Peripheral Blood Film

When a peripheral blood sample is smeared on a slide and stained, it is known as a peripheral blood film. It allows for examination of the physical characteristics of the red cells, white cells and platelets under the microscope. Additionally, it helps detect parasites or abnormal cells in the blood. Thus the peripheral blood film is an important indicator of haematological and other disease. It is, however, relatively difficult to interpret, uses terminology which can be opaque to those who do not practise haematology and can be of limited specificity depending on the abnormality that is found.

This record will provide you with an overview of the peripheral blood film result and provides glossaries of terms commonly used. It will identify possible diseases associated with particular findings but care must be taken in using the information from a blood film to reach a diagnosis; the clinical context must be kept in mind at all times. When in doubt ask advice from your local haematology service on the significance of a particular finding and how to proceed with investigation or management.¹

Go to our record on Full Blood Count (FBC) for information about:

• Red cell parameters (haemoglobin, mean cell volume (MCV), mean cell haemoglobin (MCH), mean cell haemoglobin concentration (MCHC), haematocrit (Hct), red cell count (RCC) and red cell distribution width (RDW)) and causes of abnormal haemoglobin levels
• White cell counts (WCCs) and causes of abnormalities in the differential white cell count
• Platelet counts and causes of abnormalities of these

Many labs will examine a peripheral blood film if an abnormality is detected in the FBC or if the clinical details indicate a requirement. However, if you suspect an underlying blood disorder or parasitic infection, request one directly yourself. The sample should be collected in an EDTA bottle and be as fresh as possible at the time of analysis.

• The erythrocytes (red blood cells): a note will be made of their size, shape, any membrane changes, colour and stippling. Any inclusion bodies (e.g. Howell-Jolly bodies or malarial parasites) will also be noted. Other abnormalities include red cell rouleaux, red cell nucleation and the presence of reticulocytes.
• The leukocytes (white blood cells): the number and morphology of these cells are noted, as well as abnormalities such as toxic granulation or dysplastic changes. Presence of abnormal cells is important (e.g. leukaemic blasts).
• Platelets: number, size and shape are commented on.
• Other abnormalities such as parasites, plasma cells and occasional circulating carcinoma cells may be found.

The red blood cell is a pink biconcave disc-shaped cell, about the size of a small lymphocyte nucleus. Normally, healthy cells are roughly the same size, shape and colour. Morphological abnormalities are rarely specific to one condition; they can occur in a range of problems.

A glossary of terms

The lists below are not exhaustive but examples of some of the conditions that cause the defects.

