Viral infection: A common cause of leukopenia, particularly in the absence of other cytopenias. Many viral infections can cause a transient decrease in white blood cell count, including the flu, mononucleosis, and hepatitis.

Medication-induced leukopenia: Various medications, such as antibiotics (e.g., trimethoprim-sulfamethoxazole), anticonvulsants (e.g., carbamazepine), and antihistamines, can cause leukopenia. The patient's medication history should be thoroughly reviewed.
Vitamin deficiencies: Deficiencies in vitamins like B12 or folate can lead to leukopenia, although this is less likely given the absence of anemia.
Chronic diseases: Chronic diseases such as renal failure, liver disease, or chronic infections (e.g., tuberculosis) can cause leukopenia.

HIV/AIDS: Although less likely, HIV infection can cause leukopenia, particularly in its early stages. Given the potential severity of the disease, it is crucial not to miss this diagnosis.
Bone marrow failure syndromes: Conditions like aplastic anemia or myelodysplastic syndromes can cause leukopenia and are critical to diagnose due to their potential for severe complications and the need for specific management.
Felty's syndrome: A complication of long-standing rheumatoid arthritis characterized by leukopenia, which is important to recognize due to its association with significant morbidity.

Congenital neutropenias: Rare genetic disorders that affect neutrophil production, such as Kostmann syndrome.
Large granular lymphocytic leukemia: A rare form of leukemia that can present with leukopenia.
Autoimmune neutropenia: An autoimmune condition where the body produces antibodies against neutrophils, leading to their destruction.
Cyclic neutropenia: A rare condition characterized by periodic episodes of neutropenia, often associated with other cytopenias.