Beyond the Newborn Screen: An Infant With Hypoxic Respiratory Failure.

Anchoring bias can narrow diagnostic reasoning in infants with nonspecific but severe presentations. We describe a 4-month-old full-term female who presented with acute hypoxic respiratory failure and significant recent weight loss. Initial evaluation revealed profound CD4 lymphopenia and hypogammaglobulinemia, prompting early concern for severe combined immunodeficiency (SCID). A normal newborn T-cell receptor excision circle screen and preserved IgA and IgM levels later reduced the likelihood of classic SCID, but the diagnostic focus remained anchored on primary immunodeficiency as the child worsened. Human immunodeficiency virus-1 (HIV-1) RNA PCR testing confirmed a markedly elevated viral load, and bronchoalveolar lavage identified Pneumocystis jirovecii. Maternal testing revealed previously undiagnosed HIV, suggesting late-term seroconversion. This case demonstrates how anchoring on SCID delayed consideration of HIV despite evolving clinical and immunologic findings and highlights the role of HIV testing at delivery in identifying perinatal HIV exposure and preventing delayed diagnosis.