What is the role of lab tests in the workup of severe combined immunodeficiency (SCID)?

Updated: Apr 28, 2021
  • Author: Robert A Schwartz, MD, MPH; Chief Editor: Harumi Jyonouchi, MD  more...
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Answer

Conduct a complete blood count (CBC) with differential to help detect lymphopenia. Children with SCID have a lymphocyte count lower than 3000/µL; however, a normal number of lymphocytes does not rule out SCID, because the lymphocytes may be nonfunctional. An absolute lymphocyte count lower than 2500/µL in an infant definitely warrants further workup, but any infant with severe infection or opportunistic infection should have the full initial workup.

Obtain total serum immunoglobulin (Ig) levels, including IgG, IgA, IgM, and IgE. Immunoglobulin levels, especially IgM levels, can be low. However, soon after birth, IgG levels may be falsely elevated because of maternal IgG.

Draw lymphocyte markers at the same time as the CBC to obtain percentages and absolute counts of CD3+ T cells, CD4+ T cells, CD8+ T cells, CD19+ B cells, and natural killer (NK) cell markers (CD16 and CD56).

Lymphocyte function should be assessed by measuring responses to phytohemagglutinin, a nonspecific stimulant of T-cell proliferation, concanavalin A directed at T-cell proliferation, and pokeweed mitogen directed at T-cell and B-cell proliferation.

A complete absence of T-cell function by mitogen tests can occur in association with a normal lymphocyte count for age in some forms of SCID, including X-linked SCID (XL-SCID), in which all the lymphocytes are B cells. DiGeorge syndrome is another example in which lymphocyte counts may be higher than 2000/µL with no T-cell function, or, conversely, normal T-cell function may be observed in spite of lymphopenia.

Specific antigens, such as tetanus and Candida, stimulate lymphocyte proliferation and represent a later step in lymphocyte function than responses to the nonspecific mitogens. Healthy young infants may not respond well to these specific antigens due to lack of exposure and/or immature T-cell functions.

Another T cell function used for screening is their ability to proliferate in response to allogeneic cells; this response aids in defining the type of SCID but also is relevant to determining the need for immunosuppressive therapy in preparation for stem cell reconstitution. Additional activators of lymphocyte proliferation are phorbol myristate acetate (PMA) with ionomycin or anti-CD3 and anti-CD28.

Cellular hallmarks that help differentiate between various forms of SCID, as well as other combined immune deficiencies that are sometimes severe enough to be classified as SCID, are as follows:

  • X-linked SCID - Lymphopenia occurs primarily from the absence or near absence of T cells (CD3+) and NK cells; variable levels of B cells occur, which do not make functional antibodies

  • JAK3 deficiency - Lymphopenia occurs primarily from the absence or near absence of T cells (CD3+) and NK cells; normal or high levels of B cells occur, which do not make functional antibodies

  • Adenosine deaminase (ADA) deficiency - Lymphopenia occurs from the death of T and B cells secondary to the accumulation of toxic metabolites in the purine salvage pathway; functional antibodies are decreased or absent

  • ZAP-70 deficiency - Lymphopenia occurs because of the absence of CD8+ T cells; as in all types of SCID, no antibody formation is present

  • Reticular dysgenesis - Lymphopenia occurs from the absence of myeloid cells in the bone marrow; red blood cells and platelets are present and functioning

  • Omenn syndrome - Normal or elevated T-cell numbers are present, but these are of maternal, not fetal, origin; B cells are usually undetectable, NK cells are present, and the total Ig level is markedly low with poor antibody production; eosinophils are elevated, as is total IgE

  • Purine nucleoside phosphorylase (PNP) deficiency - Lymphopenia occurs from the death of T cells secondary to the accumulation of toxic metabolites in the purine salvage pathway; this deficiency differs from ADA deficiency because circulating B cells are normal in number, but B-cell function is poor, as evidenced by the lack of antibody formation; PNP deficiency can be severe enough to be classified as SCID

  • Bare lymphocyte syndrome - The lymphocyte count is normal or mildly reduced, CD4+ T cells are decreased, and CD8+ T cells are normal or mildly increased; B-cells are normal or mildly decreased, but the ability to make antibodies is decreased; bare lymphocyte syndrome is sometimes classified as SCID

  • IL-2 deficiency - Normal, or near normal, numbers of T cells exist (both CD4+ and CD8+), but they fail to proliferate in vitro when stimulated with mitogens unless IL-2 is added to the culture medium; production of functional antibody is decreased; IL-2 deficiency may be severe enough to be classified as SCID

Determine the ADA and PNP levels in lymphocytes, erythrocytes, or fibroblasts. Measurement of leukocyte ADA enzyme activity is both sensitive and specific for the detection of ADA-deficient SCID.

Consider X-inactivation studies to determine whether the SCID is X-linked. Approximately 50% of patients have sporadic mutations with no history of affected family members.

Perform molecular studies to identify any specific known genetic defects or to identify new defects. These tests are now commercially available. If identifying a laboratory to perform these tests is difficult, consult a referral center for primary immune deficiency to assist in this matter.

Even when SCID is not suspected until the infant’s death, lymphocyte markers, mitogen responses, and DNA studies can still be carried out. Anticoagulated blood should be saved because lymphocytes are viable for at least 48 hours after death. An autopsy to assess the thymus and peripheral lymphoid tissues, including the spleen, gut, and tonsils, is needed.

Compromise of other hematopoietic cell lines is observed in reticular dysgenesis, in which myeloid cells are decreased, and platelets and erythrocytes may be deficient. Autoimmune hemolytic anemia can complicate forms of SCID in which autoimmune phenomena are present. Hypoplastic anemia occurs in cartilage-hair hypoplasia.

Patients with SCID are anergic. However, the reliability of delayed hypersensitivity skin testing depends on adequate exposure to the antigen. Candida and tetanus are the most useful antigens, but exposure requires 4-6 weeks, and more than 1 immunization is required in the case of tetanus. Mumps and Trichophyton antigens are of minimal use in infants.

T-cell defects can be difficult to define. The clinical manifestations of T-cell–associated opportunistic infections, such as mycobacteria, cytomegalovirus (CMV) and associated viruses, and P jiroveci, are usually interpreted by immunologists as defining a T-cell defect, even in the presence of apparently adequate mitogen responses (eg, IKK-γ deficiency for which impaired T-cell receptor [TCR]–mediated signaling is present despite normal mitogen responses).

Somech and Roifman suggest mutation analysis in patients with apparently normal immunologic tests to diagnose atypical cases of γC deficiency. [35]

When a T-cell disorder is suspected, the Immune Deficiency Foundation has a consultative service for physicians. Laboratories in Seattle (the University of Washington), Boston (Children’s Hospital), and New York City are funded to provide molecular analysis (Jeffrey Modell Foundation) or they can assist in contacting other research facilities.

To exclude HIV infection, perform HIV-DNA testing using polymerase chain reaction (PCR) testing; because of maternal antibody, anti-HIV tests are of no value in this setting. To help exclude congenital infection, perform serum testing of IgM against any suspected infection.


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