Causes of Leukopenia in Healthy Adults
In otherwise healthy adults presenting with isolated leukopenia, the most common etiologies are autoimmune conditions (including autoimmune thyroid disease), iron deficiency anemia, and medications, with autoimmunity accounting for over half of all cases.
Systematic Approach to Etiology
Most Common Causes in Non-Neutropenic Leukopenia (ANC ≥1500/µL)
When leukopenia occurs without significant neutropenia, the following etiologies predominate 1:
- Iron deficiency anemia (21.8% of cases) 1
- Autoimmune thyroid disease (21.8% of cases) 1
- Other autoimmune/autoinflammatory diseases (17.3% of cases) 1
- Medications (12.7% of cases) 1
- Infections (5.5% of cases) 1
Most Common Causes in Neutropenic Leukopenia (ANC <1500/µL)
When neutropenia accompanies leukopenia, the distribution shifts 1:
- Other autoimmune/autoinflammatory diseases (17.0% of cases) 1
- Hematopoietic disorders (11.9% of cases) 1
- Iron deficiency anemia (10.2% of cases) 1
- Medications (8.5% of cases) 1
- Autoimmune laboratory abnormalities (8.5% of cases) 1
Key Clinical Considerations
Benign Ethnic Neutropenia
Benign ethnic neutropenia (BEN) is a hereditary, asymptomatic condition most commonly observed in individuals of African ancestry, characterized by stable neutrophil counts between 1000-1500 cells/µL without increased infection risk 2. This condition:
- Remains stable over time with low infection rates comparable to the general population 2
- Presents across all age groups (children, younger adults, older adults), suggesting hereditary transmission 2
- Does not result from absent or reduced chronic low-grade inflammation 3
Autoimmunity as a Dominant Factor
When isolated antinuclear antibody positivity is included, autoimmune diagnoses or laboratory findings account for 53.8% of all isolated leukopenia cases 1. This makes autoimmunity the single most important category to evaluate systematically.
Medication-Induced Leukopenia
Document all current medications, as drug-induced leukopenia occurs in 8.5-12.7% of cases 1. Notably:
- Intravenous iron therapy can induce leukopenia in 1.91% of patients, neutropenia in 0.95%, and lymphocytopenia in 0.76%, though clinical significance is limited with minimal infection risk 4
Essential Diagnostic Workup
Initial Laboratory Assessment
Obtain the following to exclude reversible causes 5:
- Complete blood count with differential and peripheral smear review 5
- RBC folate and serum vitamin B12 to exclude megaloblastic anemia 5
- Serum iron, total iron-binding capacity, and ferritin to exclude iron deficiency 5
- Thyroid function tests given the high prevalence of autoimmune thyroid disease 1
- Autoimmune serologies (ANA, other relevant antibodies) 1
- Lactate dehydrogenase, bilirubin, and haptoglobin to exclude hemolysis 5
Timing and Persistence
Cytopenias must persist for ≥4-6 months before pursuing extensive workup for hematopoietic disorders (or ≥2 months if specific karyotype or bilineage dysplasia is present) 5. This prevents overdiagnosis and unnecessary invasive testing.
When to Pursue Bone Marrow Examination
Bone marrow examination is indicated when 5, 1:
- Neutropenia is present (physicians order bone marrow more frequently in neutropenic patients) 1
- Cytopenias persist beyond the appropriate observation period 5
- Peripheral smear shows dysplastic features 5
- No reversible cause is identified after comprehensive evaluation 5
Critical Pitfalls to Avoid
- Do not immediately pursue bone marrow biopsy in mild, isolated leukopenia without neutropenia; complete the reversible causes workup first 5, 1
- Do not overlook ethnic background; BEN is a diagnosis of exclusion in appropriate populations and requires no treatment 2
- Do not dismiss iron deficiency as a cause even in the absence of anemia, as it is the leading single etiology in both neutropenic and non-neutropenic groups 1
- Do not forget to assess infection risk systematically in neutropenic patients, though most healthy adults with mild leukopenia have infection rates similar to the general population 2