Elevated Blood Lead Levels and Blood Lead Screening Among US Children Aged One to Five Years: 1988–1994

DISCUSSION

This analysis indicated that ∼10% of US children 1 to 5 years of age had received blood lead testing before their participation in NHANES III in 1988–1994. Presumably, virtually all of these tests were screens or follow-ups of screens among asymptomatic children, because symptomatic lead poisoning is rare in the United States. This result is a lower estimate than that found in a 1994 national survey in which 24% of parents interviewed reported that their children had undergone lead screening tests.¹⁸ Two factors may have contributed to this discrepancy. One is that screening rates may have risen over the 1988–1994 period in response to the universal screening recommendations issued by CDC in 1991 and by the American Academy of Pediatrics in 1993.^4,9 If this is the case, then the response to the recommendations was greater in later years, because the NHANES III data did not show a significantly higher screening rate in phase 2 (1991–1994) than in phase 1 (1988–1991). Unfortunately, it was not possible to analyze the NHANES data by year of interview to assess whether this was the case.

A second possibility is that the estimates were affected by selective nonparticipation in one or both surveys. The response rate for the 1994 telephone survey was 56%, compared with over 90% for NHANES III.^15,18 It is possible that parents who chose to participate in the 1994 survey were more likely than nonparticipants to have children who were screened for lead or more likely to remember previous blood lead testing of their children.

Blood lead screening is generally conducted in a pediatrician's office as part of a routine, well-child visit. Because blood may be drawn at these visits for routine laboratory analyses and anemia testing as well as for lead testing, parents may not always be aware that a lead test is being performed. Thus, the estimates of testing prevalence found in this study, as well as in the 1994 telephone survey, may have been biased downward. However, a review of Medicaid claims records for 1994–1995 indicated a screening rate similar to that reported in NHANES III.¹⁹

The estimated prevalence of elevated blood lead levels may also be slightly inaccurate because blood lead data were missing for 21% of NHANES III participants 1 to 5 years of age. The primary reason for missing data was parental refusal of the blood draw.¹⁵Data were more likely to be missing for children living in the Northeast region of the United States and for younger children, both of whom tend to have higher blood lead levels, and for phase 1 recipients, who were sampled when blood lead levels were higher. Thus, the prevalence estimate may have been biased downward. However, data were also more likely to be missing for children who were sampled in the fall and winter months, for those who were not anemic, and for those who had an annual household income ≥$20 000. Blood lead levels were lower in the winter and spring, and both anemia and lower income were significantly associated with elevated blood lead levels on the bivariate analyses. Thus, the overall direction or degree of any bias is unclear.

Correlates of an elevated blood lead level in this analysis included being 1 to 2 years of age, being of minority race/ethnicity, having a lower income, and living in an older home, all of which are well-known risk factors whose relationships to elevated blood lead levels have been demonstrated in previous analyses of NHANES III data.^1,5,6 Living in the Northeast was also shown to be a risk factor in a previous NHANES III analysis.⁵ Season did not predict continuously modeled blood lead concentrations in children in another NHANES III analysis but has been shown to predict blood lead levels elsewhere.^6,20 Note that because NHANES III physical examinations were generally conducted in the northern United States during the warmer months and in the southern United States during the cooler months, the increased risk associated with residence in the Northeast and Midwest regions of the United States might be overestimated if the categorical coding for season incompletely controlled for the NHANES III examination schedule.

Being on Medicaid, not having a regular doctor, living in a household whose head has less than a high school education, and having a history of anemia were also independent risk factors in this analysis but were not evaluated in the previous NHANES III studies. Anemia has previously been shown to be correlated with elevated blood lead levels.²¹ Being on Medicaid is a close correlate of poverty, another well-known risk factor for lead exposure that probably reflects the condition of the child's home. Heads of household with less education may be less aware of lead hazards and, therefore, less able to protect their children, or lower education may be a proxy marker of lower socioeconomic status and poorer housing. Similarly, having a regular doctor may be a proxy for higher socioeconomic status or a more orderly lifestyle in which children are less likely to be exposed to lead hazards.

