Neonatal heel prick
The neonatal heel prick or Guthrie test is a screening test done on newborns. It consists in making a pinprick puncture in one heel of the newborn and soaking the blood into pre-printed collection cards known as Guthrie cards.
The classical Guthrie test is named after Robert Guthrie, an American bacteriologist and physician, who devised it in 1962. The test has been widely used throughout North America and Europe as one of the core newborn screening tests since the late 1960s. The test was initially a bacterial inhibition assay, but is gradually being replaced in many areas by newer techniques such as tandem mass spectrometry that can detect a wider variety of congenital diseases.
The blood samples can be used for a variety of metabolic test to detect genetic condictions, including:
- Phenylketonuria, a disorder where an error in amino acid metabolism can impair brain development (PKU)
- Thyroid stimulating hormone (TSH) to detect congenital hypothyroidism and hence prevent cretinism.
- Immunoreactive Trypsinogen (IRT) to detect cystic fibrosis.
- Medium-chain acyl-coenzyme A dehydrogenase deficiency (MCADD)
- Sickle-cell disease 
- Biotinidase deficiency
- Congenital adrenal hyperplasia
The test uses the growth of a strain of bacteria on a specially-prepared agar plate as a sign for the presence of high levels of phenylalanine, phenylpyruvate, and/or phenyllactate. The compound B-2-thienylalanine will inhibit the growth of the bacterium Bacillus subtilis (ATCC 6051) on minimal culture media. If phenylalanine, phenylpyruvate, and/or phenyllactate is added to the medium, then growth is restored. Such compounds will be present in excess in the blood or urine of patients with PKU. If a suitably-prepared sample of blood or urine is applied to the seeded agar plate, the growth of the bacteria in the test will be a positive indicator for PKU in the patient.
To prepare the sample for application, a small amount of blood (from a heel puncture, for example) or urine (from a diaper, for example) is applied to a piece of filter paper. Then a small disc is punched from the center of the spot of blood or urine, and the disc applied to the surface of a seeded, minimal-medium agar plate that contains added beta-2-thienylalanine. If the sample contains phenylalanine, phenylpyruvate, and/or phenyllactate then these compounds will diffuse into the agar medium. If their concentrations are high enough (as with the excess levels seen with PKU), bacteria will grow under the disc, but not elsewhere. Generally an overnight incubation is enough to determine whether phenylalanine, phenylpyruvate, and/or phenyllactate are present in unusual concentrations in blood or urine.
It is recommended that the screening test be performed when the infant is between 48 and 72 hours of age. False positives and negatives can sometimes occur when the screening tests are performed before 48 hours.
With genetic tests becoming more common, a wide variety of tests may use the blood drawn by this method. Many neonatal units (SCBUs) now use this method to carry out the daily blood tests (blood count, electrolytes) required to check the progress of ill neonates.
- Heel stick wound
- Newborn screening
- Dried blood spot testing
- Guthrie Cards, Catalyst (ABC1), 29 May 2003.
- Julia A. McMillan; Ralph D. Feigin; Catherine DeAngelis; M. Douglas Jones (1 April 2006). Oski's pediatrics: principles & practice. Lippincott Williams & Wilkins. pp. 162–. ISBN 978-0-7817-3894-1. Retrieved 16 April 2010.
- , University of Illinois Chicago, 09 September 2013.
- Newborn screening guidelines, Government of Victoria, 13 December 2007.