Atypical Lymphocytes on Peripheral Blood Smear

Definition

Classic Morphology (Peripheral Smear Features)

Fig. 8.16 - Atypical lymphocytes in infectious mononucleosis (Robbins, Cotran & Kumar Pathologic Basis of Disease)

Downey Classification (Historical Subtypes)

Peripheral Blood CBC Findings in Infectious Mononucleosis

• Absolute lymphocytosis: >60% of total WBCs are lymphocytes
• Atypical lymphocytes: 5-80% of the lymphocytes are atypical
• Diagnostic threshold: >10% atypical lymphocytes on differential is highly suggestive
• Mild thrombocytopenia may be present (immune-mediated)

Causes of Atypical Lymphocytes on Smear

Distinguishing Atypical Lymphocytes from Blasts (Clinically Critical)

Diagnostic Workup When Atypical Lymphocytes Are Found

1. Monospot test (heterophile antibody) - rapid screen for EBV
2. EBV-specific antibodies: VCA-IgM (acute), VCA-IgG (past), EBNA (late/past)
3. CMV serology if Monospot negative
4. Toxoplasma IgM/IgG if clinical suspicion
5. HIV testing (primary HIV can cause mononucleosis syndrome)
6. LFTs - transaminases elevated in ~80% of EBV mononucleosis
7. Flow cytometry if atypical lymphocytes are >20% or morphology is blastoid (rule out lymphoma/leukemia)

Differentiating Type III Atypical Lymphocytes from Acute Leukemic Blasts

The Core Problem

• Large cell size
• Dispersed/fine chromatin
• Prominent nucleolus
• Basophilic cytoplasm

Feature-by-Feature Morphologic Comparison

The Single Most Important Morphologic Clue

"A pleomorphic spectrum of normal to variant lymphoid morphologies should be reassuring for a reactive lymphocytosis."

• Tietz Textbook of Laboratory Medicine, 7th ed.

• Small normal lymphocytes
• Type I (monocytoid) cells
• Type II (classic Downey) cells
• Type III (immunoblast) cells
• Occasional plasma cells

Chromatin Pattern - Deeper Look

CBC/Clinical Context Clues (Essential Adjuncts)

When Morphology Is NOT Enough: Ancillary Testing

Summary: The Practical Approach at the Microscope

CBC Interpretation: 11-Month-Old Male "Kartik"

Reading the CBC

• Microcytic hypochromic red cells
• Atypical lymphocytes noted previously
• Occasional target cells - now confirmed

Synthesis: What This Picture Tells You

1. High reticulocyte count (3.1%) - this is the key distinguishing feature
2. Target cells on smear
3. High RDW (18.6%) - mixed population of cell sizes
4. Reactive thrombocytosis
5. Age of presentation: 11 months

Why Iron Deficiency Anemia (IDA) is unlikely as the sole diagnosis:

Most Likely Diagnosis: Thalassemia Syndrome

• Microcytic hypochromic anemia (MCV 71, MCH 20)
• Elevated reticulocytes (compensatory erythropoiesis due to hemolysis/ineffective erythropoiesis)
• Target cells on smear
• High RDW (anisocytosis)
• Presentation at 11 months of age
• Reactive thrombocytosis

"The peripheral blood smear shows the same features seen in β-TM, including anisocytosis, hypochromia, target cells, basophilic stippling, and nucleated RBCs." - Tietz Textbook of Laboratory Medicine, 7th ed.

Differential Diagnosis and Correlation

Critical Next Steps - Workup Required

1. Hemoglobin Electrophoresis / HPLC (Most Important)

• β-Thalassemia major: Absent Hb A, elevated Hb F, variable Hb A2
• β-Thalassemia intermedia: Reduced Hb A, elevated Hb F, elevated Hb A2
• Hb E/β-thal: Hb E peak (co-elutes with Hb A2 on HPLC), elevated Hb F
• HbH disease: HbH peak (fast-moving band), reduced Hb A

2. Iron Studies (Serum Iron, TIBC, Serum Ferritin)

• Rules out concurrent IDA (common co-morbidity, especially in developing countries)
• In thalassemia: iron stores are normal or elevated (not low)
• In IDA: ferritin low, TIBC high

3. Peripheral Smear Review (Manual Differential)

• Nucleated RBCs (very suggestive of thalassemia major/intermedia)
• Basophilic stippling (ribosomal aggregates - characteristic of thalassemia)
• Schistocytes (hemolysis)
• Polychromasia (elevated reticulocytes)

4. Parental CBC + HPLC

• Both parents should be tested
• β-Thalassemia minor pattern in one or both parents (low MCV, normal Hb, elevated Hb A2 ≥4%) strongly supports the diagnosis
• "Family studies on both parents and siblings should be performed, and the classical β-thalassemia minor pattern should be found in the parents." - Tietz, 7th ed.

