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The Blood Picture - From Basics to Clinical

A complete teaching guide drawn from Henry's Clinical Diagnosis, Tietz Textbook of Laboratory Medicine, Histology (Eroschenko), Robbins Pathology, Harrison's, and Goldman-Cecil Medicine.

PART 1 - What is the "Blood Picture"?

The term blood picture refers to the full interpretation of a peripheral blood smear (PBS) combined with the complete blood count (CBC). It is the most ordered investigation in medicine and serves as a window into the bone marrow, immune system, nutritional status, and dozens of systemic diseases.

PART 2 - The CBC: Numbers First

The CBC is generated by automated flow cytometry-based analyzers that count ~10,000 cells per category. The key parameters:

Parameter	Normal Range	What it Reflects
RBC count	M: 4.5-5.5 × 10¹²/L; F: 4.0-5.0 × 10¹²/L	Total circulating red cells
Hemoglobin (Hb)	M: 13.5-17.5 g/dL; F: 12-15.5 g/dL	O₂-carrying capacity
Hematocrit (HCT/PCV)	M: 41-53%; F: 36-46%	Packed cell volume
MCV	80-100 fL	Average RBC size
MCH	27-33 pg	Hb per cell
MCHC	31-36 g/dL	Hb concentration per cell
RDW	<14.5%	Variation in RBC size (anisocytosis)
WBC	4.5-11.0 × 10⁹/L	Total leukocytes
Platelets	150-400 × 10⁹/L	Thrombocytes

The rule of thirds: Hb × 3 ≈ HCT, and HCT × 3 ≈ RBC count (roughly). A quick bedside check for CBC consistency.

Source: Histology - A Text and Atlas (Eroschenko), p. 728-729

PART 3 - Making and Reading the Blood Film

Preparation

Blood is anticoagulated in EDTA and a wedge film (or coverslip preparation) is made. It is stained with Romanowsky stain (Wright-Giemsa). The film must be:

Well-made (no artefacts, uniform thickness)
Properly stained
Evaluated systematically: low power scan first (10x), then high power (40-100x oil immersion)

The Systematic Approach

When you pick up a blood film, go through these 5 steps in order:

Scan at low power - estimate WBC count, check for clumps, parasites, large abnormal cells
Evaluate RBC morphology - size, shape, colour, inclusions
Perform differential WBC count - 100 cells minimum; note morphology
Estimate platelet count - ~1 platelet per 10-30 RBCs = normal
Note any abnormal cells - nucleated RBCs, blasts, parasites - these are NEVER normal in peripheral blood

PART 4 - Normal RBC Morphology

The reference image - normal RBCs:

Normal RBC morphology showing central pallor ~1/3 diameter

Normal RBCs are:

Normocytic: MCV 80-100 fL, diameter ~6-8 µm (roughly the size of a small lymphocyte nucleus)
Normochromic: MCHC 31-36 g/dL; central pallor = approximately 1/3 of the cell diameter
Uniform in shape: biconcave discs - this gives them flexibility to squeeze through capillaries
Uniform in size: minimal anisocytosis (RDW <14.5%)

Source: Tietz Textbook of Laboratory Medicine, 7th Ed., p. 2989

PART 5 - Abnormal RBC Morphology (The Core of the Blood Picture)

This is the most clinically useful part. Each abnormal shape has a differential diagnosis.

5.1 - Size Changes

Abnormality	Feature	Key Causes
Microcytes	RBC <6 µm, MCV <80 fL, increased central pallor	Iron deficiency anemia, thalassemia, anemia of chronic disease, sideroblastic anemia
Macrocytes	RBC >8 µm, MCV >100 fL	B12/folate deficiency, liver disease, hypothyroidism, MDS, chemotherapy (methotrexate)
Anisocytosis	Variation in size	Elevated RDW; seen in iron deficiency (one of the earliest findings), mixed deficiencies

Pearl: In iron deficiency, anisocytosis (high RDW) appears BEFORE the MCV falls. An elevated RDW with normal MCV should prompt iron studies.

5.2 - Shape Changes (Poikilocytosis)

Cell	Description	Mechanism	Causes
Spherocyte	Small, round, no central pallor	Loss of membrane	Hereditary spherocytosis, AIHA, ABO incompatibility
Elliptocyte / Ovalocyte	Oval/cigar-shaped	Spectrin mutation	Hereditary elliptocytosis; also iron deficiency (pencil cells = hypochromic elliptocytes)
Target cell (Codocyte)	Bullseye appearance	Excess membrane or decreased Hb	Thalassemia, HbC disease, liver disease, post-splenectomy, iron deficiency
Sickle cell (Drepanocyte)	Elongated crescent/scythe	HbS polymerisation under low O₂	Sickle cell disease (pathognomonic)
Schistocyte	RBC fragments, helmet cells	Mechanical shearing in small vessels	TTP, HUS, DIC, prosthetic heart valves, HELLP syndrome
Acanthocyte (Spur cell)	Irregular spicules of varying length	Membrane lipid abnormality	Abetalipoproteinemia, severe liver disease, McLeod phenotype
Echinocyte (Burr cell)	Regular spicules, uniform	Crenation (often artefact)	Uremia, liver disease, often artefact (EDTA effect)
Stomatocyte	Slit-like central pallor (mouth shape)	Altered Na/K permeability	Hereditary stomatocytosis, alcohol, liver disease
Teardrop cell (Dacrocyte)	Teardrop-pear shape	Forced through stiff marrow stroma	Myelofibrosis, thalassemia, infiltrative marrow disease
Bite cell (Degmacyte)	Semicircular bite out of cell	Heinz body removed by spleen	G6PD deficiency, unstable hemoglobins, oxidative drug exposure

