Neutropenia

Neutrophils are an important component of the immune system and play a significant role in the eradication of infections. Low numbers of circulating neutrophils, referred to as neutropenia, predispose the body to recurrent infections or sepsis, though patients can also be asymptomatic. Most causes of neutropenia are acquired, including neutropenia that is secondary to infections, bone marrow disorders, and/or adverse effects of many drugs. The diagnosis is made using CBC and peripheral blood smears. Management is aimed at preventing and treating infections.

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Overview

Definition

Neutropenia is a decrease in the number of circulating neutrophils in the blood, which is typically defined as an absolute neutrophil count (ANC) of:

  • < 1500 cells/µL in adults and children > 1 year of age:
    • Mild neutropenia: 1000–1500 cells/µL
    • Moderate neutropenia: 500–1000 cells/µL
    • Severe neutropenia: < 500 cells/µL  
    • Agranulocytosis: < 100–200 cells/µL
  • < 1000 cells/µL in children 2 weeks to 1 year of age
  • < 5000 cells/µL in infants during their 1st few days of life

Neutrophils

Neutrophils are the most common of all the leukocytes. Leukocytes are WBCs and are a major component of the immune system.

  • Neutrophils respond rapidly to the site of infection.
  • Neutrophils are granulocytic leukocytes:
    • Contain enzymes, oxidants, and proteins, within 4 types of granules, all of which are formed during neutrophil differentiation:
      • Azurophil (primary) granules: contain antibacterial proteins like myeloperoxidase and CAP37; function primarily within the phagolysosomes
      • Specific granules: contain antibacterial proteins, such as lactoferrin, neutrophil gelatinase–associated lipocalin, cathelicidin, and lysozyme, and proteases, such as collagenase
      • Gelatinase granules: contain antibacterial protein, lysozyme, and the proteases gelatinase (matrix metalloproteinase 9) and leukolysin
      • Secretory granules: contain transmembrane receptors (e.g., tumor necrosis factor receptors and interferon-α receptors) that integrate into the plasma membrane of neutrophils as exocytosis takes place
    • Can produce neutrophil extracellular traps (NETs): 
      • Made up of networks of extracellular microfibers, mostly composed of DNA from disrupted neutrophils 
      • NETs bind and degrade bacteria and other pathogens.
  • Other terms:
    • Often referred to as PMNs
    • Younger and immature neutrophils known as “bands” or “stab cells” 
  • Typically reported on a CBC as a percentage of the WBC count
  • ANC:  
    • The number of neutrophils (as opposed to the percentage of WBCs) circulating per µL of blood 
    • ANC = WBC cells/µL × percent (segmented neutrophils + bands)
    • Example: 
      • WBC = 5600 cells/µL; neutrophils = 5%; bands = 2%
      • ANC = 5600 × 7% = 392 cells/µL
  • Normal ANC levels: vary by age
Table: Normal ANC levels by age
Age groupNormal range (cells/µL)Percent of WBCs that are neutrophils
At 1 day of life5000–21,000Approximately 60%
At 1 month of life1000–9000Approximately 35%
≥ 1 year of age1500‒8500Approximately 31%
≥ 10 years of age1500–8000Approximately 40%–70%

Clinical significance of neutropenia

  • The correlation between ANC and infectious risk applies only to conditions in which the bone marrow neutrophil reserve is diminished, such as hematologic toxicity from chemotherapy.
  • With non–chemotherapy-related mild neutropenia (ANC, 1000–1500  cells/µL), there is no significantly increased risk of infection.
  • With severe neutropenia (ANC, < 500/µL), there is a significant risk of infection; fever should be managed on an inpatient basis with parenteral antibiotics, even with few clinical signs of infection.

Epidemiology

Prevalence:

  • 1%–10% in healthy, asymptomatic individuals
  • Higher in those with certain medical conditions:
    • Autoimmune disorders: systemic lupus erythematosus (SLE), rheumatoid arthritis
    • Connective tissue diseases
  • Varies by ethnicity:
    • Blacks: 4.5%–10.5%
    • Whites: 0.79%
    • Mexican Americans: 0.38%

Neutropenia may occur as an isolated feature or in association with other hematologic abnormalities and/or bone marrow defects (e.g., megaloblastic anemia or leukemia).

