"Idiopathic neutropenia" in very low birthweight infants.

We employed a standardized investigative approach to evaluate four cases of "idiopathic neutropenia" in very low birthweight infants. The evaluation included maternal anti-neutrophil antibodies, a marrow aspiration, and a three-day trial of recombinant granulocyte colony-stimulating factor (rG-CSF). All patients had neutropenia at or shortly following delivery, and remained neutropenic (generally <500/microl) for 1-9 weeks until rG-CSF was administered. Blood and bone marrow studies in all 4 indicated that the neutropenia was: (i) the kinetic result of diminished neutrophil production; (ii) not alloimmune; (iii) not cyclic; and (iv) not associated with recognized inborn errors, bacterial or viral infections, or medications. All responded to rG-CSF by transiently increasing their blood neutrophils to normal, and the neutropenia resolved in all cases with time. It is likely that "idiopathic neutropenia" in this population represents a common phenotype of several different causes, rather than a single entity. Some cases, perhaps the majority, will respond to a short course of rG-CSF administration.