Chronic Lymphocytic Leukaemia (CLL)

CLL is the commonest leukaemia of adults in the Western world and a favourite NEET PG topic for its instantly recognisable smear (smudge cells), a fixed immunophenotype (CD5+CD19+CD23+), and several named complications (Richter transformation, AIHA). This note packs the high-yield identifiers, staging systems and management you are most likely to be tested on.

Definition & classification

Chronic lymphocytic leukaemia is a monoclonal proliferation of mature, small, functionally incompetent B lymphocytes that accumulate in the blood, bone marrow, lymph nodes and spleen. The cells are immunologically "naive-looking" but biologically inert, so the patient is left immunodeficient despite a high lymphocyte count.

CLL and small lymphocytic lymphoma (SLL) are the same disease differing only in the site of the bulk of tumour:

Feature	CLL	SLL
Predominant compartment	Peripheral blood + marrow	Lymph nodes / tissue
Diagnostic blood criterion	≥5 × 10⁹/L monoclonal B cells	<5 × 10⁹/L in blood
Immunophenotype	CD5+CD19+CD23+	Identical
WHO view	One entity ("CLL/SLL")	One entity ("CLL/SLL")

A pre-malignant state, monoclonal B-cell lymphocytosis (MBL), is defined as a clonal B-cell count <5 × 10⁹/L with no lymphadenopathy, organomegaly, cytopenia or symptoms. MBL progresses to overt CLL at ~1–2% per year.

High-yield: The diagnostic threshold for CLL is a clonal B-lymphocyte count of ≥5000/µL (5 × 10⁹/L) sustained for at least 3 months. Below this, with no other disease features, it is MBL — not CLL.

WHO classifies CLL/SLL under mature (peripheral) B-cell neoplasms. It is distinct from acute lymphoblastic leukaemia, which involves immature lymphoblasts (TdT+) — a classic distractor.

Epidemiology & etiology

• Median age ~70 years; rare below 40. A disease of the elderly.
• Male predominance (~2:1).
• Commonest leukaemia in adults in Western countries; notably rarer in East Asia, a difference that persists after migration, implicating genetics over environment.
• Strongest known risk factor is a positive family history (genetic predisposition). Unlike most haematological malignancies, CLL has no proven link to ionising radiation.

The cell of origin is a mature B lymphocyte. Two molecular subsets exist based on the somatic hypermutation status of the immunoglobulin heavy-chain variable region (IGHV):

IGHV status	Cell of origin	Prognosis
Mutated IGHV	Post-germinal-centre B cell	Favourable
Unmutated IGHV	Naive (pre-germinal-centre) B cell	Worse

Pathophysiology

The malignant clone survives because of defective apoptosis, not rapid proliferation. The anti-apoptotic protein BCL-2 is overexpressed, so cells accumulate over years — explaining the "chronic," indolent course in many patients. (This is the rationale behind the BCL-2 inhibitor venetoclax.)

B-cell receptor (BCR) signalling drives survival, with Bruton tyrosine kinase (BTK) as a central node — the target of ibrutinib and acalabrutinib.

The accumulating clone produces three downstream problems:

1. Marrow replacement → anaemia, thrombocytopenia, neutropenia.
2. Hypogammaglobulinaemia → recurrent infections (the leading cause of death in CLL).
3. Immune dysregulation → autoimmune cytopenias (AIHA, ITP).

Cytogenetics & prognosis

FISH abnormalities are strongly prognostic and frequently asked:

Abnormality	Frequency	Prognostic meaning
del(13q14) (isolated)	~55%	Best prognosis
Trisomy 12	~15%	Intermediate
del(11q) (ATM)	~10%	Poor; bulky nodes
del(17p) / TP53 mutation	~7%	Worst; resistant to chemo-immunotherapy

High-yield: del(17p)/TP53 mutation predicts resistance to standard chemo-immunotherapy (FCR) and mandates a targeted agent (BTK or BCL-2 inhibitor) up front. Isolated del(13q) carries the best prognosis.

Clinical features

CLL is frequently an incidental finding — a lymphocytosis on a routine blood count in an asymptomatic elderly person. When symptomatic:

• Painless, symmetrical lymphadenopathy (rubbery, mobile, non-tender).
• Hepatosplenomegaly (splenomegaly more prominent).
• Constitutional "B symptoms": fever, drenching night sweats, weight loss >10%, fatigue.
• Recurrent infections from hypogammaglobulinaemia and impaired cell-mediated immunity — herpes zoster reactivation is characteristic.
• Autoimmune cytopenias: warm autoimmune haemolytic anaemia (AIHA) and immune thrombocytopenia.

High-yield: Warm AIHA is the classic autoimmune complication of CLL → a positive direct Coombs (DAT) test, spherocytes, raised reticulocytes, raised LDH and unconjugated bilirubin, and low haptoglobin. The autoantibody is typically IgG reacting at 37 °C.

Diagnosis & investigation of choice

Approach: CBC showing lymphocytosis → peripheral smear → flow cytometry (immunophenotyping) is the investigation of choice → confirm clonality.

