Many children and pregnant women may need to get new lead tests because one of the most common lab tests may have given falsely low readings, the Food and Drug Administration said Wednesday.
They may have mistakenly been given a clean bill of health when in fact they had dangerously high lead levels, the FDA said.
The blood tests were made by Magellan Diagnostics and it's the only FDA-approved test used in most doctor's offices, the FDA said.
Any adult or child who had blood drawn for a lead test since 2014 may have to be re-tested, the FDA said. Finger-prick tests are exempt for now.
It's a big issue because of the Flint, Michigan, water crisis, in which at least 100,000 people were exposed to dangerous levels of lead in their drinking water.
Studies have shown many U.S. public water supplies are contaminated by lead.
Related: Here's What Lead Can Do to You
"The FDA is deeply concerned by this situation and is warning laboratories and health care professionals that they should not use any Magellan Diagnostics' lead tests with blood drawn from a vein," said the FDA's Dr. Jeffrey Shuren.
"Children are particularly vulnerable to lead exposure due to the effect on their developing brains and organ systems," the Centers for Disease Control and Prevention said.
"Today at least 4 million households have children living in them that are being exposed to high levels of lead. There are approximately half a million U.S. children ages 1-5 with blood lead levels above 5 micrograms per deciliter (µg/dL), the reference level at which CDC recommends public health actions be initiated," it added.
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A blood test is the only way to know if a child has too much lead in his or her body. Lead poisoning does not cause obvious symptoms.
"No safe blood lead level in children has been identified. Lead exposure can affect nearly every system in the body," the CDC said.
FDA spokesperson Deborah Kotz said not everyone who got a lead test needs to worry immediately. "The FDA believes that most people will not be affected by this issue, as a majority of Magellan lead tests currently in use in the United States are conducted using blood obtained from a finger or heel stick," Kotz said.
But the FDA is also checking to make sure those tests are accurate. "All of Magellan's LeadCare tests can be used with blood obtained from a finger or heel stick," Kotz said.
Doctors who have given patients these test should re-test them, the CDC said.
"CDC recommends that healthcare providers re-test patients who are younger than 6 years of age … and had a venous blood lead test result of less than 10 micrograms per deciliter (µg/dL) analyzed using a Magellan Diagnostics' LeadCare analyzer at an onsite or at an offsite laboratory," the agency said.
"CDC also recommends that healthcare providers re-test currently pregnant or lactating women who had a venous blood lead test performed using a Magellan Diagnostics' LeadCare analyzer."
Lead kills developing brain cells and the consequences are permanent.
That can include lower IQ scores, poor school performance, inattention, impulsive behavior, aggression and hyperactivity. Studies increasingly link conduct disorder, delinquency, and criminal behaviors to lead toxicity.
Older children and adults are less vulnerable, but high levels of lead can also affect a mature brain and, in very high doses, prove fatal.
"Most children are tested for lead poisoning with a finger or heel stick, which is not impacted by today's warning," said Dr. Fernando Stein, president of the American Academy of Pediatrics.
"Parents of children age 6 and younger who have had a venous blood lead test, meaning blood was drawn from their arm, should talk with their pediatrician about whether their child needs to be re-tested."