• Acanthocytes are spiculated (spikey) red cells that are found in some cases of α-β-lipoproteinaemia, chronic liver disease and α-thalassaemia trait. There is also a hereditary acanthocytosis.
• Anisocytosis is variation in red cell size which may occur in thalassaemia, iron deficiency or megaloblastic anaemias.
• Basophilic stippling describes the presence of small granular bodies within the red cell cytoplasm and occurs when there is disordered and accelerated erythropoiesis so that red cells with immature cytoplasm are released into the circulation. It may be found in lead poisoning, thalassaemia or other causes of significant anaemia.
• Bite cells occur in G6PD deficiency and in oxidative haemolysis.
• Burr cells type of echinocyte: found in patients with uraemia.
• Cabot's rings are circular or figure-of-eight structures in red cells that stain red with Wright's stain and are thought to represent nuclear membrane remnants; they are found in similar conditions to Howell-Jolly bodies (see below).
• Cigar cell see elliptocyte.
• Codocytes see target cells.
• Crenated cells type of echinocyte: most often a storage or EDTA artefact.
• Dacrocytes see teardrop cells.
• Dimorphic picture/appearance describes heterogeneity in the size of red blood cells, usually with two distinct populations. It can be found in partially treated iron deficiency, mixed deficiency anaemias (e.g. folate/B12 and iron together), following red cell transfusion or in cases of sideroblastic anaemia.
• Drepanocytes see sickle cells.
• Echinocytes are cells with many blunt spicules. They may be artefactual - see crenated cells or pathological - see burr cells.
• Elliptocytes oval (ovalocyte) or cigar shaped cells (cigar cell) that occur in hereditary elliptocytosis, myeloproliferative disorder and myelodysplastic syndrome. Subtype: pencil/rod cell.
• Erythroblasts (normoblasts) are immature, nucleated red cells seen in the peripheral blood in leucoerythroblastic anaemia, haemolysis, hypoxia and marrow infiltration.
• Fragmented red cells see schistocytes.
• Heinz bodies are denatured haemoglobin due to oxidative damage. They are never seen in normal individuals as they are removed by the spleen. A small number may therefore be seen post-splenectomy and also with the use of anti-oxidant drugs or sulphonamides, in G6PD deficiency and with unstable haemoglobin.
• Howell-Jolly bodies are nuclear remnants found in red cells normally removed by the spleen and seen after splenectomy, in cases of megaloblastic and iron-deficiency anaemias and rarely in cases of leukaemia.
• Hypochromia is impaired staining of red cells seen commonly in iron deficiency anaemia and also in thalassaemia and sideroblastic anaemias.
• Hyposplenic film is a description of the collection of abnormalities found in these patients. They include Howell-Jolly bodies, target cells, occasional nucleated red blood cells, lymphocytosis, macrocytosis and acanthocytes. There may also be evidence of infectious mononucleosis, any viral infection, toxoplasmosis and drug reactions.
• Leptocytes see target cells.
• Macrocytosis is the presence of abnormally large red cells found when erythropoiesis is disordered or when red cells are released prematurely from the marrow. They occur in alcohol excess and liver disease, megaloblastic anaemia or as a consequence of haemolysis. They are also found in B12 or folate deficiency, pregnant women and newborn infants, following use of antimetabolites, hypothyroidism, chronic respiratory failure and aplastic anaemia.
• Mexican hat cells see target cells.
• Microcytosis is the presence of abnormally small red cells often found in association with hypochromia in iron deficiency anaemia. They are also seen in thalassaemia trait, congenital sideroblastic anaemia and in the anaemia of chronic disorders if these are long-standing.
• Mouth cell see stomatocyte.
• Normoblast see erythroblast.
• Ovalocyte see elliptocyte.
• Pappenheimer bodies are phagosomes, containing ferruginous granules, found in red blood cells in diseases such as sideroblastic anaemia, haemolytic anaemia and sickle cell disease. They are also found in carcinomatosis and after splenectomy. They may contribute to spurious platelet counts by electro-optical counters.
• Pencil cell type of elliptocyte: occur where there is iron deficiency anaemia, thalassaemia trait and syndromes and in pyruvate kinase deficiency.
• Polychromasia is the heterogeneous staining of red blood cells of different ages, with younger cells appearing blue, that occurs after haemorrhage, haemolysis, dyserythropoiesis and treatment with haematinics such as iron and vitamin B12.
• Poikilocytosis is the general term used for variation in cell shape.
• Reticulocytosis is the presence of > 0.8–2% of total red cell count in the form of reticulocytes. See below.
• Rod cell see pencil cell.
• Rouleaux are stacked/clumped groups of red cells caused by the presence of high levels of circulating acute-phase proteins which increase red cell 'stickiness'. They are often an indicator that a patient has a high ESR and are seen in infections, autoimmune conditions, chronic inflammation, paraproteinaemia and myeloma.
• Schistocytes are red cells fragmented by their passage through intravascular strands of fibrin, found in cases of intravascular haemolysis, renal failure and thrombotic thrombocytopaenic purpura amongst others.
• Sickle cells also known as drepanocytes, are found in sickle cell anaemia and other sickle syndromes but not in sickle trait. Their name describes their shape.
• Spherocytes are overly-round or spheroid red cells that usually indicate active haemolysis and are also found where there is a delayed post-transfusion reaction, DIC and post-splenectomy. They are seen more rarely in cases of hereditary spherocytosis.
• Spur cells see acanthocytes.
• Stomatocyte an oval or rectangular area of central pallor, sometimes referred to as a "mouth" arises as a result of loss of concavity on one side. They can occur in liver disease, electrolyte imbalance and hereditary stomatocytosis.
• Target cells - also known as codocytes, leptocytes or mexican hat cells. They are red cells with a central area of increased staining, surrounded by a ring of hypodense staining and then a further ring of dense staining at the edge of the cell, giving an appearance akin to an archery target. This occurs as a result of the shift in equilibrium between the erythrocyte and cholesterol. They may be found in liver disease, thalassaemia or sickle-cell disease. They occur occasionally in small numbers in iron deficiency anaemia and sometimes following splenectomy.
• Teardrop red cells are found in myelofibrosis, metastatic marrow infiltration and myelodysplastic syndrome.