Pathophysiology

Normal physiology review

Mature neutrophils are produced by precursors in the bone marrow.
Neutrophils can be found in three compartments:

  • Bone marrow; neutrophils here can be classified as:
    • Proliferative: developing
    • Stored: mature neutrophils
  • Blood; neutrophils here can be classified as:
    • Circulating 
    • Marginated: adherent to the vascular endothelium or located within the spleen
  • Tissues: Neutrophils leave the blood randomly after about 6–10 hours to enter the tissues, where they act to destroy pathogens or undergo apoptosis.

Mechanisms that lead to neutropenia

  • Decreased production/differentiation in bone marrow:
    • Damaged hematopoietic precursors (e.g., chemotherapy, radiation, infection)
    • Ineffective granulopoiesis (e.g., megaloblastic anemia)
    • Bone marrow infiltration (e.g., leukemia)
  • Margination: 
    • Shift of circulating PMNs to the vascular endothelium or spleen
    • Example: endotoxin can cause margination → ↓ circulating neutrophils (sometimes referred to as pseudoneutropenia)
  • Enhanced peripheral immune destruction:
    • Drug reactions
    • Autoimmune process

Etiology

Neutropenia is most commonly acquired, often after infections or as a side effect of a number of different medications. Rarely, neutropenia may be “primary” or congenitally inherited as part of a less common syndrome.

Acquired neutropenia: non–drug-induced

  • Infections (most common form of acquired neutropenia):
    • Viral, including:
      • CMV
      • EBV
      • Influenza
      • Viral hepatitis
      • HIV
    • Bacteria, including:
      • Bacterial sepsis
      • Tuberculosis
      • Rickettsial infections
    • Parasitic:
      • Toxoplasmosis
      • Malaria
  • Hematologic malignancies (typically present with pancytopenia):
    • Leukemias
    • Other lymphoproliferative disorders
    • Myelodysplastic syndrome
    • Myelofibrosis
  • Bone marrow suppression:
    • Aplastic anemia
    • Ionizing radiation
    • Malignant infiltration
    • Medications (see below)
  • Hypersplenism
  • Nutritional causes:
    • Vitamin B12 or folate deficiency (megaloblastic anemia; may be seen in alcoholism)
    • Copper deficiency
  • Autoimmune process and/or associated with rheumatologic disorders:
    • SLE
    • Rheumatoid arthritis
    • Sjögren syndrome
    • Crohn’s disease
    • Granulomatosis with polyangiitis
    • Transfusion reactions
  • Complement activation:
    • Hemodialysis using nonbiocompatible membranes
    • Acute respiratory distress syndrome
  • Chronic idiopathic neutropenia

Acquired neutropenia: drug-induced

Some medications, including many chemotherapy agents, produce predictable dose-dependent myelosuppression. Many other medications are associated with severe isolated neutropenia (known as idiosyncratic drug reactions (IDRs)), typically occurring within 3 months after starting the medication.

Medications associated with myelosuppression (partial list, includes many chemotherapy and immunosuppressive agents):

  • Methotrexate
  • Cyclophosphamide
  • Cisplatin, carboplatin
  • Etoposide
  • Doxorubicin
  • Colchicine
  • Azathioprine
  • Ganciclovir

Medications associated with IDR neutropenia (partial list, most common are in bold): 

  • Antibiotics: Penicillins (especially penicillin G), cephalosporins, sulfonamides, macrolides, vancomycin, dapsone
  • Antimalarials: chloroquine, quinine
  • Antifungals: amphotericin B, flucytosine
  • Antivirals: acyclovir, oseltamivir, ganciclovir
  • Antiinflammatories: NSAIDs, sulfasalazine, penicillamine
  • Thionamides (thyroid medications): methimazole, propylthiouracil (PTU)
  • Cardiovascular drugs: ticlopidine, procainamide, propranolol, digoxin, ACE inhibitors, hydralazine
  • Diuretics: thiazides, acetazolamide, spironolactone
  • Psychotropics: clozapine, phenothiazines, tricyclic antidepressants
  • Antiepileptics: phenytoin, carbamazepine, valproic acid
  • Nonmyelosuppressive anticancer agents: rituximab, tyrosine kinase inhibitors

Congenital or primary neutropenias

Neutropenia is associated with multiple rare syndromes. Examples include:

  • Benign familial (ethnic) neutropenia
  • Hereditary (or congenital) neutropenia
  • Kostmann syndrome
  • Cyclic neutropenia
  • Shwachman-Diamond syndrome

Clinical Presentation

Patients with neutropenia will either be asymptomatic, present with infections, or present with symptoms associated with the underlying cause of their neutropenia. 

