An adult patient presents with generalized lymphadenopathy and blood film shows 70% immature looking lymphocytes. What should be the next best investigation?
An adult patient presenting with generalized lymphadenopathy and blood film shows 70% immature looking lymphocytes is highly suggestive of chronic lymphocytic leukemia. Immunophenotyping can be one of the best ways to differentiate between CLL and other B cell neoplasms.
Important points about CLL

Most of the patients are often asymptomatic at diagnosis. When symptoms appear, they are nonspecific and include easy fatigability, weight loss, and anorexia. Hepatosplenomegaly and generalized lymphadenopathy are present in 50% to 60% of symptomatic patients.
The immunophenotype of CLL is distinct. The tumor cells express the pan-B cell markers CD19 and CD20, as well as CD23 and CD5, the latter a marker that is found on a small subset of normal B cells. Low-level expression of surface Ig (usually either IgM or IgM and IgD) is also typical.
Genotyping/karyotyping
Immunophenotyping
Bone marrow
Peripheral smear study