Source: Tietz Textbook of Laboratory Medicine, Table 76.1, pp. 2989-2990; Harrison's Principles of Internal Medicine 22E

PART 6 - RBC Inclusions

Inclusion	Stain	Composition	Causes
Howell-Jolly bodies	Romanowsky (dark purple dot)	DNA remnant (chromosome fragment)	Post-splenectomy, functional asplenia (sickle cell), megaloblastic anemia
Basophilic stippling	Romanowsky (blue dots)	Aggregated ribosomes	Lead poisoning, thalassemia, megaloblastic anemia, pyrimidine 5'-nucleotidase deficiency
Heinz bodies	Supravital stain (brilliant cresyl blue) - NOT visible on Romanowsky	Denatured/oxidised Hb	G6PD deficiency, unstable hemoglobins, alpha-thalassemia
Pappenheimer bodies	Romanowsky and Prussian blue	Iron granules (siderosomes)	Sideroblastic anemia, post-splenectomy, hemolytic anemia
Cabot rings	Romanowsky (ring/figure-8)	Mitotic spindle remnant	Severe megaloblastic anemia, lead poisoning
Malaria parasites	Romanowsky	Plasmodium sp.	Malaria (ring forms in P. falciparum; enlarged cells in P. vivax/ovale)

PART 7 - WBC Morphology on the Blood Film

The Differential - Normal Ranges

Cell	%	Absolute count × 10⁹/L	Key morphology
Neutrophil	50-70%	1.8-7.0	2-5 lobes, coarse chromatin, pale granules
Lymphocyte	20-40%	1.0-4.8	Round nucleus, scant cytoplasm
Monocyte	2-8%	0.2-0.8	Kidney-shaped nucleus, grey cytoplasm
Eosinophil	1-4%	0.04-0.4	2-lobed, large orange-red granules
Basophil	0-1%	0-0.1	Large dark blue/purple granules obscure nucleus

Neutrophil Morphology in Detail

Segmented vs. Band Neutrophil:

Segmented (mature): 2+ lobes connected by thin filaments (thread-like, no width)
Band (immature): horseshoe/U-shaped nucleus, thicker connection between lobes - filament has width

Segmented and band neutrophils on peripheral blood smear (Henry's Clinical Diagnosis)

(Here you can see two neutrophils with lobulated nuclei and pale granular cytoplasm, surrounded by normocytic normochromic RBCs and a platelet cluster.)

Clinically Important WBC Changes

Left shift: appearance of immature granulocytes in blood (bands, metamyelocytes, myelocytes) - seen in sepsis, severe infection, myeloproliferative disorders

Toxic granulation: coarse, dark-blue primary granules in neutrophils - indicates severe bacterial infection, sepsis, or toxic states

Dohle bodies: pale blue cytoplasmic inclusions in neutrophils (rough ER remnants) - sepsis, burns, pregnancy

Hypersegmentation (>5 lobes): classic for megaloblastic anemia (B12/folate deficiency). Defined as: >5% of neutrophils with 5+ lobes, OR any neutrophil with 6+ lobes.

Megaloblastic anemia - hypersegmented neutrophil with 6+ lobes, oval macrocytes in background

Reactive lymphocytosis: atypical lymphocytes (Downey cells) - large, irregular, with abundant blue cytoplasm that indents around adjacent RBCs - seen in EBV (infectious mononucleosis), CMV, viral hepatitis

PART 8 - Platelet Assessment on the Film

Estimation rule: ~7-20 platelets per oil immersion field = normal count (corresponds to 150-400 × 10⁹/L)

Platelets (arrows) on peripheral blood smear - round to oval, 2-4 µm, fine purple granules

Normal platelets: 2-4 µm, round to oval, contain fine purple granules. Central "granulomere" and peripheral pale "hyalomere" may be seen.

Giant platelets (>3 µm): ITP (immune thrombocytopenic purpura), Bernard-Soulier syndrome, myeloproliferative neoplasms

Platelet clumping: artefact from EDTA causing pseudothrombocytopenia - check by repeating in citrate tube

Hypogranular platelets: myeloproliferative neoplasms (especially CML, ET)

Source: Henry's Clinical Diagnosis, p. 645

PART 9 - Clinical Integration: Reading the Film Diagnostically

This is where it all comes together. Use the MCV as your first branch point.