Infections

The primary symptoms related directly to neutropenia involve infections. Viral infections may cause neutropenia, while patients with moderate to severe neutropenia are more likely to develop a bacterial or fungal infection. Patients with neutropenia may present with:

  • Recurrent infections (e.g., recurrent otitis media)
  • Opportunistic infections (e.g., candidal rash) 
  • Serious infections with severe neutropenia (ANC < 200 cells/µL):
    • Sepsis
    • Life-threatening GI and pulmonary infections

Most common organisms causing recurrent, opportunistic, and/or serious infections:

  • Gram-negative rods: 
    • Escherichia coli
    • Klebsiella spp.
    • Pseudomonas aeruginosa
  • Gram-positive cocci (especially in patients with indwelling catheters): 
    • Staphylococcus spp.
    • Streptococcus viridans
  • Candida spp., especially after neutropenic patients receive treatment with broad-spectrum antibiotics

Clinical findings will depend on the location of infection and causative organism and most commonly may include:

  • Fever
  • Fatigue
  • Lymphadenopathy
  • Skin findings: 
    • Rash
    • Cellulitis, ulcers, and/or abscesses
    • Jaundice (may suggest liver disease secondary to viral hepatitis)
  • ENT findings:
    • Recurrent mouth ulcers
    • Gingivitis
  • Lung findings (if pneumonia is present): 
    • Cough
    • Dyspnea
    • Dullness to percussion
  • Musculoskeletal:  
    • Joint pain and/or swelling
    • Myalgias

Note: Patients are not at increased risk for viral or parasitic infections, as these infections are not neutralized by neutrophils.

Other findings on history and physical exam

  • May be asymptomatic
  • History of medications that may cause neutropenia
  • Symptoms related to the primary cause, for example:
    • SLE: 
      • Joint pain/swelling
      • Rash
      • Hematuria
      • Pleuritic pain
    • Myelofibrosis:
      • Bone pain
      • Fatigue and dyspnea due to concurrent anemia

Diagnosis

Laboratory studies

  • CBC with differential:
    • Determines the severity of neutropenia
    • Determines whether neutropenia is isolated or associated with other hematologic abnormalities (helps narrow differential diagnosis); examples include:
      • Eosinophilia → drug reactions/allergies or parasitic infections
      • Lymphocytosis → leukemias, lymphomas
      • Lymphopenia → immunodeficiency
      • Megaloblastic anemia → folate or vitamin B12 deficiency
  • Peripheral blood smear:
    • Confirm the reduced number of neutrophils
    • Hypersegmented neutrophils → folate or vitamin B12 deficiency
    • Atypical lymphocytes → viral cause
    • Abnormalities consistent with leukemias (e.g., smudge lymphocytes, hair-like projections, or azurophilic granules)
  • Erythrocyte sedimentation rate (ESR) and CRP: since classic findings of infection or inflammation may be inapparent
  • Other laboratory tests to consider based on history and exam findings:
    • Blood and urine cultures if febrile
    • Stool testing if patient has infectious diarrhea
    • Testing for rheumatologic causes:
      • Antinuclear antibodies 
      • Anti-DNA antibodies
      • Complement levels
    • Screen for nutritional deficiencies: 
      • Serum copper
      • Folate and vitamin B12 levels
    • Screen for HIV infection
    • Genetic testing if congenital syndromes are suspected
Peripheral blood smear showing normochromic RBCs with anisocytosis:poikilocytosis - neutropenia

Peripheral blood smear for a patient with systemic lupus erythematosus (SLE) showing a paucity of neutrophils

Image: “Peripheral blood smear” by Melissa Zhao. License: CC BY 4.0

Other tests

Other tests to consider based on clinical presentation include:

  • Chest X-ray to evaluate for infection or malignancy
  • Ultrasound or CT of abdomen for liver and spleen imaging
  • Bone marrow aspiration and biopsy if malignancy is suspected

Management

General management

Management depends on the cause and degree of the neutropenia.

