Newborns: Why is the PKU test done twice?

Can you please explain the reason why we test for PKU before a baby has even had a chance to consume any milk? A breastfed baby doesn't get milk until about the third or fourth day. Why not just wait a week and test once, rather than test in the hospital and again in two weeks? They say the first PKU test is not certain, isn't it because it is really to early to test?


Robert Steele

Robert W. Steele, MD, is a board certified pediatrician at St. John's Regional Health Center in Springfield, MO. He graduated from medical... Read more

What an outstanding question! You certainly have done your homework because indeed the PKU test done in the hospital is usually done when it is too early to test. So, of course, the question becomes "Why do we do it?" The answer is not as simple as it might seem. The screening test for PKU is a nice example of how public policy medicine clashes with individual medical care. Let me explain:

Every test we do in medicine has two properties, sensitivity and specificity. The sensitivity of a test is how well the test will pick up on the fact that the person actually has the disease. The specificity of the test is how well the test does at being negative when the person does not have the disease. Confused? Let me give you a couple of examples:

  1. A teenage girl fears she has been infected with the virus which causes mononucleosis. The doctor tells her she is going to perform a fancy new test to find out if she has mono or not. It happens that this new test has a very high sensitivity but a quite low specificity. The test comes back positive. Does she really have mono? The answer is maybe but maybe not. The test's sensitivity allows it to pick up every person who has mono but its lack of specificity means it will be falsely positive for a lot of people who actually don't have it. Her's might be a false positive.

    Now let's say the test comes back negative. Does she really not have mono? The answer is she probably does not have mono. Since its sensitivity is so high, if she had it, the test would have picked it up.

  2. A mom brings in her 7 year old son in to see the doctor for a sore throat. A new test for detecting strep throat has just come out. Its sensitivity is pretty low but its specificity is very high. The test comes out negative. Does he really not have strep? The answer is he actually might have strep but because the test's lack of sensitivity may have given a false negative result.

    Now, let's say it comes out positive. Does he really have strep? The answer is yes, he probably does. Since the test's specificity is so high, you can be assured that a positive result is specific for strep and not a false positive.

So, how does this all tie in with your PKU question? Well, while it would be nice to have a test which is very high both in specificity and sensitivity, it usually doesn't work out that way. Most tests that are high for both of these are rather expensive to do. The PKU test is a screening test meaning thousands of babies get this test. If it were expensive, the states would go bankrupt. Therefore, the state usually puts most of its emphasis on a test that is high in sensitivity at the expense of specificity. In other words, the state realizes there may be some babies that will have a positive PKU when really they don't have the disease. But they will catch most every baby that really does have the disease.

Misty, I hope I haven't totally confused the issue. But if I have, let me summarize the answer to your question. For most babies, checking a PKU within the first 48 hours is too soon for the very reasons you stated in your question. However, because of the high sensitivity of the PKU test, there are some babies with PKU who will have a positive test within 48 hours. Most states feel it is best to test everyone before they leave the hospital because unfortunately not all babies get their two week checkup. Since it is impossible to predict who will get their PKU recheck at two weeks and who won't make it to the doctor, the states choose to test everyone. There are some states that will allow the parents to waive the PKU test in the hospital. Therefore, parents should inquire about this option. But be sure to get the test done within two weeks of age and preferably on the fourth or fifth day of life.

I hope this helps.

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