Peripheral Smear: RBC Shapes & Inclusions
Read the red cell: shapes and inclusions that name the disease
Reading the smear and choosing the stain
The normal red cell. On a routine Wright-Giemsa smear a normal RBC is a biconcave disc about the size of a small lymphocyte nucleus, with central pallor filling roughly one-third of its diameter. Changes in pallor, outline and contents each point to a mechanism.
Three kinds of stain. Some inclusions show on the routine stain, some only with a supravital stain applied to living cells before fixation, and iron inclusions are confirmed with Prussian blue.
| Stain | Reveals |
|---|---|
| Wright-Giemsa (routine) | Howell-Jolly bodies, basophilic stippling, Pappenheimer bodies (as blue granules), Cabot rings, polychromasia |
| Supravital (new methylene blue, crystal violet) | Reticulocyte RNA network, Heinz bodies, HbH inclusions |
| Prussian blue | Iron: Pappenheimer bodies (siderocytes), ringed sideroblasts and iron stores in marrow |
Reticulocytes. Newly released RBCs keep ribosomal RNA for about a day. New methylene blue precipitates it into a blue reticulum; on Wright stain the same cells look larger and blue-gray (polychromasia). The normal count is about 0.5–2.5%. A high count, once corrected for the degree of anemia, means an active marrow responding to hemolysis or bleeding; a low count means underproduction.
Abnormal shapes
| Shape | Mechanism | Classic causes |
|---|---|---|
| Spherocyte | Membrane lost relative to volume; no central pallor, ↑MCHC, ↑osmotic fragility | Hereditary spherocytosis (ankyrin, spectrin, band 3); warm AIHA (macrophages nibble IgG-coated membrane) |
| Target cell | Membrane surplus relative to Hb; ↓osmotic fragility | Thalassemias, HbC and HbSC disease, liver disease (cholesterol loading), asplenia (no splenic trimming) |
| Sickle cell | Polymerized deoxy-HbS forms rigid crescents | HbSS, HbSC (HbC disease shows blunt rod-shaped crystals) |
| Schistocyte (helmet cell) | Shearing by fibrin strands or turbulent flow | TTP, HUS, DIC, HELLP, malignant hypertension; mechanical valves, severe aortic stenosis |
| Teardrop cell (dacrocyte) | Deformed squeezing out of fibrotic or infiltrated marrow | Primary myelofibrosis, myelophthisis (metastatic cancer, granulomas), thalassemia |
| Acanthocyte (spur cell) | Membrane lipid imbalance; few irregular spikes | Severe liver disease, abetalipoproteinemia |
| Echinocyte (burr cell) | Many short, regular projections | Uremia, pyruvate kinase deficiency, drying artifact |
| Bite cell | Splenic removal of Heinz bodies | G6PD deficiency |
| Macro-ovalocyte | Impaired DNA synthesis | B12 or folate deficiency |
Spherocytes. The direct Coombs test separates the two main causes: negative in hereditary spherocytosis, positive in warm AIHA.
Schistocytes. Fragmentation means intravascular hemolysis, with high LDH, low haptoglobin and hemoglobinuria. With thrombocytopenia, normal PT and aPTT favor TTP or HUS, while prolonged clotting times and low fibrinogen favor DIC.
Marrow invasion. Teardrop cells travel with a leukoerythroblastic smear: nucleated RBCs plus immature granulocytes (myelocytes, metamyelocytes). In myelofibrosis the marrow aspirate is often a dry tap, and splenomegaly reflects extramedullary hematopoiesis.
Abetalipoproteinemia. Loss of microsomal triglyceride transfer protein (MTTP) prevents assembly of apoB-containing lipoproteins, causing fat malabsorption, vitamin E deficiency with ataxia and retinitis pigmentosa, and acanthocytes.
Macrocytes. Macro-ovalocytes (MCV above 100 fL, often far higher) with hypersegmented neutrophils mark megaloblastic anemia, where nuclear maturation lags behind cytoplasm; Howell-Jolly bodies and Cabot rings may accompany. Round macrocytes without hypersegmentation suggest alcohol, liver disease or reticulocytosis.
Rouleaux. Excess plasma protein (a myeloma paraprotein, or high fibrinogen in inflammation) neutralizes the negative surface charge that keeps RBCs apart, so they stack like coins and the ESR rises.
Inclusions
- Howell-Jolly bodies: a single round, dark-purple nuclear (DNA) remnant normally pitted out by splenic macrophages; seen after splenectomy, in functional asplenia (sickle cell autosplenectomy) and in severe megaloblastic anemia. They warn of risk from encapsulated organisms (S. pneumoniae, H. influenzae type b, N. meningitidis), so vaccination matters.
- Heinz bodies: oxidized, precipitated hemoglobin, visible only with a supravital stain. Oxidants (fava beans, primaquine, dapsone, sulfonamides, infection) overwhelm scarce NADPH and reduced glutathione in G6PD deficiency; splenic macrophages then remove the Heinz bodies, leaving bite and blister cells. Unstable hemoglobins also form them.
- Basophilic stippling: fine-to-coarse blue dots of aggregated ribosomal RNA spread through the cell. Causes are lead poisoning (lead inhibits pyrimidine 5′-nucleotidase, as well as ALA dehydratase and ferrochelatase), thalassemias, sideroblastic anemia, alcohol use and inherited pyrimidine 5′-nucleotidase deficiency.
- Pappenheimer bodies: a few small iron granules clustered near the cell edge (siderocytes), Prussian blue-positive; seen in sideroblastic anemia and after splenectomy.
- Ringed sideroblasts: marrow erythroblasts whose iron-loaded mitochondria encircle the nucleus (at least 5 granules around at least one-third of it). Causes include X-linked ALAS2 defects, vitamin B6 deficiency (isoniazid), alcohol, lead, copper deficiency and SF3B1-mutated myelodysplasia.
- Cabot rings: thin red-violet loops or figure-8s in megaloblastic anemia and lead poisoning.
Look-alike traps
- Spherocyte vs microcyte: iron-deficient microcytes have more central pallor; spherocytes have none.
- Spur vs burr: acanthocytes have a few uneven spikes of varying length (liver disease, abetalipoproteinemia); echinocytes have many short, evenly spaced projections (uremia, pyruvate kinase deficiency, artifact).
- Rouleaux vs agglutination: rouleaux are orderly stacks that disperse when plasma is replaced by saline; agglutinates are irregular clumps from antibody bridging, classically IgM cold agglutinins (Mycoplasma, EBV, lymphoma), which disperse on warming to 37 °C and falsely raise the MCV while lowering the RBC count.
- Stippling vs Pappenheimer bodies: stippling is RNA scattered throughout the cell; Pappenheimer bodies are a few iron granules at one edge that turn Prussian blue.
- Howell-Jolly vs Heinz bodies: Howell-Jolly bodies show on routine stain; Heinz bodies need a supravital stain.
Howell-Jolly bodies, target cells and Pappenheimer bodies together on one smear point to an absent or nonfunctioning spleen.