The peripheral blood smear: what to look for

Interpretation

A drop of blood is spread on a glass slide, stained and looked at under the microscope to see the shape of the blood cells.

Updated Nov 13, 2026

What it is

The peripheral blood smear (blood film) is a drop of blood spread thinly on a slide, stained (usually with a Romanowsky stain such as Wright or May-Grünwald-Giemsa) and examined by eye. It answers questions the analyser cannot: what the cells look like. A smear is requested when the full blood count is unexplained or flagged, when haemolysis, a microangiopathy, a marrow disorder, leukaemia or a blood parasite is suspected, and to check a low platelet count. It is read in the thin area near the feathered edge, where cells lie side by side without overlapping. Red cells are described by size (normal red cells are about the size of a small lymphocyte nucleus), colour (central pallor about one third of the cell), shape (poikilocytosis) and inclusions. White cells are checked for number, the differential and abnormal forms such as blasts. Platelets are estimated and checked for clumps and giant forms. Many shape changes are not specific on their own; the pattern, together with the blood count, reticulocytes and the clinical picture, gives the answer. A few findings are urgent: schistocytes with low platelets, blasts, and malaria parasites.

Reference values and concepts

Read in the monolayer near the feathered edge; too thick, cells overlap and look like rouleaux; too thin, they are flattened and lose central pallor.
Size: microcytes and macrocytes; anisocytosis is variation in size (matches a high RDW). Colour: hypochromia is wide central pallor; polychromasia (blue-grey cells) reflects reticulocytes.
Poikilocytosis is variation in shape: schistocytes, spherocytes, target cells, teardrop cells, sickle cells, elliptocytes, acanthocytes and echinocytes.
Inclusions: Howell-Jolly bodies (nuclear remnants), basophilic stippling (ribosome remnants), Pappenheimer bodies (iron granules), Heinz bodies (denatured haemoglobin, seen only with a supravital stain), parasites.
White cells: left shift, toxic granulation and Döhle bodies in infection; smudge cells in chronic lymphocytic leukaemia; blasts in acute leukaemia.
Rouleaux (red cells stacked like coins) reflect high plasma proteins, such as a paraprotein in myeloma.

Reading it step by step

Check the film is well made and stained, and know the blood count first — the film answers a question the count has raised.
Scan at low power along the edges and tail — platelet clumps, large abnormal cells and rouleaux are found there.
Move to the monolayer and use high power — only cells lying side by side can be judged for shape and pallor.
Red cells: size, colour and central pallor, then shape, then inclusions — this order keeps the description complete.
Note polychromasia and nucleated red cells — they show an active or stressed marrow, as in haemolysis or bleeding.
White cells: estimate the count, perform the differential and look for blasts, dysplasia or toxic changes — leukaemia is often found here.
Platelets: estimate the number, size and any clumps — a clumped sample needs a repeat count in a citrate tube.
Put it together with reticulocytes and haemolysis tests and write a short report — a list of shapes is not a conclusion.

Findings on the film and what they suggest

Finding	What it looks like	Main causes
Schistocytes	Fragments, helmet or triangular shapes	TTP, HUS, DIC, mechanical heart valves
Spherocytes	Small, dense, no central pallor	Hereditary spherocytosis, warm autoimmune haemolysis, burns
Bite cells and Heinz bodies	Bite-shaped notch; inclusions on supravital stain	G6PD deficiency, oxidant drugs
Target cells	Central dot of haemoglobin, like a target	Thalassaemia, liver disease, haemoglobin C, after splenectomy
Teardrop cells	Cell drawn out to one end	Myelofibrosis, marrow infiltration
Sickle cells	Crescent-shaped red cells	Sickle cell disease
Howell-Jolly bodies	Single small dark dot in the red cell	Asplenia or hyposplenism
Basophilic stippling	Fine blue dots across the cell	Lead poisoning, thalassaemia
Hypersegmented neutrophils	Six or more nuclear lobes	B12 or folate deficiency
Rouleaux	Red cells stacked like coins	Myeloma, high plasma proteins

Patterns and common mistakes

Haemolysis pattern: polychromasia with spherocytes (immune or hereditary), schistocytes (mechanical) or bite cells (oxidative).
Target cells: thalassaemia, liver disease, haemoglobin C and after splenectomy; acanthocytes (spur cells): severe liver disease.
Basophilic stippling: lead poisoning and thalassaemia; Pappenheimer bodies: sideroblastic anaemia and after splenectomy.
Leukoerythroblastic film (nucleated red cells, young white cells) with teardrop cells: marrow fibrosis or infiltration by cancer.
Common mistakes: reading the thick end, calling echinocytes (often an artefact of drying or old samples) a disease, and missing a few schistocytes.
A normal film does not exclude malaria: if suspected, thick and thin films are repeated, and a rapid antigen test can help.

Next step

Schistocytes with low platelets: call a senior and haematology now, and check creatinine, LDH, bilirubin, haptoglobin and clotting.
Blasts or a leukoerythroblastic picture: urgent haematology review; a bone marrow examination is usually needed.
Malaria parasites: report the species and parasite percentage, and start treatment the same day per local protocol.
Haemolysis without schistocytes: direct antiglobulin test, G6PD assay when the episode settles, and family history.
Ask the laboratory for a haematologist review of any film you are not sure about, and record what you asked.

Test yourself

Think of the answer, then tap the question to see it.

Why is the blood smear read near the feathered edge and not at the thick end?

Only in the thin monolayer do cells lie side by side without overlapping, so size, shape and central pallor can be judged. At the thick end cells pile up and can mimic rouleaux.

Which inclusion shows that the spleen is absent or not working?

Howell-Jolly bodies, small nuclear remnants normally removed by the spleen; they are seen after splenectomy and in functional asplenia, for example in sickle cell disease.

A 25-year-old has fever, confusion, Hb 8 g/dL, platelets 18 × 10⁹/L and many schistocytes. What is the concern and what do you do?

A thrombotic microangiopathy, most urgently TTP. Call a senior and haematology now, check LDH, creatinine, bilirubin and clotting, and avoid platelet transfusion unless advised.

A platelet count of 40 × 10⁹/L is reported in a well patient with no bleeding. What should the film be checked for?

Platelet clumps, which give a falsely low count (pseudothrombocytopenia) in EDTA tubes. If present, repeat the count in a citrate tube.

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Normal and Abnormal Complete Blood Count With Differential (StatPearls (NCBI Bookshelf), 2024); Poikilocytosis (StatPearls (NCBI Bookshelf), 2026); Erythrocyte Inclusions (StatPearls (NCBI Bookshelf), 2025).

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