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The newborn screening hearing and pulse oximetry results are ready immediately, and the blood screening results are available within a week. As part of the blood screening, doctors measure the T cell receptor excision circles, or TRECs, and the TREC screening results are either normal or abnormal.

  • A normal TREC result, also called negative, in-range, or low-risk, means that it is unlikely that a baby has a health condition.
  • An abnormal TREC result, also called positive, out-of-range, or high-risk, means that a baby needs more testing to see if they have a health condition.

If the results are normal, families are often not notified, but it is always important to confirm the newborn screening results of the baby with the primary healthcare provider.

If the results are abnormal, the provider will contact the family. The family should follow the provider’s directions about what to do next. 

Abnormal TREC Results

Abnormal TREC results could mean your child has SCID or it could mean your child has a different condition associated with the immune system. These other conditions are often not as severe as SCID, but it's important to find out about them and test for them. If your child is diagnosed with low T cells and it’s not SCID, please continue to pursue testing and medical intervention. Keep in mind that some of the medical steps taken overlap those of SCID, at least in the beginning.

Doctors discover conditions related to low T cells in the same way they discover SCID ­– through newborn screening. The newborn screening test for SCID measures the number of T cell receptor excision circles, or TRECs, in the blood of the baby. The immune system requires T cells to function. The number of TRECs is directly related to the number of newly formed T cells in a baby’s blood. Low or no T cells means your baby’s immune system is not functioning properly.

If the TREC test is low, it is considered an abnormal TREC test, and it suggests that the number of T cells in the blood of a baby is low.

Follow-up testing for T cells

To determine if the number of T cells in the blood is actually low, a follow-up test, called a flow cytometry for lymphocyte subsets, is performed. In this test, the actual number of T cells and other types of lymphocytes, including B cells and Natural Killer cells, are counted. 

Sometimes the results of this test show the baby has a low number of T cells, which is a condition called T cell lymphopenia or T cell deficiency. In a minority of cases, the T cell lymphopenia is a result of SCID, but in most cases, the T cell numbers are not low enough to be called SCID.

Clinical immunologists

Babies with T cell lymphopenia – both those with SCID and those with non-SCID T cell lymphopenia – need to be evaluated by a clinical immunologist. A clinical immunologist is a specialist in disorders of the immune system. The clinical immunologist can determine if the low T cell count requires further evaluation and treatment.

The clinical immunologist will first determine if the low T cell count is caused by:

  • The inability of the thymus to produce enough T cells, which sometimes resolves on its own
  • A medical condition that causes a loss of T cells including (but not limited to):
    • Birth defects of the heart or digestive tract
    • Disorders in the lymphatic system, which can cause a loss of T cells outside of the lymphatic system and the bloodstream
  • Prematurity in infants, which requires additional TREC screening as the baby becomes older

For names of specific diagnoses which result in T cell lymphopenia and for a more detailed explanation of low T cell results, click here.

In any of these situations, you should still seek an opinion from a clinical immunologist who can determine why the T cell count is low and if your child requires further testing or treatment.

Further testing

The additional testing may include blood work to:

  • Study the function of the baby’s T cells
  • Determine the ability of the baby to make antibodies, a blood protein that fights infection
  • Conduct genetic tests to look for abnormality (mutation) in a gene that is important for the normal function of the immune system

What if you are not in a location where there is a highly skilled clinical immunologist who has experience treating SCID? In that case, you should consider reaching out to find a clinical immunologist that practices at a hospital that is part of the Primary Immune Deficiency Treatment Consortium (PIDTC). Here is the link to the PIDTC, where you can find participating clinical centers and names and contact information for clinical immunologists.


The treatments of children with low T cell numbers are decided on a case-by-case basis. Depending on the cause, the specialist may prescribe prophylactic medicine, which is given to the baby to prevent infection. Prophylactics may include immunoglobulin (which provides infection-fighting antibodies), antibiotics, and antivirals or antifungal medications.

The most important thing to remember is that if your baby has a low T cell count, you should always follow up with a skilled clinical immunologist, which, if not available locally, can be found by using the link to the PIDTC.