Ask Dr. Matt: Child Metabolic Disorder Screening

QUESTION: A friend of mine has a child who has a metabolic disorder that wasn’t diagnosed with routine newborn screening even though screening for the disorder does exist. How could that happen?

ANSWER: Newborn screening in the United States first began in the 1960s with a simple heel-stick blood test for phenylketonuria, a disease affecting approximately 1 in 25,000 newborns in the United States. With early restriction of the amino acid phenylalanine, children can lead perfectly normal lives; without the special diet they will likely develop severe mental retardation.

As a result of this screen, thousands of children have gone on to reach their full potential and live normal lives. Since the PKU test was first developed, there have been dozens of other tests created to identify disorders of metabolism, immune and endocrine dysfunction, many of which can be performed with the same simple test as the original PKU test.

So why doesn’t every state in the nation screen for all of these diseases?

Like many issues in public health, this excellent question has a very complex answer. While running a test may be simple enough, responding to the answer is a much more involved thing. Let me give a simple statistical demonstration — stick with me here.

Any test has a measure of its sensitivity — the test’s likelihood of being positive when a newborn has the disease — and its specificity — the test’s likelihood of being negative when a newborn does not have the disease. No test is perfect, but say we’re lucky enough to have one with 99 percent sensitivity and 99.9 percent specificity. That sounds excellent. But remember, we’re often searching for very rare abnormalities of about 1 in 100,000 newborns. On average, each year in the U. S. four million babies are born. So if we ran a screen for a 1-in-100,000 newborn disease, our test would identify 39 of 40 babies who had the disease (hooray for science!), but it would mis-identify 3,999 healthy babies as having the disease. These are known as false positives and create a lot of very worried parents. Studies show the effects of a false positive can be long-lasting, with parents continuing to perceive their child as abnormal and going on to utilize more health care for the rest of that child’s life. That cost, combined with the expense of ruling out a false positive, and including the quality-of-life cost for the parents (and they do have ways to calculate that) means it costs a ton — financially and emotionally — to prevent a disease that only affects 1 in 100,000 kids. Multiply this effect over the many different newborn screens that have been developed, and you can see there is a dilemma.

Those are the facts. But to the parent of a child with that 1-in-100,000 disease, the argument doesn’t seem very satisfactory and I would not even attempt to make it. That’s why I am relying on the data analyzed by The Newborn Screening Task Force formed in 1998, which has gone to painstaking lengths to develop a recommendation for universal newborn screening to be implemented in the U.S. It has identified 29 diseases to be included in a universal newborn screen.  This list of diseases includes errors of metabolism, endocrine disorders, various anemias and diseases such as cystic fibrosis. The prevalence of these diseases ranges from 1 in 1,000 to less than 1 in 100,000, which results in roughly 115,000 false positives as a result of doing all the screens.

Progress is being made toward implementing these recommendations and some states are closer than others. Washington is on its way with approval this May of 15 tests to be added to the 10 tests already being performed.

The March of Dimes has been a big supporter of newborn screening progress. Check out its Website at and search for the newborn screening section.

Matt Thompson is a pediatrician in Spokane.

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