The Peripheral Blood Film and Differential White Count

Introduction

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 is of limited specificity depending on the abnormality that is found. This article is useful as a glossary of terms used in discussing the findings of the peripheral blood film; it may also help to indicate possible diseases associated with particular findings in the blood film. Care must be taken in using the information from a blood film to reach a diagnosis, and 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

Cellular abnormalities seen on the peripheral blood film²

• Acanthocytes are spiculated (spikey) red cells that are found in some cases of abetalipoproteinaemia and α-thalassaemia trait.
• 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.
• 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.
• Burr cells are irregularly shaped red cells that may be found in patients with uraemia
• 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 transfusion, or in cases of sideroblastic anaemia.
• Howell-Jolly bodies are nuclear remnants found in red cells after splenectomy, in cases of megaloblastic and iron-deficiency anaemias, and rarely in cases of leukaemia. 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.
• Hypochromia is impaired staining of red cells seen commonly in iron deficiency anaemia, and also in thalassaemia and sideroblastic anaemias.
• 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.
• Leptocytes are synonymous with target cells and 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. They may be found in liver disease, thalassaemia, or sickle-cell disease. They occur occasionally in small numbers in iron deficiency anaemia.
• 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.
• 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, megaloblastic anaemia or as a consequence of haemolysis.
• Microcytosis is the presence of abnormally small red cells often found in association with hypochromia in iron deficiency anaemia.
• Myelocytes, promeyelocytes, metamyelocytes and normoblasts are immature white cells seen in a leucoerythroblastic picture (see above).
• Normoblasts are immature, nucleated red cells seen in leucoerythroblastic anaemia, haemolysis, hypoxia and marrow infiltration.
• Pappenheimer bodies are the granules seen in siderocytes found in carcinomatosis and after splenectomy.
• Poikilocytosis is the variation in cell shape seen in iron deficiency anaemia.
• Polychromasia is the heterogeneous staining of RBCs of different ages, with younger cells appearing blue, that occurs after haemorrhage, haemolysis, dyserythropoiesis and treatment with haematinics such as iron and vitamin B₁₂.
• Reticulocytosis is the presence of >0.8–2% of total red cell count in the form of reticulocytes. They are young, oversized red cells that are present when the marrow is actively producing red cells. They are present after haemorrhage, haemolysis or following treatment of deficient patients with haematinics.
• Right shift is characterised by the presence of hypersegmented polymorphonucleocytes (>5 lobes to their nucleus), seen in liver disease, uraemia and megaloblastic anaemia.
• 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 etc.
• Schistocytes are red cells fragmented by their passage through intravascular strands of fibrin, found in cases of intravascular haemolysis.
• Spherocytes are overly-round or spheroid red cells that usually indicate active haemolysis. They are seen more rarely in cases of hereditary spherocytosis.

Neutrophils (polymorphs or polymorphonucleocytes)

• Normal range 2–7.5 X 10⁹/l, comprising 40–75% of WBCs, although absolute values are of more use ion interpretation than relative percentage counts
• Raised in:
  • Bacterial infections
  • Trauma
  • Surgery
  • Burns
  • Haemorrhage
  • Inflammation
  • Infarction
  • Polymyalgia rheumatica
  • Polyarteritis nodosa
  • Myeloproliferative disorders
  • Certain drugs, e.g. steroids
  • Large increase in numbers may be seen in leukaemias, disseminated malignancy and severe childhood infections.
• Decreased in:
  • Viral infections
  • Brucellosis
  • Typhoid
  • Kala-azar
  • TB
  • Certain drugs, e.g. carbimazole, sulphonamides, methotrexate
  • Severe sepsis (consumption by attempt to combat infection)
  • Hypersplenism
  • Systemic lupus erythematosus
  • Rheumatoid arthritis (destroyed by chronic inflammatory process)
  • Vitamin B12 or folate deficiency
  • Bone marrow failure (impaired production)

