This patient most likely has lymphoma, and excisional or core needle biopsy of an enlarged lymph node should be done next to establish a tissue diagnosis. Optimally, an excisional biopsy should be performed to preserve lymph node architecture which is important in differentiating reactive lymphadenopathy from lymphoma. Core needle biopsy is able to sample some structural aspects of the lymph node, and may be used for deep lymph nodes in place of excision. This patient's presentation of asymptomatic but progressive lymphadenopathy, splenomegaly, and lymphocytosis is highly suggestive of lymphoma. To determine the subtype of lymphoma and to guide therapy, the biopsy specimen is sampled for histopathologic, cytogenetic, and fluorescence in situ hybridization (FISH) analysis, as well as immunophenotype and gene expression profiling. Routine blood studies include a complete blood count with differential, erythrocyte sedimentation rate, and serum chemistry studies, including serum urate level. Serum lactate dehydrogenase, β₂-microglobulin, and immunoglobulin levels should also be determined. Screening for viral infections, including hepatitis B and C, HIV, human T-cell lymphotrophic virus type 1, human herpesvirus-8, and Epstein-Barr virus (and, when indicated, screening for bacterial infection due to Helicobacter pylori), needs to be performed because these infections can be causative drivers of lymphoma. As active infections may reduce lymphoma response rates and duration, it is essential to treat both the lymphoma and any underlying infections.

Although bone marrow biopsy, generally iliac crest bone marrow biopsy, is needed to complete the evaluation, excisional or core needle biopsy should be done first to establish a tissue diagnosis prior to staging.

Fine-needle lymph node biopsy should not be used because it will not preserve the architecture of the lymph node that is required for the diagnosis of lymphoma.

Patients with aggressive lymphoma presenting with involvement of the testes, sinuses, bone marrow, and ocular sites have an increased risk of central nervous system involvement and require lumbar puncture for cerebrospinal fluid examination. This procedure is only appropriate following the diagnosis and staging of non-Hodgkin lymphoma.

PET scanning is performed to complete staging but, again, should not be done until a tissue diagnosis is established. Early repeat PET scanning (after two to three cycles of chemotherapy) provides important prognostic information for patients with Hodgkin lymphoma but not for those with non-Hodgkin lymphoma.