The End of the Road for Lead Screening?

George W. Citroner

Disclosures

May 21, 2019

How Much Should We Worry About Lead?

Five years ago, the residents of Flint, Michigan, saw a startling change in their drinking water. It had turned brown, foul-smelling, and bad-tasting. Eventually we learned why. To save money, the city's water supply was switched to the highly corrosive Flint River, exposing the population (including 9000 children) to lead-contaminated drinking water for a year and a half. This wake-up call refocused the nation's attention on the problem of lead toxicity. Speculation that Flint was only the tip of the iceberg and that water supplies delivered through ancient pipes across the country could be similarly tainted prompted new questions about lead screening, particularly for young children and pregnant women.

Lead screening seeks to identify children with the heaviest exposure to this neurotoxin. Recent efforts to reduce lead exposure (deleading gasoline and paint, reducing lead in food, and educating the public about the hazards of lead) have been successful, yet lead remains a significant environmental hazard. Children in at least 4 million US households are exposed to unsafe levels of lead, and roughly a half million of those aged 1-5 years have blood lead concentrations higher than 5 µg/dL, the level now deemed to require public health intervention.

Lead Screening, Prevention, and Treatment

Against this backdrop, the US Preventive Services Task Force (USPSTF) updated its 2006 guidance on routine lead screening, examining current evidence on the benefits and harms of screening for and treating elevated blood lead levels among children and pregnant women.

An elevated blood lead level serves as a prompt for clinical monitoring, prevention, and treatment.[1] Historically, two methods have been used to identify those with elevated blood lead levels: screening questionnaires and blood lead testing.

The USPSTF found that traditional screening questionnaires and clinical tools fail to predict which asymptomatic children (aged 1-5 years) or pregnant women have elevated blood lead levels. Direct testing of the concentration of lead in the blood, however, accurately identifies those who have been exposed to lead. Capillary blood is the preferred medium for this (although it is associated with more false-positive results), and venous blood is used to confirm high capillary blood lead levels.

The USPSTF also reviewed studies on the prevention of lead exposure (counseling and nutritional interventions) and on chelation treatment to lower high blood lead concentrations. The evidence was inadequate to show any short-term benefits (eg, reduction in blood lead levels) or long-term benefits (eg, improved health outcomes) of these interventions. The bottom line: The USPSTF could not determine the balance of benefits and harms of lead screening among asymptomatic pregnant women and children younger than age 5 years.

How Much Lead Is Too Much?

Joseph F. Hagan, Jr, MD, clinical professor of pediatrics at the University of Vermont College of Medicine, often tries to stump his students by asking them to define a normal blood lead level. "They always fall for it and say under 5 μg/dL. I tell them that a normal lead level is actually zero; you shouldn't have any lead, which offers no physiologic benefit and can be harmful." He remembers that early in his medical career, a blood lead level below 25 μg/dL was considered acceptable. In subsequent years, this threshold was lowered to 10 μg/dL and then to the current level of 5 μg/dL.

The 5 μg/dL reference value was set using 2007-2010 National Health and Nutrition Examination Survey (NHANES) data and calculating the 97.5th percentile of blood lead levels in children. More recent (2011-2014) NHANES data indicate that the 97.5th percentile is now 3.5 µg/dL.[2] Accurately measuring such low blood lead levels is challenging for laboratories, but research suggests that even very low levels of lead, once considered innocuous, can permanently harm health and neurodevelopment.[3] The Centers for Disease Control and Prevention has not officially lowered the blood lead threshold to 3.5 µg/dL, but this step could be taken in the future.

The Status of Lead Screening Today

It has been suggested that the idea of lead screening has outlived its usefulness, and that our resources would be better spent on the primary prevention of environmental lead exposure.[4] However, the Flint water crisis was a reminder that unexpected sources of lead exposure can crop up at any time, and screening for elevated blood lead levels can uncover previously unrecognized sources of lead contamination in a community.[1]

Therefore, clinicians should be wary of misinterpreting the USPSTF's conclusion that the evidence is insufficient to balance the benefits and harms of screening, emphasized Michael L. Weitzman, MD, who said, "The updated recommendations should in no way discourage us from continuing to screen." A professor of pediatrics and environmental medicine at NYU Langone, Weitzman wrote an editorial[5] about the updated USPSTF lead screening recommendations, calling for more rigorous research on lead screening.

But until that happens, and given the irreversible effects of elevated blood lead levels in young children and pregnant women, other groups continue to recommend or require routine lead screening:

  • The American Academy of Pediatrics periodicity schedule for 2019 recommends that a risk assessment for lead exposure be performed at well-child visits at age 6, 9, 12, 18, and 24 months, and then yearly until age 6. Blood lead testing is done only if a risk assessment is deemed positive.

  • The American College of Obstetricians and Gynecologists does not recommend universal blood lead testing in pregnancy. However, because lead readily crosses the placenta and can be passed to an infant in breastmilk, a risk assessment for lead exposure should be performed at the earliest contact with pregnant or lactating women, and blood lead testing should be performed if a single risk factor is identified.

  • The Centers for Medicare & Medicaid Services (CMS) stipulates that all children enrolled in Medicaid must undergo blood lead screening tests (not just a risk assessment) at ages 12 and 24 months, as well as any child between ages 24 and 72 months with no record of a previous blood lead screening test. Some states are petitioning CMS to allow targeted rather than universal lead testing of children.[6]

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