What Is Neutropenia?
Neutropenia is a medical condition characterised by an abnormally low level of neutrophils in the blood. Neutrophils are a type of white blood cell and are a cornerstone of the immune system's defence against bacterial and fungal infections. When their numbers fall below safe thresholds, the body becomes highly vulnerable to infections that a healthy immune system would normally clear without difficulty.
In clinical practice, neutropenia is defined by an absolute neutrophil count (ANC) — a measure of how many neutrophils are present per microlitre of blood. Normal ANC ranges from roughly 1,500 to 8,000 cells/µL. Clinicians generally classify neutropenia as follows:
| Severity | ANC (cells/µL) | Infection Risk |
|---|---|---|
| Mild | 1,000–1,500 | Slightly increased |
| Moderate | 500–1,000 | Moderately increased |
| Severe | <500 | Significantly increased |
| Profound (agranulocytosis) | <100 | Severe — life-threatening risk |
What Causes Neutropenia?
There are several causes of neutropenia, ranging from temporary to chronic:
- Chemotherapy: The most common cause in oncology settings. Cytotoxic drugs damage the bone marrow, reducing neutrophil production.
- Radiation therapy: Particularly when large areas of bone marrow-containing bone are irradiated.
- Bone marrow disorders: Conditions like aplastic anaemia, myelodysplastic syndromes (MDS), or leukaemia can impair normal blood cell production.
- Autoimmune conditions: The immune system may mistakenly attack neutrophils.
- Certain medications: Some antibiotics, antipsychotics, and anti-thyroid drugs can suppress neutrophil production.
- Viral infections: Influenza, HIV, hepatitis, and other viruses can temporarily suppress bone marrow function.
- Severe infections: Paradoxically, overwhelming sepsis can consume neutrophils faster than the body can replace them.
- Nutritional deficiencies: Severe deficiencies in vitamin B12 or folate can impair blood cell production.
Recognising the Symptoms
Neutropenia itself does not always cause noticeable symptoms. Many patients discover they are neutropenic only through routine blood tests. However, the consequences of neutropenia — infections — produce symptoms that should prompt immediate medical attention:
- Fever of 38°C (100.4°F) or above — a key warning sign in neutropenic patients
- Chills and sweating
- Sore throat or mouth sores
- Skin infections, redness, or swelling
- Cough, shortness of breath (possible lung infection)
- Pain or burning during urination
- Unusual fatigue or feeling generally unwell
A fever in a neutropenic patient is a medical emergency known as febrile neutropenia, requiring prompt hospital assessment and intravenous antibiotics.
How Is Neutropenia Diagnosed?
Diagnosis is made through a full blood count (FBC), sometimes called a complete blood count (CBC). This blood test measures the number and proportions of different blood cells, including neutrophils. Patients on chemotherapy typically have regular FBC monitoring to track their ANC and determine when G-CSF therapy like Zarzio may be needed.
Chemotherapy-Induced Neutropenia: The Nadir
After a cycle of chemotherapy, neutrophil counts typically reach their lowest point — called the nadir — around 7–14 days after treatment. This window represents the period of highest infection risk, and it is during or approaching this phase that medications like filgrastim (Zarzio) are most commonly prescribed to accelerate neutrophil recovery.
Key Takeaways
- Neutropenia is defined by a low absolute neutrophil count and is most common in chemotherapy patients
- Severity determines infection risk, with ANC below 500 being a serious concern
- Fever in a neutropenic patient is an emergency — always contact your care team immediately
- G-CSF therapy (such as Zarzio) is a primary treatment for chemotherapy-induced neutropenia
This content is educational. If you or someone in your care has cancer and is receiving chemotherapy, follow your medical team's guidance on managing neutropenia risk.