Spontaneous resolution of severe idiopathic T cell lymphopenia.

Potential etiologies of TBNK SCID include both hematopoietic defects and thymic aplasia. The management of patients with this phenotype, identified by newborn screen, may be unclear in the absence of a genetic diagnosis. We report an infant with lymphocyte flow cytometry consistent with TBNK SCID and reduced proliferative response to phytohemagglutinin. The patient had no genetic diagnosis after targeted panel and exome sequencing. The decision to trend laboratory values rather than move immediately to hematopoietic cell transplant was made given the absence of a genetic defect and the finding of a normal thymus on ultrasound. During the course of evaluation for transplant, the patient unexpectedly had normalization of T cell number and function. This case demonstrates a role for mediastinal ultrasound and the utility of trending laboratory values in patients with severe T cell lymphopenia but no genetic diagnosis, given the small but important possibility of spontaneous resolution.