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Nov. 22, 1999 (Atlanta) -- Current screening tests in newborns for homocystinuria -- a condition that can result in mental retardation and other medical problems -- miss too many cases, according to a study in the Nov. 18 issue of the New England Journal of Medicine. The researchers recommend a change in the way doctors analyze the test results.
"If one lowers the cutoff level -- that is, increases the sensitivity of newborn screening for homocystinuria -- you identify cases that otherwise would be missed," co-researcher Harvey Levy, MD, tells WebMD. Levy is senior associate in the departments of medicine and genetics at the Children's Hospital in Boston and an associate professor of pediatrics at Harvard Medical School.
Homocystinuria is a rare disorder that affects only one out of every 200,000 infants, but if it is not detected early enough it can cause serious complications, including severe mental retardation, osteoporosis, and the formation of clots in blood vessels. The disorder is diagnosed by measuring the amount of the amino acid called methionine in the blood. The amount measured currently varies from state-to-state.
M. Judith Peterschmitt, MD, and colleagues reviewed the results of neonatal screening for homocystinuria over 32 years in New England. When blood methionine levels were at or above the established cutoff, additional specimens were analyzed to confirm the disorder.
"We might have expected to find an occasional case [of homocystinuria] but we didn't expect to find that almost all of the cases that have been identified since the cutoff level was lowered in 1990 would have been missed at the previous higher cutoff level," says Levy. "We thought one or two would have been missed, but we didn't realize that actually five out of seven would have been missed."
"Even very small rises in methionine might indicate the presence of the disease," says Levy. "These small rises would not be identified if the cutoff were too high. Most newborn screening programs are missing children with homocystinuria because of that [too high cutoff]. In fact, we think that many newborn screening programs have discontinued screening for homocystinuria because of that, when indeed, they didn't realize that they were missing these cases."
Even if the cutoff value is reduced, not all infants with homocystinuria will be identified, warn Peterschmitt and colleagues. They write that probably more than one in five affected infants will be missed.
Most states in the U.S. do not screen for homocystinuria, says Levy. "[Therefore,] if a child has any of those problems [such as skeletal abnormalities or vascular problems, parents] should suggest to the doctor that those problems may be due to a metabolic disorder such as homocystinuria, and I would urge the doctor to test the child," he says.
"There is a misconception among consumers as well as physicians that newborn screening covers all of the metabolic disorders. That is absolutely not true." He adds that if the child who has been tested is experiencing problems possibly connected with homocystinuria, the child should be tested again since the condition may have been missed the first time.
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