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Neutropenia is a reduced number of neutrophil granulocytes in the blood. Normally, one would expect a number $\geq 1500\ \frac{\text{neutrophils}}{\mu l}$ in the peripheral blood, with a typical value of $2500-4500 \frac{\text{neutrophils}}{\mu l}$.
The decreased number of neutrophils is associated with reduced immune response and hence causes an increased chance of infection. Patients with neutropenia have a risk of developing febrile neutropenia (also known as neutropenic sepsis). Febrile neutropenia is a medical emergency and hence requires immediate treatment!

Neutropenia can be easily diagnosed using a complete blood count to count the number of neutrophils in the peripheral blood. In case a lab report does not contain the absolute neutrophil count (ANC), you can also multiply the percentage of neutrophils with the leukocyte count.

The different grades of neutropenia are defined in the Common Terminology Criteria for Adverse Events (CTCAE). On this page, we use Version 5.0 published in 2017.

is defined as an absolute neutrophil count (ANC) of

$$\leq 1500\ \frac{\text{neutrophils}}{\mu l}$$ or equivalently
$$\leq 1500\ \frac{\text{neutrophils}}{mm³}$$ or equivalently
$$\leq 1.5\ billion\ \frac{\text{neutrophils}}{l}$$ or equivalently
$$\leq 1.5e9\ \frac{\text{neutrophils}}{l}$$

is defined as an absolute neutrophil count (ANC) of

$$1000\dotsb 1500\ \frac{\text{neutrophils}}{\mu l}$$ or equivalently
$$1000\dotsb 1500\ \frac{\text{neutrophils}}{mm³}$$ or equivalently
$$1000\ billion\ \dotsb 1500\ billion\ \frac{\text{neutrophils}}{l}$$ or equivalently
$$1e9\dotsb 1.5e9\ \frac{\text{neutrophils}}{l}$$

is defined as an absolute neutrophil count (ANC) of

$$500\dotsb 1000\ \frac{\text{neutrophils}}{\mu l}$$ or equivalently
$$500\dotsb 1000\ \frac{\text{neutrophils}}{mm³}$$ or equivalently
$$500\ billion\ \dotsb 1000\ billion\ \frac{\text{neutrophils}}{l}$$ or equivalently
$$0.5e9\dotsb 1e9\ \frac{\text{neutrophils}}{l}$$

$$\leq 500\ \frac{\text{neutrophils}}{\mu l}$$ or equivalently
$$\leq 500\ \frac{\text{neutrophils}}{mm³}$$ or equivalently
$$\leq 500\ million\ \frac{\text{neutrophils}}{l}$$ or equivalently
$$\leq 0.5e9\ \frac{\text{neutrophils}}{l}$$