Newborn screening: What parents need to know

Babies can look healthy at birth and still have a serious health condition, which is why newborn screening tests are completed before discharge from the hospital. A few drops of blood, taken from a baby’s heel, are tested for a number of disorders including a potentially fatal disorder of the immune system known as Severe Combined Immunodeficiency (SCID).

The immune system is made up of cells in the body that protect a person from infection. SCID is a disorder where the immune system is so weak that any infection, even a common cold, can be life-threatening. Many people remember David Vetter, affectionately known as the “boy in the bubble”. David had SCID. The good news is SCID can be successfully treated if the diagnosis is made early before serious infections develop.

Dr. John Routes, Professor of Pediatrics at the Medical College of Wisconsin, was the principal investigator for the first statewide program to screen all newborns for SCID. By 2018 (a decade later), screening for SCID had been implemented in all 50 states. We sat down with him to ask some patient questions about newborn screening and SCID.

What is newborn screening (NBS) and when does it take place?
NBS is a simple blood test that is performed to check for a variety of disorders that are not apparent after delivery. The test is usually done by heel prick approximately 24 to 48 hours after birth and before the baby leaves the hospital.

Do parents have to consent to the screening?
Most states require mandatory newborn screening. Only Wyoming requires parental consent for NBS. The majority of states require the parents to be informed about newborn screening and may permit parents to opt-out for certain reasons such as religious objections.   

 What do doctors test during the screening?  What other health conditions besides SCID would cause an abnormal screening?
Each state decides what disorders to screen for, but most states test for the disorders recommended by the Health Resources and Services Administration (HRSA) in their Recommended Uniform Screening Panel. NBS tests for disorders of metabolism (e.g. phenylketonuria-PKU), hormones (e.g. hypothyroidism), hemoglobin (e.g. sickle cell disease), and other rare but serious diseases such as severe combined immunodeficiency (SCID) and cystic fibrosis. Many states also screen for hearing loss and serious congenital heart disease, which are not screened by blood tests.   

What is the name of the particular kind of test used to identify SCID and what kind of results would indicate that SCID may be present?
The NBS test used to screen for SCID is the TREC (T cell receptor excision circle) test. TRECs are made when the immune system makes new T cells. T cells are very low or absent in SCID. Therefore, a low number of TRECS are found in babies with  SCID or other conditions associated with low numbers of T cells. 

Please discuss false-positives in the screenings. Are they common?
Sometimes an NBS test will be positive for a disorder but follow-up testing shows the baby does not have the disease — this is called a false-positive test. False-positive results are not uncommon and the percentage of false-positive results may vary from state to state depending on the specific disorders tested for. The disorders that NBS tests detect are serious and treatable.  Therefore, it is very important to identify as many babies as possible, which sometimes leads to a false-positive test result.

How long does it take for the results to come back and how are parents informed?
The time to get the test results varies from state to state but the doctor should receive the results from the state within 2-3 weeks after the sample collection. The doctor will then discuss the results with the parents.

How/when does testing happen for babies born outside of a hospital?
A midwife may collect the blood sample and send it to the state. Otherwise, the test is done at the doctor’s office or hospital.

What are the next steps parents should take if a screening comes back as abnormal?
Your health care provider will discuss the results of the NBS tests with you and the steps that should be taken to confirm the diagnosis.

If the NBS test for SCID is abnormal, the next test that will be performed is a blood test called a lymphocyte subset test and it measures the numbers of newly made T cells in the blood. If the number of T cells is normal, no further testing is usually needed. If the numbers of T cells are low, then further testing may be required to determine if the baby has SCID or another problem that can be associated with low T cells. A clinical immunologist, a doctor that specializes in the diagnosis of SCID and other primary immunodeficiencies, usually directs the testing at this point.

How long does it take to finally arrive at an accurate SCID diagnosis and while they’re waiting, what precautions should they be taking?
NBS has markedly improved the outcomes for babies with SCID by identifying the disorder early before serious infections occur. The time to a definitive diagnosis of SCID can vary, but your doctor will usually know if there is a serious problem in the immune system such as SCID within a few weeks after birth.

Your health care provider will discuss the specific precautions that should be taken while definitive testing for SCID is being performed.  Many of the precautions are similar to those recommended by the CDC during the COVID-19 pandemic such as avoidance of crowds, social distancing, avoiding sick contacts, handwashing, and for family members to wear masks when outside the home. If the initial NBS test is very suggestive of SCID, you should discuss with your provider the pros and cons of breastfeeding and the need to use boiled water for reconstitution of formula.