Of the correlates of blood lead testing, many were also risk factors for elevated blood lead levels in this analysis (being of minority race/ethnicity, living in an older home, living in the Northeast or Midwest regions of the United States, being on Medicaid, having a head of household with less than a high school education, and having a history of anemia). Of the remaining correlates of previous blood lead testing in this analysis, having a female head of household has been shown to be related to the likelihood of elevated blood lead levels in other studies.²² Increased residential mobility has also been shown to be a risk factor for elevated blood lead levels,²³ but in this analysis a greater number of moves was associated with a decreased likelihood of being screened for lead exposure. Children with parents who spoke a language other than English at home were less likely to have received blood lead testing, perhaps reflecting less access to health care among these families.

In summary, these findings indicate that children with known risk factors for elevated blood lead levels were more frequently screened. However, among children with elevated blood lead levels, only one third had been tested previously. Clearly, many practitioners did not heed CDC's 1991 call for universal screening. Several surveys of pediatricians were conducted in the early 1990s to determine screening practices. The proportion of respondents reporting that they universally screen patients ranged from 12% to 75%, with higher proportions found in later years.^24–27 The proportion reporting that they never screened patients ranged from 4% to 25%, and the proportion that screened selected patients ranged from 21% to 63%. These studies indicate that universal screening was more commonly practiced by pediatricians with more recent training, those who practiced in academic or public clinic settings, those who had a greater proportion of minority or Medicaid patients, those who believed the benefits of screening outweigh the costs, and those who reported a high prevalence of elevated blood lead levels among their patients.^24–26 Among selective screeners, indicators for screening were generally in adherence with the 1991 guidelines of the CDC and included such factors as living in or regularly visiting an older home, having a history of pica, being enrolled in Medicaid, having a parent who uses lead for work or for a hobby, and having a sibling with an elevated blood lead level.^4,24,25

Childhood lead poisoning prevention programs that are attempting to devise plans for reaching high-risk children should focus on those risk factors that can identify the greatest numbers of such children and can be assessed easily by pediatric practitioners. The candidate risk factors evaluated in terms of absolute numbers of high-risk children identified in this analysis were Medicaid status, race/ethnicity, and year the home was built. (Region of the country was not evaluated despite being a strong risk factor because of the possible bias introduced into the estimate by the survey design.) Among the factors evaluated, Medicaid status identified the greatest number of high-risk children by far. Although Medicaid-eligible children represented 30% of the NHANES III, phase 2 sample, they represented 60% of the population with elevated blood lead levels (535 000/890 000) and 93% of the population with blood lead levels ≥20 μg/dL (68 000/73 000; data not shown), the level at which children require clinical management.⁸ In addition, since 1992 the Health Care Financing Administration has required that all children receiving Medicaid benefits be tested for lead exposure.²⁸ However, among NHANES III, phase 2 (1991–1994) Medicaid-eligible participants 1 to 5 years of age, 81% did not receive blood lead screening.²⁹ Nonacceptance of federal screening policies and lack of oversight within state Medicaid programs have been identified as 2 reasons why many Medicaid-eligible children do not receive screening.¹⁹

Among the 355 000 NHANES III, phase 2 children with elevated blood lead levels who were not Medicaid-eligible, 49% (175 000) lived in homes built before 1946 and 24% (85 000) were black, non-Hispanic. Thus, focusing on the age of the child's home may be more useful than focusing on the child's race for identifying at-risk children among those who are not Medicaid-eligible. Year of housing construction has been also recommended as a factor on which health departments should base screening recommendations for neighborhoods.⁸ In addition, physicians can ask parents about the child's home during individual examinations. Not all parents know when their home was built (8% of respondents in this analysis did not know); the conservative approach for physicians would be to screen all such children.

The ideal approach to childhood lead poisoning is primary prevention by removing exposure sources, and, in fact, the prevalence of elevated blood lead levels among children 1 to 5 years of age has dropped 95% since the late 1970s because of the removal of lead from gasoline, food and drink cans, house paint, and plumbing.^1,6,30 However, remaining lead sources, such as deteriorating house paint, must be addressed on an individual, rather than on a population, level. Therefore, secondary prevention through screening to identify exposed children, followed by environmental remediation and treatment, remains important. Physicians should screen Medicaid-eligible children and should follow health department recommendations about identifying and screening other at-risk children. In areas where no health department guidelines exist, physicians should either screen all children or screen based on known risk factors.