Peripheral blood smear

• Marked mature lymphocytosis — small lymphocytes with scant cytoplasm, clumped "soccer-ball"/"cracked-earth" chromatin and an inconspicuous nucleolus.
• Smudge / smear / basket cells: fragile leukaemic lymphocytes ruptured during smear preparation. Their number correlates with tumour burden.

High-yield: Smudge cells (Gumprecht shadows) on the peripheral smear are the classic morphological clue to CLL. They are an artefact of the fragile CLL lymphocyte — adding albumin to the blood before smearing reduces them.

Immunophenotyping (flow cytometry) — investigation of choice

This is the diagnostic gold standard and confirms a monoclonal B-cell population.

Marker	CLL result
CD5	Positive (a T-cell marker aberrantly expressed)
CD19, CD20, CD23	Positive
CD200	Positive
Surface immunoglobulin (sIg)	Weak/dim
CD20, CD79b, FMC7	Weak/negative
Clonality	Light-chain restriction (kappa or lambda only)

High-yield: The classic NEET-PG immunophenotype is CD5+ CD19+ CD23+ with dim surface Ig and light-chain restriction. Co-expression of CD5 (a T-cell antigen) on a clonal B cell (CD19+) is the single most tested identifier.

The Matutes/CLL score (out of 5) distinguishes CLL from other B-cell leukaemias — one point each for CD5+, CD23+, weak sIg, CD79b/FMC7 weak/negative, and CD22/CD11c weak. Score 4–5 = CLL.

High-yield: Bone marrow biopsy is not required to diagnose CLL — peripheral blood flow cytometry suffices. Marrow may be done for prognosis or to evaluate cytopenias.

Other tests

• Hypogammaglobulinaemia on immunoglobulin assay (immune paresis).
• FISH for del(17p)/del(11q); TP53 sequencing and IGHV mutation status before treatment.
• Beta-2 microglobulin — raised level is an adverse prognostic marker (part of the CLL-IPI).

Staging — Rai & Binet (named criteria)

Two clinical staging systems, both based on lymphadenopathy, organomegaly and cytopenias, are highly testable.

Rai (USA):

Stage	Features	Risk
0	Lymphocytosis only	Low
I	+ Lymphadenopathy	Intermediate
II	+ Hepato/splenomegaly	Intermediate
III	+ Anaemia (Hb <11 g/dL)	High
IV	+ Thrombocytopenia (<100 × 10⁹/L)	High

Binet (Europe) — counts involved lymphoid areas (cervical, axillary, inguinal nodes, spleen, liver = 5 areas):

Stage	Definition
A	<3 areas involved; no anaemia/thrombocytopenia
B	≥3 areas involved; no anaemia/thrombocytopenia
C	Anaemia (Hb <10 g/dL) or thrombocytopenia (<100 × 10⁹/L)

High-yield: In both systems, the worst stage (Rai III–IV, Binet C) is defined by cytopenias from marrow failure — anaemia or thrombocytopenia. Note these must be from marrow infiltration, not autoimmune destruction, to upstage.

The modern CLL-IPI integrates TP53 status, IGHV status, beta-2 microglobulin, clinical stage and age into low/intermediate/high/very-high risk groups.

Management / drug of choice

The cardinal principle: early-stage asymptomatic CLL is NOT treated — "watch and wait." Trials show no survival benefit from treating early; treatment is reserved for active disease.

Indications to treat (IWCLL): progressive marrow failure (worsening anaemia/thrombocytopenia), massive or symptomatic splenomegaly/nodes, lymphocyte doubling time <6 months, autoimmune cytopenia refractory to steroids, or significant B symptoms.

High-yield: An absolute lymphocyte count alone — however high — is NOT an indication to treat. Treat for symptoms, marrow failure or rapid progression, not for the number.

Treatment paradigm shift (drugs of choice)

Modern first-line therapy is targeted, oral and largely chemotherapy-free:

• BTK inhibitors — ibrutinib, acalabrutinib, zanubrutinib.
• BCL-2 inhibitor — venetoclax (often + anti-CD20 obinutuzumab); risk of tumour lysis syndrome at initiation, hence dose ramp-up.
• Anti-CD20 monoclonal antibodies — rituximab, obinutuzumab.

High-yield: For del(17p)/TP53-mutated CLL, the drugs of choice are a BTK inhibitor or venetoclax — chemo-immunotherapy (FCR) is ineffective and should be avoided.

The old standard FCR (fludarabine + cyclophosphamide + rituximab) is now reserved mainly for young, fit, IGHV-mutated, TP53-wild-type patients. Supportive care — vaccination (avoid live vaccines), prompt treatment of infections, and IV immunoglobulin for recurrent infections with hypogammaglobulinaemia — is essential. Autoimmune cytopenias are treated first with corticosteroids.

Complications

1. Infections — the leading cause of death; from hypogammaglobulinaemia and immunosuppressive therapy.
2. Autoimmune cytopenias — warm AIHA (commonest), immune thrombocytopenia, pure red cell aplasia.
3. Richter transformation — see below.
4. Second malignancies — increased risk of skin cancers and other solid tumours.
5. Tumour lysis syndrome — especially on starting venetoclax.