The MCV-Based Approach to Anemia

ANEMIA (low Hb)
       |
       +--- MCV <80 fL (MICROCYTIC)
       |         Iron deficiency: ↑RDW, hypochromia, pencil cells, ↓ferritin, ↑TIBC
       |         Thalassemia: normal or ↓RDW, target cells, basophilic stippling, normal ferritin
       |         Anemia of chronic disease: usually normocytic, but can be microcytic; ↑ferritin
       |         Sideroblastic: ring sideroblasts on bone marrow; dimorphic picture on PBS
       |
       +--- MCV 80-100 fL (NORMOCYTIC)
       |         Acute blood loss, hemolytic anemia, anemia of chronic disease
       |         Aplastic anemia, renal disease (EPO deficiency)
       |         Mixed deficiency (Fe + B12 -- can cancel each other out to give normal MCV!)
       |
       +--- MCV >100 fL (MACROCYTIC)
                 Megaloblastic: oval macrocytes + hypersegmented neutrophils
                 (B12 deficiency: subacute combined degeneration; folate: neural tube defects)
                 Non-megaloblastic: round macrocytes (liver disease, alcohol, hypothyroidism, MDS)

Source: Robbins Pathologic Basis of Disease, Table 14.1; Harrison's, p. 731

Disease-Specific Patterns

Disease	Blood Film Clues
Iron deficiency anemia	Hypochromic microcytes, pencil cells (hypochromic elliptocytes), ↑RDW, thrombocytosis (reactive)
B12/Folate deficiency	Oval macrocytes, hypersegmented neutrophils (≥5 lobes), pancytopenia in severe cases
Sickle cell disease	Sickle cells (pathognomonic), target cells, Howell-Jolly bodies (functional asplenia), polychromasia
Thalassemia	Microcytes, target cells, basophilic stippling, nucleated RBCs, tear drop cells (beta-thal major)
TTP/HUS/DIC	Schistocytes (microangiopathic hemolysis) + thrombocytopenia - this is a medical emergency
Hereditary spherocytosis	Spherocytes (no central pallor), polychromasia (reticulocytosis), increased MCHC
G6PD deficiency (after oxidant stress)	Bite cells, Heinz bodies (on supravital stain), polychromasia
Malaria	Ring forms inside RBCs, multiple infections per cell (P. falciparum), enlarged pale RBCs (P. vivax)
CML	Leukocytosis with full granulocyte spectrum (myelocytes, metamyelocytes, bands), eosinophilia, basophilia
Infectious mononucleosis	Atypical lymphocytes (>10% Downey cells), lymphocytosis
ITP	Isolated thrombocytopenia, giant platelets, otherwise normal smear
Aplastic anemia	Pancytopenia, no blasts, no abnormal cells - "empty" smear
AML	Blasts (large cells with open chromatin, prominent nucleoli, Auer rods in M2/M3), +/- Auer rod clumps
CLL	Lymphocytosis of small mature lymphocytes + smudge/smear cells (fragile CLL cells rupturing during slide prep)

PART 10 - Key Terminology Summary

Term	Meaning
Anisocytosis	Variation in RBC size
Poikilocytosis	Variation in RBC shape
Polychromasia	RBCs with bluish tint = reticulocytes (young RBCs with residual RNA)
Hypochromia	Increased central pallor (>1/3 diameter) = reduced MCHC
Rouleaux	RBCs stacked like coins - seen in high fibrinogen, paraproteinaemia (myeloma)
Leukoerythroblastic reaction	Immature WBCs + nucleated RBCs in peripheral blood = bone marrow infiltration (cancer mets, myelofibrosis)
Left shift	Immature neutrophil precursors in blood = active marrow response (usually infection)
Dimorphic picture	Two populations of RBCs (normochromic + hypochromic) - sideroblastic anemia, post-transfusion, treated iron deficiency

PART 11 - A Practical Approach Summary

When you receive a blood picture report or look at a film yourself:

Check the numbers (CBC): Is there anemia? Leukocytosis/leukopenia? Thrombocytopenia? Is it one cell line or pancytopenia?
Classify anemia by MCV: Micro/Normo/Macrocytic
Check RDW: High RDW = iron deficiency or mixed; normal/low RDW = thalassemia or chronic disease
Look at the film description: What shapes? Inclusions? Color changes?
Look at the white cells: Hypersegmented? Left shift? Blasts? Atypical lymphocytes?
Look at platelets: Count estimate, size, morphology
Correlate with the clinical context: The film never stands alone - always integrate with history, examination, and other tests

Sources: Henry's Clinical Diagnosis and Management by Laboratory Methods; Tietz Textbook of Laboratory Medicine 7th Ed.; Histology - A Text and Atlas (Eroschenko); Robbins Pathologic Basis of Disease; Harrison's Principles of Internal Medicine 22E; Goldman-Cecil Medicine; Quick Compendium of Clinical Pathology 5th Ed.

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