Guthrie test
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The '''Guthrie test''', also known as the '''Guthrie bacterial inhibition assay''', is a [[medicine|medical]] test performed on newborn infants to detect [[phenylketonuria]], an [[inborn error of metabolism|inborn error]] of [[amino acid]] [[metabolism]]. The test has been widely used throughout North America and Europe as one of the core [[newborn screening]] tests since the late 1960s. In recent years it is gradually being replaced in many areas by newer techniques such as [[tandem mass spectrometry]] that can detect a wider variety of [[congenital disease]]s. The Guthrie test is named after [[Robert Guthrie]], an American [[bacteriology|bacteriologist]] and [[physician]], who devised it in 1962.
==Theory and method==
The Guthrie test is a semiquantitative [[assay]] designed to detect elevated blood levels of the amino acid [[phenylalanine]], using the ability of phenylalanine to facilitate [[bacteria]]l growth in a [[microbiological culture|culture medium]] with an inhibitor.
A drop of blood is usually obtained by pricking the heel of a newborn infant in a [[newborn nursery|hospital nursery]] on the second or third day of life. The blood is collected on a piece of [[filter paper]] and mailed to a central laboratory. A small disk of the filter paper is punched out and placed on an [[agar|agar gel]] plate containing [[Bacillus subtilis]] and B-2-thienylalanine. Each gel holds 60-80 disks. The agar gel is able to support bacterial growth but the B-2-thienylalanine inhibits bacterial growth. However, in the presence of extra phenylalanine leached from the impregnated filter paper disk, the inhibition is overcome and the bacteria grow. Within a day the bacterial growth surrounding the paper disk is visible to the eye. The amount of growth, measured as the diameter of the colony, is roughly proportional to the amount of phenylalanine in the serum. The result is read by comparing the diameter of each sample disk's colony to the colonies of a series of reference disks with standard phenylalanine content included on each large plate.
==Interpretation of results==
The Guthrie assay is sensitive enough to detect serum phenylalanine levels of 180-240 μmol/L (3-4 mg/dL). In healthy normal people, phenylalanine levels are usually under 120 μmol/L.
When an elevated level of phenylalanine is detected, the laboratory notifies the infant's physician, who explains the result to the family and arranges testing to determine the cause of the high phenylalanine levels.
There are a number of causes of [[hyperphenylalaninemia]], and less than 10% of the positive results obtained by the screening program are confirmed as due to phenylketonuria (PKU). There are other metabolic diseases that can produce hyperphenylalaninemia, but [[Type I and type II errors|false positive]] results can also occur due to unexplained mild, transient elevations, [[prematurity]], [[parenteral nutrition]], or contamination of the filter paper specimen.
False negative results can be produced by antibiotics in the blood sample. Misleadingly low results can also occur if a sample is taken too soon after birth, since phenylalanine levels rise steadily with age and protein feeding.
==History==
Phenylketonuria (PKU) was one of the earliest recognized [[inborn error of metabolism|inborn errors of metabolism]]. In populations of European ancestry, it affects about 1 in 13,000 infants. In 1934, Følling discovered that restricting phenylalanine could improve the neurologic function of children with PKU, and within a few years it was established that a diet low in [[protein]] from infancy could prevent the [[mental retardation]]. Unfortunately, PKU was often not detected until significant brain damage had occurred. The [[ferric chloride test]] was unreliable in newborns and even by the 1950s there was no reliable, practical method for mass screening and detection of this disease before harm had occurred.
[[Robert Guthrie]] (1916-1995), a [[bacteriologist]] and [[physician]] at the [[Women and Children's Hospital of Buffalo]], [[New York]], reported in 1963 the initial version of this assay. Within a decade, newborn screening programs using this test were established in most of the [[United States]] and many other countries.
==See also==
*[[Newborn screening]]
*[[Phenylketonuria]]
*[[Hyperphenylalaninemia]]
==External links==
*PMID 14063511 Abstract of original report
[[Category:Microbiology techniques]]
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