5. Complete Reticulocyte Indices

• Absolute reticulocyte count and reticulocyte production index (RPI)
• RPI >2 confirms adequate marrow response (hemolytic/ineffective erythropoiesis pattern)

Immediate Clinical Concern

• Assess for symptoms of cardiac decompensation: tachycardia, poor feeding, dyspnea, hepatosplenomegaly
• Do NOT start empirical iron until thalassemia is ruled out - iron loading in thalassemia is dangerous
• Urgent HPLC/electrophoresis before any treatment decision
• Pediatric hematology referral if thalassemia confirmed

Summary Statement

Peripheral Blood Smear Report - Reactive Lymphocytosis

Morphology Description (Standard Reporting Language)

• Enlarged lymphocytes (12-20 µm) with abundant pale gray-blue cytoplasm
• Irregular cytoplasmic borders with "skirting" or molding around adjacent red blood cells
• Oval to kidney-shaped nuclei, some with indentation or folding
• Coarse to partially dispersed chromatin with retained heterogeneity
• Prominent but variable nucleoli in larger immunoblastic forms
• Scattered azurophilic cytoplasmic granules in a subset of cells
• Cytoplasmic vacuolations in occasional cells
• Rare plasmacytoid lymphocytes with eccentric nucleus and deeply basophilic cytoplasm

Short-Form Versions (for different contexts)

1. Concise Laboratory Report Format

Reactive (atypical) lymphocytosis noted. Lymphocytes show pleomorphic activated morphology with enlarged cell size, abundant pale cytoplasm, irregular nuclear contours, and partially dispersed chromatin. Cytoplasmic molding around RBCs is present. Morphology consistent with viral reactive lymphocytosis. No blasts identified.

2. EBV/Infectious Mononucleosis-Specific

Absolute lymphocytosis with 25-30% atypical lymphocytes. Atypical cells are large (12-16 µm), characterized by abundant pale vacuolated cytoplasm, oval to folded nuclei with coarse chromatin, and scattered azurophilic granules. Cytoplasmic borders are irregular and indent adjacent RBCs (Downey Type II morphology predominates). Occasional large immunoblastic forms with prominent nucleoli present (Downey Type III). Findings are consistent with reactive lymphocytosis in the clinical context of infectious mononucleosis.

3. Bullet-Point Format (for case sheets / ward rounds)

• Lymphocytes: increased, pleomorphic reactive forms
• Cell size: large, 12-16 µm
• Cytoplasm: abundant, pale gray-blue, vacuolated
• Nuclear shape: oval/indented/folded - NOT round blasts
• Chromatin: coarse, partially dispersed (NOT fine/lacy as in blasts)
• Nucleolus: variable, present in larger forms
• RBC molding ("skirt sign"): present
• Granules: azurophilic, scattered
• Morphologic spectrum: Type I + II + III Downey cells present
• No blasts, no Auer rods, no monomorphic population

4. Morphology for the 11-Month-Old Kartik (Current Case)

Peripheral blood smear shows microcytic hypochromic red cells with moderate anisocytosis and occasional target cells. Polychromasia is noted consistent with elevated reticulocyte count. White cells show a reactive lymphocytosis with pleomorphic atypical lymphocytes displaying enlarged size, abundant cytoplasm, and partially dispersed chromatin. The RBC morphology and clinical context raise concern for an underlying hemoglobinopathy/thalassemia syndrome. Hemoglobin HPLC and iron studies recommended.

Key Phrases to Always Include in Reactive Lymphocytosis Reports

Advising Flow Cytometry in This Case

First: Is Flow Cytometry Indicated Right Now?

1. HPLC / Hemoglobin electrophoresis (for the thalassemia/hemoglobinopathy)
2. Iron studies (ferritin, serum iron, TIBC)
3. Parental CBC + HPLC

When to Advise Flow Cytometry in THIS Case

Trigger 1 - Morphology Concerns (Most Important)

Trigger 2 - CBC Parameters

Trigger 3 - Clinical Features

What Flow Cytometry Panel to Advise (If Indicated)

Panel 1: Rule Out Acute Lymphoblastic Leukemia (ALL) - Most Relevant in 11-Month-Old

• TdT positive + CD34+ + CD10+ CD19+ = B-ALL (most common pediatric leukemia)
• TdT positive + CD3+ CD7+ = T-ALL
• TdT negative, polyclonal Ig light chains, CD3/CD4/CD8 preserved ratio = Reactive

Panel 2: Rule Out AML (if myeloid involvement suspected)

How to Write the Flow Cytometry Request

Clinical Information to Write on the Request Form:

11-month-old male. Peripheral blood smear shows microcytic hypochromic anemia (Hb 7.8 g/dL, MCV 71 fL) with reactive lymphocytosis and atypical lymphocytes (pleomorphic, Type I-III Downey cells). Reticulocyte count 3.1%. Target cells present. WBC 8.2 × 10³/µL. R3 instrument flag.

Thalassemia workup pending (HPLC requested). Query: is the lymphocytosis reactive (viral) or does it represent a co-existing lymphoid malignancy? Please perform peripheral blood immunophenotyping to include TdT, CD34, CD10, CD19, CD3, CD7, CD45, CD20, kappa/lambda.

The Practical Decision Tree for THIS Case

Atypical lymphocytes in 11-month-old
            |
            ↓
Morphology pleomorphic + WBC normal + PLT normal/high?
            |
            YES → Primary workup: HPLC + iron studies
            |         Flow cytometry: NOT needed immediately
            |
            NO (monomorphic / blastoid / thrombocytopenia / high WBC)
            ↓
     Order flow cytometry URGENTLY
     Panel: TdT, CD34, CD10, CD19, CD3, CD7, MPO, CD45

In Kartik's Specific Case - Recommendation

• WBC 8.2 - normal, not elevated
• PLT 408 - elevated (reactive thrombocytosis, not thrombocytopenia)
• Morphology described as pleomorphic (reactive spectrum present)
• Clinical picture best explained by thalassemia + concurrent viral illness (common in infants)

1. Repeat CBC in 2-3 weeks shows persistent or worsening lymphocytosis
2. WBC rises above 20 × 10³/µL
3. New thrombocytopenia or anemia worsening beyond thalassemia expectation
4. Smear review shows predominantly blastoid Type III cells or monomorphic population
5. EBV/CMV serology comes back negative with no identifiable viral cause
6. Clinical: bone pain, night sweats, disproportionate hepatosplenomegaly