What is the Guthrie Test?

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  • Written By: Emma Lloyd
  • Edited By: Bronwyn Harris
  • Last Modified Date: 22 September 2019
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The Guthrie test is a blood test which is carried out on newborn babies. The test is used to detect an inherited genetic disorder called phenylketonuria. The disorder is caused by a defect in the metabolic pathway which breaks down the amino acid phenylalanine. If untreated, phenylketonuria can cause severe intellectual disability, and early detection via this test is vital.

Also known as the Guthrie bacterial inhibition assay, the Guthrie test is widely used throughout Europe, North America, and Australasia. In some countries the test is used to screen for other diseases in addition to phenylketonuria. Additional conditions that may be tested for include cystic fibrosis, congenital hypothyroidism, and sickle cell anemia. Each of these additional conditions is screened for using a different method, but the same blood sample can be used for all of the tests.

The Guthrie bacterial inhibition assay was developed in the 1960s by an American physician and microbiologist named Robert Guthrie. The test was initially devised to screen for the presence of phenylketonuria in newborn babies. When the presence of this metabolic disorder is discovered soon after birth, the affected infant can be protected from intellectual disability and other side effects with a low-protein diet that restricts phenylalanine intake.


The Guthrie test detects increased levels of phenylalanine in the blood through the use of a bacterial culture which requires phenylalanine to grow. A blood sample is taken for this test on the sixth or seventh day after birth. It is important to wait several days after birth before taking the blood sample to ensure an accurate reading.

The blood sample is incubated with a bacterial species called Bacillus subtilis and a chemical called B-2-thienylalanine, which inhibits the growth of the bacteria by acting as a phenylalanine agonist. This means it prevents the bacteria from feeding off the phenylalanine available in the culture medium. A blood sample from a baby with phenylketonuria provides enough extra phenylalanine for the bacteria to grow despite the presence of the B-2-thienylalanine agonist.

Therefore, a positive Guthrie test for phenylketonuria is one in which the bacterial culture is able to grow. When the bacterial culture cannot grow, the test is negative. When a positive test result is obtained, the baby’s physician is contacted. The physician then arranges for further testing to determine why phenylalanine levels are elevated. This is an important consideration, as phenylketonuria accounts for less than 10% of positive Guthrie test results. For example, false positive results can be obtained from infants born prematurely, or who have been treated with antibiotics in the week after birth.


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Post 3

@anamur-- The heel prick test and the Guthrie test are the same thing. It's also called a PKU test, which stands for phenylketonuria test.

It's one of the components of the newborn screening which also includes a hearing test and a physical examination. The blood sample for the Guthrie test is taken by pricking the newborn's heel and taking a very small amount of blood (a few drops) onto a card. That's why it's often referred to as that.

Most hospitals will use this sample to check for all of the conditions the article already mentioned. If any levels are elevated, they will request a new test. @feruze is right in that these samples can easily be contaminated. That's why the test has to be repeated multiple times to confirm the results.

Post 2
@anamur-- Yes, we received a similar call with my son after newborn screening and also went in for a second test. Thankfully, the results of the second test came out normal.

Please don't jump to conclusions, because like the article said, false positives often happen with this test. My son had no reason to show a false positive and the doctor said that his blood sample probably got contaminated before it was read.

Contamination of blood samples are common and sometimes mistakes happen at the hospital. They could lose the sample or it might get broken by accident and the staff wouldn't want to admit it.

I hope that all will come out well with your daughter's second test. And even if it doesn't, the disorder is preventable / treatable with a dietary change. I know that we all want our children to be perfectly healthy, but there are much worse disorders out there.

Post 1

I got a call from my newborn's doctor today, that they need to redo the Guthrie blood test for my daughter because her phenylalanine levels read slightly above normal. So we are going in tomorrow for another test and if it reads high again, she might have phenylketonuria. I'm so scared and apprehensive, I really hope that the levels will not come out high again.

Has anyone had a similar experience with their son or daughter? How many times did you have to get the Guthrie test done to know for sure that the baby has or doesn't have this disorder?

The nurse I talked to on the phone also mentioned something about a "heel prick" test. What's that? Will my daughter have to get this test too?

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