What is the Guthrie Test?

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.