A 72-year-old woman is found to have WBC 15,000/μL with 70% lymphocytes on routine CBC. Peripheral smear shows small, mature-appearing lymphocytes with scanty cytoplasm. Bone marrow is hypercellular with lymphoid infiltration. Which investigation is most specific for confirming chronic lymphocytic leukemia and determining prognosis?

Explanation

## Diagnostic and Prognostic Workup of Chronic Lymphocytic Leukemia ### Flow Cytometry: The Diagnostic Gold Standard **Key Point:** Flow cytometry is the gold standard for diagnosing CLL and must demonstrate a clonal population of CD5+ CD19+ CD23+ B lymphocytes (co-expression of T-cell marker CD5 with B-cell markers is pathognomonic). **High-Yield:** The CLL immunophenotype is: - **CD5+** (T-cell marker on B cells — key abnormality) - **CD19+** (B-cell marker) - **CD23+** (B-cell marker; distinguishes CLL from mantle cell lymphoma, which is CD5+ CD23−) - **Surface immunoglobulin weak** (monoclonal) - **CD10−** (negative; helps exclude follicular lymphoma) ### Prognostic Markers via Flow Cytometry | Marker | Prognostic Significance | Poor Prognosis | |---|---|---| | **ZAP-70** | Surrogate for IGHV mutation status | >20% expression | | **CD38** | Associated with unmutated IGHV | >30% expression | | **TP53 deletion (del 17p)** | Worst prognosis; TP53 mutation | Present | | **del 11q** | Intermediate prognosis | Present | | **Trisomy 12** | Intermediate prognosis | Present | | **Normal karyotype** | Intermediate prognosis | — | **Clinical Pearl:** ZAP-70 and CD38 are surrogate markers for IGHV (immunoglobulin heavy chain variable region) mutation status. Unmutated IGHV (reflected by high ZAP-70/CD38) predicts aggressive disease and shorter survival; mutated IGHV predicts indolent disease. ### Why Other Tests Are Insufficient 1. **Serum immunoglobulin and protein electrophoresis:** May show monoclonal spike (M-protein) in some cases, but cannot identify the clonal B-cell population or distinguish CLL from other lymphoproliferative disorders. 2. **Bone marrow trephine with reticulin:** Assesses marrow infiltration and fibrosis but does not provide immunophenotypic diagnosis or prognostic markers. 3. **Acid phosphatase/TRAP:** TRAP is positive in hairy cell leukemia (a different entity); acid phosphatase is non-specific and not used for CLL diagnosis. **Mnemonic:** **CD5-19-23 = CLL** — The triple positivity (CD5, CD19, CD23) is the immunophenotypic signature of CLL.