How would you work up and manage persistent lymphopenia in an asymptomatic patient?

Simple answer: refer to an immunologist, as this degree of lymphopenia is quite likely to represent a serious immunodeficiency. I would also send a next generation sequencing panel of immunodeficiency/immunoregulatory genes to be in process while awaiting the immunology consultation.

Long answer (from: https://www.uptodate.com/contents/approach-to-the-child-with-lymphocytosis-or-lymphocytopenia):

The causes of lymphocytopenia (i.e., an absolute lymphocyte count <1500/microL in older children or...