39 | How to feel confident looking at blood smears as a vet nurse
In this episode of the Medical Nursing Podcast, we’re kicking off a brand new series - all about haematology!
From blood smears to bone marrow biopsies, bleeding disorders to blood transfusions, we’ll spend the next few weeks working together to help you feel even more confident nursing these patients.
Blood smears are an excellent skill for us to learn - there’s no reason we can’t look at these, identify normal and abnormal cells, and perform cell counts! It’s not ‘just a vet’s job’, and the only way to feel more confident knowing what you’re looking at is to look at more and more smears and get comfortable with what’s normal and what isn’t.
But before we start looking at smears, we need to know what cells we’re looking at and refresh ourselves on what they do - exactly what we’re covering in this episode.
Let’s talk blood cells…
Our blood is made up of several major components. These include:
Red blood cells, which transport oxygen.
White blood cells, which fight infection, maintain immunity and are involved in allergic responses.
Platelets, which stem haemorrhage
Clotting factors, which stem haemorrhage.
Plasma, which contains several solutes dissolved in fluid, is the fluid in which our blood cells are suspended.
All of our blood cells are formed in the bone marrow from a single cell called a haematopoietic stem cell. This stem cell then matures into different precursor cells, eventually mature into our red blood cells, white blood cells, and platelets. This is why we’ll often sample bone marrow in some of our haematology patients - to check if the bone marrow can make new cells normally.
Red Blood Cells
Red blood cells - aka erythrocytes - transport oxygen through the bloodstream. These cells travel from the lungs to cells and tissues. Since oxygen is required for the production of cellular energy (the conversion of glucose into ATP), appropriate delivery of oxygen to cells is vital.
Erythrocytes have a specific shape and structure, which allows them to travel through blood vessels at high speeds and through tiny capillaries.
They have a classic bi-concave disc shape and are enclosed within a lipid and carbohydrate-containing cell membrane. The lipids in this membrane maintain the cell’s shape and surface area. The types of carbohydrates present determine the patient’s blood group.
The presence of haemoglobin allows erythrocytes to transport oxygen around the bloodstream. Each red blood cell contains four iron-containing haem units and four globin units. The haem units are suspended and prevented from coming into direct contact with each other, which reduces their function.
Oxygen reversibly binds to the haem units within the red blood cell after gaseous exchange occurs in the capillaries around the alveoli in the lungs.
The oxygen-rich red blood cells (now full of oxyhaemoglobin) travel to their destination via the bloodstream, releasing the bound oxygen into the cells.
At this point, carbon dioxide—a waste product of cellular respiration—is deposited into the bloodstream for transport to the lungs and removal from the body.
In normal circumstances, red cells last for approximately 2-5 months in the circulation, after which they mature. Dead and dying red blood cells are removed via the spleen.
New red cells are produced in the bone marrow, evolve into reticulocytes (immature erythrocytes), and are released into the bloodstream.
White Blood Cells
White blood cells or leukocytes are differentiated based on the presence or absence of granules in their cytoplasm:
Granulocytes have granular cytoplasm, whereas agranulocytes have no (or very few) granules within their cytoplasm. They are also mononuclear (have only one distinct nucleus), compared with granulocytes, which have an irregular, lobulated nucleus that changes shape as the cell ages.
There are five different white blood cells:
Neutrophils
Eosinophils
Basophils
Lymphocytes
Monocytes
In today’s episode, we’ll discuss what each of these cells does and the impact on our patients when they have too many or too few of them.
Neutrophils
Neutrophils are the most common white blood cell in circulation. Their primary function is to ingest and kill bacteria and fungi within the body.
They are granular (although we often do not see these granules when we examine a blood smear). As they age, their nucleus becomes more lobulated.
‘Band’ or immature neutrophils are slightly larger than mature neutrophils. They do not have a lobulated or segmented nucleus. They are more ‘U’ or ‘C’ shaped and can be differentiated from a monocyte as these cells are larger and have a thicker nucleus taking up more of the cell.
Eosinophils
Eosinophils are leukocytes with lobulated nuclei and characteristic pink granules within their cytoplasm.
Their functions include phagocytosis and destruction of bacteria, yeasts, mycoplasmas and other pathogens.