Reticulocytes⁴

These are young, oversized red cells that are present when the marrow is actively producing red cells. They are the intermediary between the nucleated red blood cell and the mature red blood cell. Small numbers of reticulocytes are found in normal peripheral blood. They are usually expressed as a percentage of total red cells.

• Increased count - they are present after haemorrhage, haemolysis, severe hypoxia, in polycythaemia of any cause and in marrow infiltration. Reticulocytes may rise during marrow recovery following chemotherapy or radiotherapy. They may also be seen following treatment of deficient patients with haematinics when they are a useful measure of response to the treatment.
• Decreased count - reticulocyte levels drop where there is marrow infiltration (leukaemia, myeloma, lymphoma or other malignancies) or due to marrow underactivity such as in iron, folate or B12 deficiency or due to autoimmune disease, malnutrition, uraemia, drugs, aplastic anaemia and red cell aplasia.

As with erythrocytes, each type of leucocyte has characteristics that can be identified on stained specimens allowing for an easy diagnosis to the well-trained eye. The first thing the haematologist will assess is the absolute number of white cells (see FBC record, link in introduction). If the count is low, subsequent stain and diagnosis may be tricky. If the count is high, an assessment is made of which cell type predominates (i.e. lymphocytes versus granulocytes) and any abnormal cells - see Glossary of terms below.

Glossary of terms

• Atypical lymphocytes see reactive lymphocytes.
• Auer rods are seen in myeloblasts and are pathognomic of acute myeloid leukaemia.
• Blast cells are abnormal, immature, nucleated precursor white cells pushed out from the marrow into the circulation by processes such as myelofibrosis or leukaemic infiltration.
• Hairy cells have fine, irregular pseudopods and immature nuclear features. They are seen only in hairy cell leukaemia.
• Hypersegmented neutrophils see right shift.
• Myelocytes, promeyelocytes, metamyelocytes are immature white cells seen in a leucoerythroblastic picture (see below).
• Left shift describes immature white blood cells that are released from the marrow when there is a cause of marrow outpouring, typically due to infection.
• Leucoerythroblastic anaemia/picture describes the presence of immature cells such as myeloblasts and normoblasts in the film. It is seen in cases of marrow infiltration - for example in metastatic malignancy, prolonged hypoxia or severe infection.
• Leukaemoid reaction is a severe, reactive leucocytosis, usually consisting of granulocytes (polymorphonucleocytes). It is seen after burns, in cases of severe infection, following an acute haemolysis or prolonged hypoxia.
• Pelger-Huet anomaly describes bilobed neutrophils which may be hereditary (when the neutrophils are functionally normal) or acquired, e.g. myelodysplastic syndrome.
• Reactive lymphocytes seen in infectious mononucleosis.
• Right shift is characterised by the presence of hypersegmented polymorphonucleocytes (> 5 lobes to their nucleus), seen in liver disease, uraemia and megaloblastic anaemia.
• Smear cells are lymphocytes whose cell membranes have ruptured in preparation of the blood film: seen in chronic lymphocytic leukaemia.
• Toxic granulation is where coarse granules are seen in neutrophils. They may be seen post-operatively, in inflammatory disorders and in severe infection.

Making the diagnosis in context⁴

The age of the patient is important: viral illnesses can produce bizarre looking film in children. The myelodysplastic syndromes and malignancies tend to occur in older people. If the patient is well, it may be worth repeating the film to see if the abnormalities have resolved. If the patient is unwell (or has lymphadenopathy or hepatosplenomegaly), then disease has to be excluded.

• Clumping is an in vitro artefact in some individuals and results in spurious blood analyser report of thrombocytopaenia. The in vivo content is normal and the platelets work well. Even a small blood clot in the EDTA bottle will affect the result. Taking blood into citrate or heparin will reveal normal platelet counts.
• Giant platelets - these occur in essential thrombocytopaenia. They result in a raised platelet distribution width (the indicator of the range in platelet size in a blood sample) unlike the normal platelet distribution width seen in reactive thrombocytosis, where there is an increased platelet count but normal sized platelets.
• Satellitosis describes platelets encircling a neutrophil. It occurs when a patient has a serum factor that reacts to the anticoagulant EDTA.

Blood films are useful for the diagnosis of:

• Babesiosis
• Malaria
• Microfilaria
• Trypanosomiasis

Some diseases require bone marrow aspiration (e.g. Leishmaniasis). However, development of sensitive monoclonal antibody techniques has made diagnosis on a peripheral blood film more of a dying art. The notable exception to this is in the diagnosis of malaria where a peripheral smear study remains the gold standard.