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?

Question

Accepted Answer

A. Flow cytometry with CD5, CD19, CD23 and assessment of ZAP-70/CD38. ## 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.

![Chronic Leukemias diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/2641.webp)

Answer

B. Serum immunoglobulin levels and serum protein electrophoresis

Answer

C. Bone marrow trephine biopsy with reticulin stain

Answer

D. Cytochemistry with acid phosphatase and tartrate-resistant acid phosphatase (TRAP)

Explanation

Diagnostic and Prognostic Workup of Chronic Lymphocytic Leukemia

Flow Cytometry: The Diagnostic Gold Standard

Prognostic Markers via Flow Cytometry

Why Other Tests Are Insufficient

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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	—