They are also involved in hypersensitivity reactions (allergic reactions) and respond to parasite burdens within the body.
Only a small number of eosinophils are present within the circulation.
Basophils
Basophils are the most uncommon white blood cell in circulation. They have lobulated nuclei and blue granules within the cytoplasm and contain histamine.
Their function is similar to that of mast cells. They play an essential role in allergic/hypersensitivity reactions in the body.
Lymphocytes
Lymphocytes are the second most common leukocyte in circulation. They play an essential role in normal immune system function.
They are agranular and have a large, rounded nucleus that takes up most of the cell.
Only 10% of lymphocytes are present in the circulation. Others are present in lymphatic tissue in other locations, such as
The lymph nodes
The spleen
The gastrointestinal tract
There are two types of lymphocytes in circulation – B and T lymphocytes.
B-lymphocytes play an essential role in humoral immunity (producing antibodies). T-lymphocytes are involved in cell-mediated immunity, meaning the destruction of antigens that have been ‘tagged’ by antibodies. So essentially, the B-lymphocytes mark antigens for destruction by the T-lymphocytes.
Lymphocytes can live for months to years in the circulation. They can leave and re-enter the circulation via the lymphatic system.
Monocytes
Monocytes are large agranular leukocytes with a ‘U’ or ‘S’-shaped nucleus and vacuolar, purple cytoplasm.
They last in circulation for 2-3 days and can move out of the circulation into peripheral tissues. Monocytes become macrophages when they enter the tissues. These phagocytose (‘eat’) pathogens, expired cells and cellular debris.
Platelets
Platelets or thrombocytes are cellular fragments, technically not cells themselves. They play a crucial role in haemostasis. They are formed in the bone marrow from large platelet precursor cells called megakaryocytes. Fragments of these cells shear off, circulating as platelets.
Primary haemostasis is the formation of a platelet plug, the first step in haemostasis before a blood clot forms. Platelets must be present in sufficient numbers and adhere together properly to perform this function.
Von Willebrand’s factor is the substance required to ensure platelets function and adhere together correctly, creating the platelet plug.
What about plasma?
Plasma is the liquid component of blood. It is responsible for carrying and transporting blood cells and other substances. Numerous biochemical substances are dissolved and transported within the plasma, such as:
Electrolytes
Hormones
Nitrogenous waste products
Glucose, and many more.
Clotting factors are also present in plasma. They play a crucial role in secondary haemostasis, where a fibrin-based blood clot forms over the plug our platelets have already made, providing a longer-lasting and stable seal over areas of haemorrhage.
So those are our cells - what happens when we don’t have enough of them?
By now, we’ve looked at what each of our different white blood cells looks like and what it does - but what does this mean for our patients?
Many of our medical patients won’t have normal levels of these. They’ll either be too low, causing immunocompromise, or too high, usually because of inflammation or infection.
To begin treating (and nursing!) these patients, we first need to understand when these levels are abnormal and what the consequences are. To do that, we need to look at the leukogram on our haematology.
What is a leukogram?
The leukogram, or white blood cell differential, shows the levels of each individual white blood cell within the bloodstream.
This is done using either an automated analyser (as part of a routine haematology test) or manually by looking at a blood smear.
Blood smear exams and manual WBC counts can help confirm the results of an automated count. Our analyser counts can be incorrect as any large or nucleated cell may be counted as a WBC. For example:
Immature neutrophils may be mistaken for monocytes
Macroplatelets (large platelets) and immature red blood cells may be mistaken for WBCs
This means that, ideally, you should always double-check your haematology analyser results with a blood smear!
Neutrophilia and neutropenia
Neutrophilia is an increase in neutrophil numbers and is the most common reason for an increased total WBC level.
A neutrophilia can either be:
Mature: where there is an increase in mature or developed neutrophils without an increase in immature or band neutrophils or
With a Left-Shift, where many immature or band neutrophils are present alongside mature cells.
Neutrophilia can be caused by several different factors, which we classify as adrenaline-induced, stress-induced, steroid-induced, or due to inflammation.
Neutropenia is a decrease in circulating neutrophils. Several factors can cause neutropenia, and these are divided into three main categories:
Decreased production within the bone marrow
Sequestration of neutrophils in the marginal pool (where they stay adhered to vessel walls)
Excessive demand or consumption of neutrophils.