Richter transformation (Richter syndrome)

Transformation of CLL into an aggressive large-cell lymphoma — most commonly diffuse large B-cell lymphoma (DLBCL) (rarely Hodgkin lymphoma).

High-yield: Suspect Richter transformation when a stable CLL patient develops rapidly enlarging asymmetric lymph nodes, a sudden rise in LDH, new B symptoms and hypercalcaemia. It carries a poor prognosis. Diagnosis is by excisional lymph node biopsy (PET-guided to the most metabolically active node).

Key differentials

CLL must be separated from other mature B-cell lymphoproliferative disorders. Immunophenotype is decisive.

Disorder	Key cell/morphology	CD5	CD23	Distinguishing marker
CLL/SLL	Small mature lymphocytes; smudge cells	+	+	CD200+, dim sIg, Matutes 4–5
Mantle cell lymphoma	Irregular nuclei	+	–	t(11;14), cyclin D1+, SOX11+
Hairy cell leukaemia	Hairy cytoplasmic projections; dry tap, pancytopenia, massive splenomegaly	–	–	TRAP+, CD11c/CD25/CD103+, BRAF V600E, annexin A1
Follicular lymphoma	Centrocytes/centroblasts	–	–/+	t(14;18), BCL2+, CD10+
Prolymphocytic leukaemia	>55% prolymphocytes, prominent nucleolus	±	–	Very high WCC, bright sIg

High-yield: CD5+CD23+ = CLL vs CD5+CD23− with cyclin D1 / t(11;14) = mantle cell lymphoma. This CD23 distinction is a recurring single-best-answer pair.

High-yield: CLL is the classic cause of warm AIHA; ALL it is not. Don't confuse with cold agglutinin disease, which is associated with Mycoplasma infection and lymphoma and gives a cold IgM, complement-only (C3d) positive DAT.

Mnemonics & eponyms

• "CLL = Crushed Little Lymphocytes" → smudge/smear cells (Gumprecht shadows).
• Immunophenotype "5, 19, 23" → CD5, CD19, CD23 positive.
• "Richter = Rapid" → rapidly enlarging node + rising LDH = transformation to DLBCL.
• Smudge cells = Gumprecht shadows; basket/smear cells are synonyms.
• Rai → R for Red cells & platelets last (anaemia stage III, thrombocytopenia stage IV).

Recently asked / exam angle

• Smudge cells on smear in an elderly man with lymphocytosis → identify the diagnosis (CLL).
• CD5+CD19+CD23+ immunophenotype → name the leukaemia (CLL); or CD5+ but CD23− with t(11;14)/cyclin D1 → mantle cell.
• Richter transformation → the aggressive lymphoma is DLBCL; diagnostic test is excisional/PET-guided lymph node biopsy.
• Positive direct Coombs test in a CLL patient → warm autoimmune haemolytic anaemia.
• Best vs worst cytogenetics → del(13q) best, del(17p)/TP53 worst.
• Indication to treat → progressive marrow failure / lymphocyte doubling time <6 months, not the absolute count.
• Diagnostic threshold → clonal B cells ≥5 × 10⁹/L; below = MBL.
• Staging → match Rai stage to feature, or Binet A/B/C to lymphoid areas + cytopenia.
• Investigation of choice → flow cytometry / immunophenotyping (bone marrow not mandatory).
• Commonest cause of death → infection (hypogammaglobulinaemia).

Rapid revision

1. CLL = monoclonal proliferation of mature, incompetent B lymphocytes; commonest adult leukaemia in the West; elderly men.
2. Diagnostic blood criterion: clonal B cells ≥5 × 10⁹/L; below threshold with no disease = MBL.
3. Smudge cells (Gumprecht shadows) on smear = fragile CLL lymphocytes; reduced by adding albumin.
4. Investigation of choice = flow cytometry; classic phenotype CD5+ CD19+ CD23+ CD200+, dim sIg, light-chain restricted.
5. Matutes/CLL score 4–5 confirms CLL; CD5+CD23− with cyclin D1/t(11;14) = mantle cell lymphoma.
6. Cytogenetics: del(13q) best, del(17p)/TP53 worst (FCR-resistant).
7. IGHV mutated = good prognosis; unmutated = worse.
8. Staging: Rai 0–IV and Binet A/B/C; worst stages defined by anaemia/thrombocytopenia from marrow failure.
9. Watch and wait in asymptomatic disease; treat for marrow failure, bulky/symptomatic disease, or doubling time <6 months — never the count alone.
10. Drugs of choice today: BTK inhibitors (ibrutinib/acalabrutinib) and venetoclax (BCL-2 inhibitor; watch tumour lysis); FCR only in young fit IGHV-mutated TP53-wild-type.
11. Richter transformation → DLBCL: rapidly enlarging node, rising LDH, B symptoms; diagnose by excisional lymph node biopsy.
12. Complications: infection (commonest cause of death), warm AIHA (positive DAT), immune thrombocytopenia, second cancers.