A blood film looks at our three cell types (erythrocytes, leukocytes & platelets) under a microscope to identify any abnormalities to give visual clues regarding the functional state of the bone marrow & any systemic diseases.
You should treat this article as a reference for where you can read up on differentials based on the blood film reports you read.
Whilst reviewing a blood film seems like a job specifically for a haematologist, being able to interpret the report of a peripheral blood film can help support your clinical assessment. As a doctor, you can correlate what you see in front of you with the blood film report much like how you’d interpret a chest x-ray or the trend in inflammatory markers.
Everything in context!
- Unfortunately, there are hundreds of different appearances and many are non-specific or multifactorial
- Therefore, upon reviewing the report consider the long list of differentials listed below & apply this to your clinical assessment of the patient in front of you
- Important factors include the age & whether the patient is clinically well
- Viral illness can cause very odd appearances in children’s blood films
- In an otherwise well patient, an abnormal appearance could be artefactual & simply may need repeating
- Older patients or those with lymphadenopathy/hepatosplenomegaly with an abnormal film are far more concerning of myelodysplastic syndromes or malignancy
Whenever in doubt, a call to a haematologist can help makes things a lot clearer.
First, look at the Full Blood Count (FBC) – this also may be known as a complete blood count or CBC. Here is a list of common abbreviations and their meaning:
|RBC||Red Blood Count||Total RBC count|
|WBC||White Blood Count||Total WBC Count|
|Hgb||Haemoglobin||Grams of Haemoglobin per decilitre (g/dL)|
|Hct||Haematocrit||The proportional volume of blood occupied by RBCs|
|MCV||Mean Corpuscular Volume||Mean RBC volume (i.e. their average size)|
|MCH||Mean Cell Haemoglobin||Amount of haemoglobin per RBC|
(Does not factor in the size of the RBC)
|MCHC||Mean Cell Haemoglobin Concentration||The concentration of haemoglobin per unit of RBC volume.|
(Chromia linked to cell size – more accurate result if MCV is abnormal)
|RDW||Red Cell Distribution Width||The variation in RBC size|
|Plt||Platelets||Total Platelet Count|
|MPV||Mean Platelet Volume||Mean platelet volume (i.e. size)|
|PDW||Platelet Distribution Width||The variation in platelet size|
The report usually then comments on the appearance of each cell type and then finally any other abnormalities.
Normal, healthy erythrocytes (RBCs) are pink, biconcave discs, roughly all the same size, shape and colour (all have a “central pallor” which indicates the biconcave shape). Morphological abnormalities are rarely specific to a single condition but do give clues about the functional problem causing the abnormality.
Abnormalities of Erythrocytes
Written below are the most common abnormalities and their significance.
There is a lot of information here. The functional significance is useful to learn but given the non-specific nature of the abnormalities, the listed pathologies are better used as a reference.
The haematologist will sometimes give an opinion about differential diagnoses based on the film appearance and the clinical information provided, but it is useful for you to be able to recognise the abnormalities and patterns yourself. For example:
“Hyposplenic film” is a description of the collection of abnormalities found in these patients. They include Howell-Jolly bodies, target cells, occasional nucleated RBCs, lymphocytosis, macrocytosis and acanthocytes. There may also be evidence of infectious mononucleosis, any viral infection, toxoplasmosis and drug reactions.
- As with Erythrocytes, the number and morphology of each type of leucocyte is assessed in the peripheral smear.
- Leucocytosis is an increase in the number of WBCs and leukopenia is a decrease in the number of WBCs.
- Neutrophils are the most abundant type of leukocytes (40-70%), followed by lymphocytes (20-45%), eosinophils (1-6%), monocytes (2-10%) and basophils (<1%).
- Increases or reductions in any WBC type may be either relative (normal white cell count) or absolute (abnormal white cell count)
Abnormalities of Leukocytes
Thrombocytopaenia (decrease in number) can result from:
- Reduced production (bone marrow failure syndromes)
- Increased peripheral destruction (thrombotic micro-angiopathies)
- Increased splenic sequestration (hypersplenism)
Thrombocytosis (increase in number) can result from:
- Major surgeries
- Acute haemolysis
- Connective tissue disorders
- Use of cytokines, and certain drugs
- Malignant conditions especially myeloproliferative neoplasms (polycythaemia vera, myelofibrosis, essential thrombocythaemia)
Megathrombocytes (increased size) are caused by hyperactivity of megakaryocytes due to increased demand commonly due to B12/folate deficiency or myeloproliferative disorders, or more rarely due to congenital thrombocytopaenic conditions (Bernard-Soulier syndrome, May-Haggelin anomaly (along with Dohle-doy inclusions) or Wiskott-Aldrich syndrome).
Platelet hypogranularity is seen in myelodysplastic syndrome and myelofibrosis, along with megakaryocytes.
- Parasite, fungal, or circulating carcinoma cells might be seen
Useful Resources & References
- Buku app – free haematology app
- Labpedia - Red Blood Cells
- Haematology Learning
- Peripheral Blood Film - a review, PubMed Article
- Lab tests online - Blood film
- Patient - peripheral blood film
- Medschool - hypochromia
- Memorang - Anaemia
- Mind the Bleep - Reviewing and Requesting Bloods
- Pathology Student
- Labpedia - Anaemia
- Mind the Bleep - Jaundice and deranged LFTs
- Laboratoryinfo - Anisocytosis
- Haematology Atlas
Written by Dr Kristen Sellick (FY3)
Reviewed by Dr Ailsa Gemmell (IMT1)
Edited by Mr Mudassar Khan (Y3 Medical Student)
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