Chronic Lymphocytic Leukemia (CLL): This is the most common diagnosis associated with a CD5+ monoclonal B-cell population. CLL is characterized by the clonal expansion of mature B cells that express CD5, along with other specific markers. The presence of CD5 on B cells is a hallmark of CLL, making it the first consideration in the differential diagnosis.

Mantle Cell Lymphoma (MCL): MCL is another B-cell lymphoma that expresses CD5. It is less common than CLL but should be considered, especially if the patient presents with lymphadenopathy or splenomegaly and specific cytogenetic abnormalities like t(11;14).
Monoclonal B-cell Lymphocytosis (MBL): MBL is a condition characterized by the presence of small clones of B cells with a CLL-like phenotype, including CD5 expression. It is often considered a precursor to CLL and can be found in healthy individuals, but its clinical significance varies.

Richter's Transformation: This is a transformation of CLL into a more aggressive lymphoma, often diffuse large B-cell lymphoma (DLBCL). Although not typically CD5+, the transformation can sometimes retain CD5 expression. Missing this diagnosis could lead to a significant delay in appropriate treatment.
Prolymphocytic Leukemia (PLL): PLL is a rare and aggressive leukemia that can express CD5. It is crucial to distinguish PLL from CLL due to differences in prognosis and treatment.

Hairy Cell Leukemia: Although typically CD103+, some variants of hairy cell leukemia can express CD5. It's a rare consideration but should be kept in mind, especially with specific clinical and morphological features.
Lymphoplasmacytic Lymphoma/Waldenström Macroglobulinemia: These conditions are characterized by the proliferation of lymphoplasmacytic cells and can rarely express CD5. They are typically associated with the production of IgM paraprotein and have distinct clinical features.