Why newborn screening for severe combined immunodeficiency is essential: a case report.

Physicians caring for infants in the first months of life need to know the normal ranges for absolute lymphocyte counts (ALCs) during that age. Any ALC <2500/microL is potentially pathogenic in early infancy and should be evaluated. We report the case of a 4-month-old white girl with a 2-month history of an oral ulcer, intermittent fever, recurrent otitis, decreased appetite, weight loss, and a new respiratory illness with hypoxemia. She had been in an in-home day care since birth. The patient's primary care physician had seen her frequently and obtained blood counts, but her persistent lymphopenia had not been appreciated. The infant was ultimately diagnosed with T(-)B(-)NK(+) (lacking both B and T lymphocytes and having primarily natural killer [NK] cells), recombinase-activating gene 2 (RAG2)-deficient severe combined immunodeficiency (SCID). However, because she had already developed 2 difficult-to-treat viral infections (parainfluenza 3 and adenovirus), she did not survive long enough to receive a bone marrow transplant. Newborn screening would not only have made the diagnosis at birth but would have led to measures to protect her from becoming infected before she could receive a transplant. Newborn screening would also reveal the true incidence of SCID and define the range of conditions characterized by severely impaired T-cell development. Until screening for SCID and other T-cell defects becomes available for all neonates (either by quantifying T-cell receptor excision circles in Guthrie spots or using other tests that quantify T cells), all pediatricians should know the normal range for ALCs according to age. Recognition of the characteristic lymphopenia of SCID can facilitate early diagnosis.