Algorithm for the classification workup of nodal PTCLs. The current algorithm for diagnosing PTCL requires immunophenotypic study with a panel of markers that, together with viral analysis (HTLV1, EBV), will orient the pathologist to consider and diagnose specific entities. In ambiguous cases, sequencing studies may help diagnose some entities, particularly follicular helper T-cell lymphoma. PTCL, NOS is established when other specific entities are excluded. Phenotypic analysis or analysis by GEP may subdivide patients with PTCL, NOS, but this subclassification is not routinely incorporated into clinical diagnosis and requires further studies for clinical validation. ATLL, adult T-cell leukemia/lymphoma; GrB, granzyme B; Per, perforine.