Vitamin B12 or Folate Deficiency: This condition often presents with leukocytosis (an increase in white blood cell count) and macrocytosis (high MCV, or mean corpuscular volume, indicating larger than normal red blood cells). The deficiency affects the production of red blood cells, leading to anemia and can also impact white blood cell production.

Chronic Myeloid Leukemia (CML): CML can cause an increase in white blood cell count and may also lead to an increase in MCV due to the abnormal proliferation of myeloid cells in the bone marrow.
Myelodysplastic Syndromes (MDS): These are a group of disorders caused by poorly formed or dysfunctional blood cells, often presenting with macrocytic anemia and can have a variable white blood cell count.
Alcohol Abuse: Chronic alcohol consumption can lead to macrocytosis and may also cause leukocytosis due to the body's response to alcohol-induced inflammation and potential infections.

Acute Myeloid Leukemia (AML): Although less common than CML, AML is a critical diagnosis to consider due to its aggressive nature and the need for prompt treatment. It can present with leukocytosis and variable effects on MCV.
Sepsis: Infection leading to sepsis can cause leukocytosis, and in some cases, especially if there's a component of anemia or nutritional deficiency, MCV might be elevated.

Primary Myelofibrosis: A rare bone marrow disorder that can lead to an increase in white blood cell count and changes in red blood cell indices, including MCV.
Hypersplenism: Conditions leading to an overactive spleen can result in the sequestration and destruction of blood cells, potentially leading to leukocytosis and changes in MCV, though this is less directly related to high MCV.
Certain Medications: Some medications can cause macrocytosis and leukocytosis as side effects, such as certain antibiotics, anticonvulsants, and chemotherapeutic agents.