Chronic Myeloid Leukemia (CML): This condition is characterized by the uncontrolled growth of myeloid cells in the bone marrow, leading to elevated levels of WBC, RBC, Hgb, Hct, neutrophils, lymphocytes, and monocytes. CML often presents with leukocytosis (high WBC count) and can also cause increases in other blood cell lines due to bone marrow overproduction.

Polycythemia Vera (PV): A myeloproliferative disorder that leads to the overproduction of all three blood cell types (RBC, WBC, and platelets), resulting in elevated counts. Patients with PV may present with symptoms related to increased blood viscosity and thrombotic events.
Reactive Leukocytosis: A response to infection, inflammation, or other stressors that can cause an increase in WBCs, including neutrophils, lymphocytes, and monocytes. While it typically doesn't cause as significant an elevation in RBCs and Hgb as seen in myeloproliferative disorders, it can be considered in the differential, especially if there's a clear precipitating factor.
Chronic Infection or Inflammation: Chronic conditions such as tuberculosis, endocarditis, or rheumatoid arthritis can lead to a sustained increase in WBCs, including lymphocytes and monocytes, due to the body's ongoing immune response.

Acute Myeloid Leukemia (AML): Although less likely than CML to present with elevated RBC and platelet counts, AML is a critical diagnosis not to miss due to its aggressive nature and the need for prompt treatment. It can occasionally present with leukocytosis and increases in other cell lines.
Myeloproliferative Neoplasm (MPN) with Blast Crisis: A transformation of an MPN (like PV or essential thrombocythemia) into a more aggressive phase resembling acute leukemia. This condition requires immediate recognition and treatment.

Primary Myelofibrosis (PMF): A myeloproliferative neoplasm characterized by the gradual replacement of bone marrow with fibrotic tissue, leading to ineffective hematopoiesis. Early stages might present with elevated blood cell counts due to extramedullary hematopoiesis, although this is less common.
Congenital Polycythemia: Rare genetic disorders that lead to increased erythropoiesis and can result in elevated RBC counts. These conditions might also affect other cell lines, though this is less typical.
Familial Erythrocytosis: Similar to congenital polycythemia, these are rare genetic conditions that primarily affect RBC production but can occasionally be associated with increases in other blood cell types.