Chemotherapy-induced marrow suppression and neutropenic sepsis

Chemotherapy drugs are cytotoxic (they kill cells), and they cannot tell a healthy cell that divides quickly from a cancer cell. The progenitor cells in the bone marrow divide rapidly too, so they are damaged along with the tumour.

When the progenitors are damaged, production falls across the blood cells. The red cell shortage gives anaemia (tiredness and breathlessness), the white cell shortage gives neutropenia (too few neutrophils, the white cells that kill bacteria) and the platelet shortage gives thrombocytopenia (too few of the cell fragments that plug leaks in blood vessels, so bruising and bleeding). Low red cells, white cells and platelets together are called pancytopenia, which in a patient on chemotherapy points to a problem at the level of the bone marrow.

Neutrophils live only a short time and cannot multiply once released, so when the bone marrow stops supplying them the count falls within days. With few neutrophils the body loses its main defence against bacteria, and the usual local signs of infection may be muted even though fever is usually present.

Neutropenia is the most dangerous, because it raises the risk of neutropenic sepsis (a serious infection in a patient with very few neutrophils). Fever after chemotherapy is neutropenic sepsis until proven otherwise, a time-critical emergency that needs urgent hospital assessment and the Sepsis Six bundle (a set of six urgent actions that includes antibiotics) within the first hour, without waiting for blood results.

A tumour can alter blood counts too: tumour bleeding or cancer invading the marrow alters counts by other mechanisms, so these are not effects of chemotherapy.