Severe Congenital Immune Deficiencies: Tests and Diagnosis

What tests are used to diagnose severe congenital immune deficiencies (SCID) and SCID variants?

  • A complete blood count that could show very low to absent lymphocytes (classic SCID) or low platelets.  
  • A T and B cell count in the blood (they are low to absent in classic SCID).
  • Quantitative immunoglobulin levels (IgG, IgA, IgM, IgE) are low to absent in classic SCID.
  • Proliferation to mitogens that tests for normal T cells, usually low or absent in SCID.
  • A physical exam which, in addition to assessing for infection and failure to thrive, could also identify physical findings that could help the diagnosis.

Specific tests could be done to look for a specific protein on the T and B cells and its function). Most specific tests are genetic analyses to look for the defective SCID genes. This is often guided by the results of the above general tests. Results from genetic screening tests can take weeks or months.

Newborn screening for SCID

This is a special test that has recently been offered in some states (including Colorado) as part of standard newborn testing for inherited diseases. This test looks for the presence of new T cells. If none are detected, the child should be referred to a pediatric immunologist, who will perform the above general tests and confirm if the child has SCID. Once the diagnosis of SCID is made, your child may be admitted to the hospital to be protected from disease.

How do providers at Children’s Hospital Colorado make a diagnosis?

The diagnosis of SCID is made by identifying the genetic defect that is characteristic of that disease. We can now identify almost 80% of SCID patients’ genetic defects, and there are some children who will have a newly discovered gene defect that causes SCID.

Currently, genetic testing is relatively slow and quite expensive, so it is important to get pre-approval from insurance. There are ongoing improvements in the testing and the computer programs that analyze the huge amount of information generated during genetic testing. The hope is that in the future, complete genetic testing for a class of diseases (i.e. SCID) will be available, practical, timely and affordable.