Neonatal Screening of Preterm Infants: Metabolism and Nutrition Factors, Part 1


Introduction to Newborn Screening (NBS)

All newborns are screened for a variety of rare genetic disorders one or two days after birth. The heritable diseases chosen as part of the newborn screening program in each state are based in part on the recommended guidelines1 by the Secretary of the Department of Health and Human Services (HHS).

The benefits of newborn screening are clear. Detection of rare genetic disorders in newborns leads to earlier diagnosis and treatments with improved outcomes.

Disorders screened include abnormalities in hormone metabolism such as Congenital Hypothyroidism or Congenital Adrenal Hyperplasia. Other inherited diseases that are part of newborn screening and are more familiar to the public include Cystic Fibrosis and Sickle Cell Anemia.

The largest category of disorders are inborn errors of metabolism (body chemistry). Collectively, these diseases account for more than 2/3 of the newborn screening panel of approximately 35-40 diseases analyzed from a few drops of blood. They range from defects in the metabolism of amino acids, fatty acids, lipids, and carbohydrates.

In addition to these lab-based screening tests from dried blood spots, two other conditions require physical testing and therefore are hospital-based. They are newborn hearing and congenital heart disease screening.

False Alarms

Many new parents are unaware that genetic screening is performed on their newborns unless they receive a phone call from their doctor or nurse counselor that asks them to bring their newborn to the clinic or doctor’s office for a repeat screening test and possible evaluation.

This is a relatively infrequent event for all newborns (less than 2-3 percent of newborn screens require repeat testing). Most repeat tests come back normal. We call the false alarm of the original screening test a false positive test.

True positive results are those that are confirmed by follow up testing and receive a clinical diagnosis for a rare disease. The combined frequency for one of all the diseases screen is about 1 in 1000 newborns.

The combined false positive rate for all diseases screened is at least 10x higher than true confirmed positives. Most of these false alarms are from a specific population of newborns: the preterm infants that require neonatal intensive care. For the lowest birth weight and earliest gestational age infants, i.e. 23-29 weeks, or more than 10 weeks premature, this frequency of false results is much higher than for a normal term newborn.

Newborn Screening and Premature Infants

For premature newborns, the issue of false results is complex and a matter that requires more discussion.

Based on public health recommendations, blood samples (a few drops of blood placed on filter paper) are collected before 24 hours of age, which by many state testing protocols for term newborns is too early. The recommended collection time is day 2 or 3 of age to allow inborn errors of metabolism to be better detected and avoiding being missed (a false negative).

Unfortunately, with this early sample, very-low birth weight (VLBW) and very premature infants (born at 23-29 weeks gestational age) have a higher frequency of false positive results compared to term newborns.

Many preterm infants have low levels of the thyroid hormone, thyroxin (T4), and a high level of thyroid stimulating hormone (TSH). These two positive test results trigger a presumption of hypothyroidism. (Initial positive screening results for a disease are all called “presumptive positives” because we presume the baby has the disease until it is ruled out that there is no inherited disease present.)

Presumptive positive results require follow-up by the neonatal intensive care unit staff to evaluate the infant and obtain another blood sample to be sent back to the newborn screening lab. This occurs while the infants are being intensively cared for with intravenous lines of nutrition, special breathing apparatus, temperature regulation, and more to keep this infant alive.

Making Matters Worse, IV Nutrition, and False Positives

Preterm infants are provided intravenous nutrition shortly after birth. This type of nutrition is prepared in a saline bag containing dextrose and minerals. A concentrated mixture of essential amino acids is added to the bag during preparation in the pharmacy to achieve the dosages of all components, i.e. dextrose and amino acids) prescribed by the neonatologist. In addition, supplemental lipid mixtures like medium chain fatty acids or linoleic acid as well as other components (i.e. L-carnitine) may be added at this time.

This intravenous mixture may create false results if the blood sample is contaminated by this mixture. This can happen by collecting blood from an IV line instead of a heel stick. Elevations of amino acids may also occur if amino acids are not distributed to the tissues rapidly, a site collection close to the dosing point or amino acids have accumulated in blood.

Elevations of some acylcarnitines may also occur from supplements such as L-carnitine. Organic acids (a metabolite of some amino acids) form acylcarnitines. High doses of L-carnitine or amino acids or both can produce higher levels of these metabolites. Acylcarnitines are important biomarkers of several inborn errors of metabolism. These abnormalities are particularly noticeable in the first or second week during maximum parenteral nutrition doses.

Solutions to False Result Reduction

One approach to reducing false positives is to withhold intravenous nutrition for a few hours before a sample is collected. The theory behind this method is to ensure that the concentrated amino acids in the parenteral solution has distributed from the blood into tissues. It is based on a presumption that amino acids concentrations are temporarily high during TPN administration. This strategy however does not account for metabolites that are formed in the tissues because of parenteral nutrition such as acylcarnitines. It also requires an interruption of the administration of nutrition for a few hours. Another approach for the first day of life is to collect a sample before nutrition is started.

A better choice for reducing false results is to use a specialized interpretation protocols that considers nutrition information and how it is applied for each individual preterm infant using data from the patient record. Interpretation must also be based on the time of collection (age of infant). For example, metabolically speaking, most infants have a normal amino acid profile at birth. False positive newborn screening results are due to hypothyroid due to prematurity.

It is the second repeat specimen (requested because of the first abnormal result) that is collected at a new time, 7-10 days after birth, and during a different nutritional strategy (maximum TPN doses). A new interpretation of newborn screening data is required using protocols based on nutrition and the age of the infant. Unfortunately, this is not done in newborn screening labs even though a specialized interpretation for older infants more than a week after birth is standard of care. Part of the problem is that the goals of public health labs are to detect rare disease, and not go beyond newborn screening and monitor nutrition and metabolism or preterm infants in the NICU.

Many preterm infants have not just one repeat, or even two repeat specimens, but sometimes up to four with monitoring for more than a month before a metabolic disease of genetic origin is ruled out. For example, a second false result due to elevated amino acids or organic acid acylcarnitines requires a third repeat at about four weeks of age.

This third analysis may be abnormal for L-carnitine deficiency if the infant was not provided L-carnitine during maximum TPN administration between 1 and 2 weeks of life. It is not until a fourth specimen is received at around the time of discharge (6 weeks of age) that a normal result is finally achieved. Based on data from two clinical trials, new approaches to interpretation may reduce these false results by making them true results for the neonatologists’ utilization in the practice of nutrition.


Newborn screening is part of the health care of a preterm infant in the NICU. Unlike term newborns, the incomplete maturation of organ systems in preterm infants cause a host of problems which neonatology is focused on overcoming. It is not surprising that there is a higher rate of abnormal metabolic screening results.

The clinical data that make up the newborn screening are almost entirely correct. The issue is the interpretation of the results. There are solutions to these false presumptions that can reduce the burden on the NICU in follow up or provide a better understanding of what the results mean. The data which is part of the results in every newborn screen can also be to better understand nutrition in a preterm infant. This will be part of a discussion in part 2 of this blog.

About the Author

Donald H. Chace, PhD, MSFS, FACB is the Chief Scientific Officer for Medolac, a public benefit corporation. He is one of the primary developers of newborn metabolic screening using tandem Mass Spectrometry. Developed 25 years ago with the first screening publication in Clinical Chemistry that describes the MS-based newborn screening of PKU, the method is now used to screen millions of infants per year, worldwide. Dr. Chace is an expert in metabolism and clinical chemistry using mass spectrometry as well as microsample analysis, e.g. the dried blood spot. Learn more about Dr. Chace by visiting his LinkedIn profile.