Granulocyte-Colony Stimulating Factor (GCSF) is a growth hormone that can be administered to neutropenic patients. G-CSF stimulates the release of neutrophils from the bone marrow into the bloodstream. This is often done during or after chemotherapy to reduce the risk of (life-threatening) bacterial infections. G-CSF also leads to activation and delayed apoptosis of already existing neutrophils. Because of these combined effects, a large increase in neutrophil numbers can occur in a short period of time with left shift and toxic granulation.

This specimen is from a patient three days after a GCSF dose, due to the delayed apoptosis, the very clear picture of hypersegmentation can be seen here.