Lymphocytes

• Normal range 1.3–3.5 X 10⁹/l, comprising 20–45% of WBCs
• Raised in:
  • Viral infections, e.g. EBV, CMV, rubella
  • Toxoplasmosis
  • Whooping cough
  • Brucellosis
  • Chronic lymphocytic leukaemia
  • Large numbers of abnormal/atypical Lymphocytes are characteristically seen in EBV infection (these are T-lymphocytes reacting against EBV-infected B-lymphocytes).
• Decreased in:
  • Steroid therapy
  • Systemic lupus erythematosus
  • Uraemia
  • Legionnaire's disease
  • AIDS
  • Marrow infiltration
  • Post chemotherapy/radiotherapy

Eosinophils

• Normal range 0.04–0.44 x 10⁹/l, comprising 1–6% of WBCs
• Raised in:
  • Asthma/allergy
  • Parasitic infestations (especially invasive helminths)
  • Polyarteritis nodosa
  • Skin disease such as eczema, pemphigus, urticaria
  • Malignant diseases (including eosinophilic leukaemia)
  • Following irradiation
  • Löffler's syndrome
  • During the convalescent phase of infections
  • As part of the hypereosinophilic syndrome
  • Eosinophilia-myalgia syndrome

Monocytes

• Normal range 0.2–0.8 X 10⁹/l. comprising 2–10% of WBCs
• Raised in:
  • Acute and chronic infections (especially TB, brucellosis, protozoan disease)
  • Malignant disease (especially M4 & M5 acute myeloid leukaemia and Hodgkin's disease)
  • Myelodysplasia

Basophils

• Normal range up to 0.01 X 10⁹/l, comprising 0–1% of WBCs
• Raised in:
  • Viral infections
  • Urticaria
  • Hypothyroidism
  • Post-splenectomy
  • Chronic myeloid leukaemia
  • Ulcerative colitis
  • Malignancy
  • Systemic mastocytosis (or urticaria pigmentosa)
  • Haemolysis
  • Polycythaemia rubra vera

The normal platelet count is 150–400 X 10⁹/l. Below is a list of the common or important causes of raised or decreased platelet counts, which is by no means exhaustive.

Causes of thrombocytopenia (decreased platelet count)

• Decreased platelet production:
  • Hypoplasia of megakaryocytes:
    • Aplastic anaemias
    • Leukaemias
    • Myelofibrosis
    • Marrow invasion, e.g. granulomata, metastatic tumour, leukaemia
    • Viral infections
    • Ionising radiation causing marrow suppression
    • Chemical toxicity, e.g. chemotherapy, toxins, medication-induced, alcohol excess
  • Ineffective thrombopoiesis:
    • Vitamin B₁₂ deficiency
    • Folic acid deficiency
• Increased platelet destruction:
  • Immune-mediated platelet destruction:
    • Drug-induced immune thrombocytopenia
    • Alloimmune thrombocytopenia, e.g. neonatal, post-transfusion
    • Autoimmune thrombocytopenia, e.g. idiopathic immune thrombocytopenia, secondary immune thrombocytopenia due to infections, rheumatological diseases and lymphoproliferative disorders.
  • Non-immune mediated platelet destruction:
    • Disseminated Intravascular Coagulation
    • Prosthetic intravascular devices
    • Thrombotic thrombocytopaenic purpurae
    • Extracorporeal circulation devices
• Increased splenic sequestration:
  • Splenomegaly
  • Portal hypertension

Causes of thrombocytosis/thrombocythaemia (increased platelet count)

• Essential or primary thrombocytosis:
  • This is defined as a non-reactive chronic myeloproliferative disorder that causes chronic elevation of platelet count.
• Reactive or secondary thrombocytosis:
  • Acute infective or inflammatory disorders
  • Chronic inflammatory disorders, e.g. tuberculosis, rheumatological disorders
  • Post-splenectomy or splenic hypofunction/hypoperfusion or congenital asplenia
  • Trauma
  • Acute haemorrhage
  • Iron-deficiency anaemia
  • Some leukaemias (particularly CLL or CML)