat-producing states"