  • Discontinue causative drugs.
  • Replete causative vitamin deficiencies (vitamin B12, folate, and/or copper).
  • Monitor ANC on serial CBCs:
    • For a majority of patients, the neutropenia will be due to an infection or medication.
    • Repeat the CBC to ensure resolution of the neutropenia following improvement of infection/discontinuation of offending agents.
    • If neutropenia persists, further workup is indicated.
  • Infections:
    • Treat infections with appropriate agents.
    • Prophylaxis against infection:
      • Handwashing to prevent infection
      • Avoid exposure to sick people. 
      • Regular dental care
  • Autoimmune-associated neutropenia: Consider IV immunoglobulin (IVIG) and/or steroids if neutropenia is severe and because of their autoimmune condition.
  • Granulocyte-colony stimulating factors (G-CSFs):
    • Naturally stimulates neutrophil production
    • Examples: filgrastim, pegfilgrastim
    • Indicated in:
      • Acute febrile neutropenia
      • Prophylactic use in chemotherapy patients at risk for neutropenia
      • Children with severe congenital neutropenia and/or bone marrow failure
      • Cyclic neutropenia if ANC < 200 and infections are present
  • Bone marrow transplantation: may be curative for children with Kostmann syndrome
  • Indications for immediate hematology referral: 
    • Blasts or other findings suggesting hematologic malignancy seen on peripheral blood smear
    • ANC < 200
    • Pancytopenia

Management of neutropenic fever in cancer patients

Neutropenic fever is the most common life-threatening complication of cancer therapy.

  • Empirical antibiotic therapy on presentation is imperative to improve outcomes and decrease mortality. 
  • A causative organism is identified only ⅓ of the time, and therefore antibiotics are aimed at treating a broad spectrum of expected pathogens.
  • Prophylactic antibiotics and growth factor support at the onset of neutropenia have been shown to benefit only a small subgroup of high-risk cancer patients receiving chemotherapy.

Differential Diagnosis of Primary Neutropenias

  • Benign familial (ethnic) neutropenia: inherited cause of mild to moderate neutropenia, most commonly seen in individuals of African descent. These patients may not generate leukocytosis during infection but their presentation is otherwise normal. They do generate a fever and tachycardia during infection (similar to controls), do not have an increased incidence of infection, and do not have an increased risk for febrile neutropenia secondary to myelosuppressive therapy. The bone marrow reserves of these patients are normal.
  • Hereditary neutropenia/congenital neutropenia: condition resulting from mutations in 1 of several genes, typically inherited in an autosomal dominant fashion and often leading to severe neutropenia. This condition is more common in infants and young children. The symptoms include mouth sores, frequent fever, pneumonia, ear infections, and rectal sores. If the condition is not treated immediately, a child may begin losing teeth as a result of severe gum infections.
  • Kostmann syndrome: inherited cause of severe congenital neutropenia (ANC is often < 200). It is inherited in an autosomal recessive pattern, though the causative genes are unknown. Kostmann syndrome is an aggressive condition in which patients are unable to effectively make neutrophils. Patients typically get very sick and may require bone marrow transplantation, which is curative.
  • Cyclic neutropenia: autosomal dominant cause of episodic neutropenia in both children and adults. Episodes occur approximately every 3 weeks (range, 12–35 days) and continue for 3–6 days in a single cycle. The symptoms include fever, infections, and ulcers. Many children improve after puberty.
  • Shwachman-Diamond syndrome: very rare syndrome presenting in infancy, inherited in an autosomal recessive pattern. The syndrome is characterized by pancreatic insufficiency, metaphyseal dysostosis (flaring at the ends of long bones with constriction and sclerosis of the diaphysis), growth retardation, and neutropenia with or without anemia or thrombocytopenia.

References

  1. Berliner, N. (2020). Approach to the adult with unexplained neutropenia. UpToDate, Retrieved April 23, 2021, from https://www.uptodate.com/contents/approach-to-the-adult-with-unexplained-neutropenia
  2. Coates, T.D. (2021). Overview of neutropenia in children and adolescents. UpToDate, Retrieved April 23, 2021, from https://www.uptodate.com/contents/overview-of-neutropenia-in-children-and-adolescents
  3. Ahmed, N.M., Palazzi, D.L. (2020).  Evaluation of children with non-chemotherapy-induced neutropenia and fever. UpToDate. Retrieved April 23, 2021, from https://www.uptodate.com/contents/evaluation-of-children-with-non-chemotherapy-induced-neutropenia-and-fever

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