Neonatal heel prick
The neonatal heel prick is a blood-collection procedure done on newborns. It consists of making a pinprick puncture in one heel of the newborn to collect their blood. This technique is used frequently as the main way to collect blood from neonates. Other techniques include venous or arterial needle sticks, cord blood sampling, or umbilical line collection. This technique is often used for the Guthrie test, where it is used to soak 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 in 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.
Detected diseases
The blood samples can be used for a variety of metabolic tests to detect genetic conditions, including:- Immunoreactive trypsinogen to detect cystic fibrosis.
- Maple syrup urine disease a rare disorder where an error in metabolism inhibits the breakdown of amino acid leucine, isoleucine and valine. It can impair brain development.
- Medium-chain acyl-coenzyme A dehydrogenase deficiency
- Phenylketonuria, a disorder where an error in amino acid metabolism can impair brain development
- Sickle-cell disease
- Thyroid stimulating hormone or Thyroxin to detect congenital hypothyroidism and hence prevent cretinism.
- Isovaleric acidemia
- Homocystinuria
- 17-hydroxy-progesterone to detect adrenogenital syndrome, also known as congenital adrenal hyperplasia
- Galactosemia
Mechanism
To prepare the sample for application, a small amount of blood or urine 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, 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.
Timing
The blood spot sample should be taken between 48 and 72 hours of age for all babies regardless of medical condition, milk feeding and prematurity. For the purpose of screening, date of birth is day 0. False positives and false negatives can sometimes occur when the screening tests are performed before 48 hours.When the immunoassay method is used as a screening method for quantifying 17α-hydroxyprogesterone in dried blood spots, it exhibits a significant rate of false positive results. As per the clinical practice guideline issued by the Endocrine Society in 2018, employing LC-MS/MS to measure 17OHP and other adrenal steroid hormones is recommended as a supplementary screening approach to enhance the accuracy of positive predictions.
With genetic tests becoming more common, a wide variety of tests may use the blood drawn by this method. Many neonatal units now use this method to carry out the daily blood tests required to check the progress of ill neonates.