Peripheral Blood Film

The peripheral blood film is a Romanowsky stained thin smear examined under the microscope for red cell, leucocyte and platelet morphology. It gives the diagnosis, or directs the workup, in many haematological conditions and reveals findings that automated counters miss, such as blasts, parasites and schistocytes.

Indications. Unexplained anaemia, cytopenia or leucocytosis; analyser flags; suspected haemolysis, leukaemia, thrombotic microangiopathy, hyposplenism, malaria or other blood parasites; fever after travel.

Specimen. Fresh EDTA blood, with the film made within 2 to 3 hours. A film made from a finger prick avoids EDTA artefact.

• Technique. Assess smear quality and stain. Read the feather edge, where cells lie in a single layer without overlap.
• Red cells. Size, haemoglobinisation, shape, inclusions, polychromasia, agglutination or rouleaux.
• Leucocytes. Differential, maturity, nuclear segmentation, granulation, blasts and abnormal lymphoid cells.
• Platelets. Number estimate, size, clumping and granulation.
• Parasites. Examine the whole film, including the edges.

Red cells.

• Microcytes, hypochromia and pencil cells: iron deficiency.
• Target cells: thalassaemia, haemoglobin C and S disease, iron deficiency, liver disease, hyposplenism.
• Spherocytes: hereditary spherocytosis, autoimmune haemolytic anaemia, ABO incompatibility, burns.
• Schistocytes: microangiopathic haemolysis (TTP, HUS, DIC, malignant hypertension) and mechanical heart valves. A count over 1% supports thrombotic microangiopathy.
• Sickle cells: sickle cell disease. They are not seen in the trait except in severe hypoxia.
• Bite and blister cells and Heinz bodies: G6PD deficiency, unstable haemoglobins.
• Tear drop cells: myelofibrosis, marrow infiltration, thalassaemia.
• Rouleaux: paraproteinaemia and inflammation.
• Agglutination: cold agglutinin disease.
• Basophilic stippling: lead poisoning, thalassaemia, myelodysplasia.
• Howell-Jolly bodies: hyposplenism and asplenia.
• Macro-ovalocytes: megaloblastic anaemia.
• Polychromasia: reticulocytosis.
• Acanthocytes and stomatocytes: liver disease, abetalipoproteinaemia, hereditary stomatocytosis, alcohol.

Leucocytes.

• Left shift, toxic granulation, Döhle bodies and vacuolation: bacterial infection and sepsis.
• Hypersegmented neutrophils (over 5 lobes): megaloblastic anaemia.
• Blasts and Auer rods: acute leukaemia.
• Smear cells with small mature lymphocytes: chronic lymphocytic leukaemia.
• Reactive lymphocytes: viral infection, particularly EBV and CMV.
• Hairy cells: hairy cell leukaemia.
• Bilobed neutrophil nuclei (Pelger-Huët): inherited, or acquired in myelodysplasia.
• Leucoerythroblastic picture (immature granulocytes with nucleated red cells): marrow infiltration, myelofibrosis, severe illness.

Platelets. Giant platelets suggest immune thrombocytopenia, myeloproliferative neoplasms or MYH9 disorders. Grey platelets suggest a granule disorder. Clumping indicates pseudothrombocytopenia, and satellitism around neutrophils is another EDTA artefact.

Parasites. Malaria, babesiosis, trypanosomes and microfilariae can be identified on thick and thin films.

Critical findings needing immediate action. Blasts, schistocytes with thrombocytopenia, malaria parasites (particularly Plasmodium falciparum with high parasitaemia) and Auer rods need urgent haematology discussion.

Pitfalls.

• Crenation, drying artefact, stain precipitate and EDTA-induced change mimic pathology.
• Thick areas give false spherocytes, and thin areas give false target cells.
• Poor smear technique causes smudge cells that can be misread as leukaemia.
• Delayed filming alters morphology.
• Interpretation depends on observer experience, and is more reliable when analyser flags and clinical details are available.

Special groups. Newborns have nucleated red cells, polychromasia and macrocytes at birth. Lymphocytes dominate in infants and young children, and reactive forms are common. In pregnancy, mild changes resembling iron deficiency may appear.

Follow up tests. Reticulocyte count, direct antiglobulin test, ADAMTS13 activity, bone marrow examination, flow cytometry, malaria rapid test and PCR, haemoglobin electrophoresis.