Peripheral Blood Smear (PBS)
🩸 Peripheral Blood Smear (PBS) / Blood Cell Morphology
The most fundamental and essential test for evaluating blood cell abnormalities
Peripheral Blood Smear (PBS) is performed by spreading a drop of peripheral blood on a glass slide, staining it, and examining it under a microscope.
Unlike automated hematology analyzers, PBS allows direct visual assessment of cell morphology, immature cells, and findings suggestive of hematologic malignancies—making it one of the most diagnostically valuable hematology tests.
1️⃣ Purpose of the PBS / Morphology Examination
✔ 1) Confirming Abnormal CBC Results
PBS is indicated when automated analyzers generate abnormal flags such as:
- WBC abnormal scatter / immature cells
- Platelet clumping
- Abnormal RBC morphology
- Suspected blasts or atypical lymphocytes
These flags require manual morphological verification.
✔ 2) Evaluating Blood Cell Morphology
PBS detects abnormalities that automated instruments cannot measure:
- Abnormal RBC shape and size
- Toxic changes in neutrophils
- Blasts in leukemia
- Atypical lymphocytes
- Giant platelets or platelet clumps
✔ 3) Diagnosing Hematologic Disorders
PBS plays a central role in evaluating:
- Types of anemia (iron deficiency, hemolytic, macrocytic, etc.)
- Leukemia and lymphoma
- Bone marrow disorders (aplastic anemia, dysplasia)
- Neutrophil changes in infection or sepsis
2️⃣ Test Method (Staining & Microscopy)
✔ 1) Smear Preparation
- Place a drop of EDTA blood near one end of the slide
- Use a spreader slide at ~30° to push the drop forward
- A good smear forms a smooth feathered edge
Quality of smear significantly affects interpretation.
✔ 2) Staining
Common stains:
- Wright
- Giemsa
- Wright–Giemsa
These highlight cytoplasm, nuclei, and granules for easier differentiation.
✔ 3) Microscopic Examination
- 10×: Evaluate smear quality and general distribution
- 40×: Screen for abnormal cells
- 100× oil immersion: Detailed morphology of RBCs, WBCs, and platelets
3️⃣ Key Morphologic Findings in PBS
🔴 1) Red Blood Cells (RBC)
✔ What to Assess
- Size: microcytes, macrocytes
- Shape: schistocytes, spherocytes, target cells
- Color: hypochromia
- Variation: anisopoikilocytosis
✔ Clinical Interpretation Examples
- Microcyte + Hypochromia → Iron deficiency anemia
- Spherocyte → Hemolytic anemia (especially autoimmune hemolysis)
- Schistocyte → DIC, TTP, HUS (microangiopathic hemolysis)
- Target cell → Liver disease, thalassemia
- Howell–Jolly body → Asplenia
⚪ 2) White Blood Cells (WBC)
✔ What to Assess
- Maturity (segmented vs band)
- Toxic changes (toxic granulation, Döhle bodies, vacuoles)
- Atypical lymphocytes
- Presence of blasts
✔ Clinical Interpretation
- Toxic changes → Severe bacterial infection
- Atypical lymphocytes → Viral infections (EBV, CMV)
- Blast increase → Suggestive of acute leukemia, requires urgent evaluation
🟣 3) Platelets
✔ What to Assess
- Platelet size (giant platelets)
- Aggregation or clumping
- Morphologic abnormalities
✔ Clinical Interpretation
- Platelet clumping → Spurious thrombocytopenia on CBC
- Giant platelets → Seen in MDS, Bernard–Soulier syndrome
4️⃣ Important Considerations for PBS Interpretation
✔ 1) Always correlate PBS with automated CBC results
Morphology + CBC indices provide accurate diagnosis.
Example:
RDW ↑ + schistocytes → consider hemolytic anemia.
✔ 2) Evaluate for platelet clumping
If platelet clumps are present, low platelet count may be artifactual.
✔ 3) Examine the feathered edge
Certain findings are often concentrated at the edge:
- Malaria parasites
- Large blasts
- Giant platelets
✔ 4) Consider medication effects
- Steroids → Neutrophilia
- Chemotherapy → Neutropenia, not blast reduction
✔ 5) Avoid misinterpretation due to artifacts
Poor smear quality may cause:
- Pseudohypochromia
- Water artifact
- Overstaining or understaining
Proper slide preparation is essential.
🧾 References
- Henry’s Clinical Diagnosis and Management by Laboratory Methods, 24th ed.
- Rodak’s Hematology: Clinical Principles and Applications, 6th ed.
- Bain BJ. Blood Cells: A Practical Guide.
- CLSI H20-A2: Reference Leukocyte Differential Count and Evaluation of Instrumental Methods.
- Hoffbrand AV. Essential Haematology.