Causes of decreased production include drug-induced side effects, bone marrow disease, infectious diseases and toxin ingestion. Chemotherapy is a classic example of drug-induced neutropenia.
Sequestration is caused by anaesthesia, anaphylaxis and endotoxic shock.
Neutrophil consumption can be caused by severe, acute, overwhelming infection, neoplasia and immune-mediated disease.
Patients with neutropenia are less able to deal with an infection, so we need to be very careful when nursing them.
Eosinophilia and eosinopenia
Eosinophilia is an increase in eosinophils. It is most commonly caused by parasitic infections or inflammatory or allergic diseases of the gastrointestinal tract, respiratory tract, urogenital tract, and skin.
Eosinopenia is an absolute decrease in eosinophil levels. This can be challenging to confirm because the normal range on many haematology analysers goes down to zero!
Most of the time, low eosinophil levels are because of stress or steroid administration.
Basophils and basophilia
Basophilia is an absolute increase in circulating basophils.
This is usually seen in combination with eosinophilia since both of these cells are involved in responding to allergies.
The most common causes of basophilia include:
Heartworm (not seen frequently in the UK, but can be seen in travelling dogs)
Allergic respiratory diseases
Parasitic infections
Some cancers
Basopenia is not considered relevant. Many healthy animals have no basophils in their bloodstream, and most haematology normal ranges go down to zero!
Lymphocytosis and lymphopenia
Lymphocytosis is an increase in lymphocyte numbers, generally due to physiologic or age-related causes.
Physiologic lymphocytosis occurs due to an adrenaline response (particularly in cats), usually due to stress, fear or excitement.
Age-related lymphocytosis occurs in young animals under six months of age. These patients have a higher lymphocyte count than adults.
Lymphopenia, or low lymphocyte levels, is usually seen as part of the stress leukogram due to stress or steroid use.
Other causes include:
Active infection
Loss of lymphocytes (e.g. in chylothorax patients)
Congenital immunodeficiencies.
Monocytes and monocytosis
Monocytosis is an increase in circulating monocytes and is seen due to two leading causes.
The first is stress, which is a common cause of monocytosis, especially in dogs. Acute or chronic inflammatory or infectious diseases are another common cause.
Like basophils, monocytopenia has no clinical significance. This is because healthy patients can have no circulating monocytes, and our analyser reference ranges can go down to zero.
Nursing considerations for WBC disorders
When it comes to nursing the white blood cell disorder patients, we need to think about:
Evaluating blood smears and double-checking our haematology machine values
Protecting patients from infection where significant decreases in white blood cells are present
Minimising fear and stress in our patients to prevent it from interfering with haematology results
Treating the underlying cause of the disorder by administering steroids, antibiotics, or other specific treatments depending on the underlying disease
Educating clients on the importance of parasite prevention
So that’s a refresher of all of the cells we see on our blood smears, what they look like, what they do, and how they form!
Remember - you don’t need to be a vet to look at these samples and identify these cells. The more you look at them, the more you’ll identify what’s normal - making those abnormalities even easier to spot.
You don’t need to know the exact term for every morphological change (though if you want to, that’s great!) - you just need a good idea of what’s normal and what’s not - and you get that through practice.
Are you currently examining smears in practice? I’d love to know—drop me a DM on Instagram, and let’s chat! And if not, why not grab the microscope and take a peek?!
Practice, practice, practice looking at those cells and noticing the difference between each WBC. You can also perform practice cell counts and double-check yourself against your patient’s haematology results as a rough guide!
Did you enjoy this episode? If so, I’d love to hear what you thought - screenshot it and tag me on Instagram (@vetinternalmedicinenursing) so I can give you a shout-out and share it with a colleague who’d find it helpful!
Thanks for learning with me this week, and I’ll see you next time!
References and Further Reading
Day, MJ and Kohn, B. 2012. BSAVA Manual of Canine and Feline Haematology and Transfusion Medicine. 2nd ed. Gloucester: BSAVA.
EClinPath. 2013. Haematology. Available from: http://eclinpath.com/hematology/
EClinPath, 2013. Individual WBC. Available from: http://eclinpath.com/hematology/leukogram-changes/leukocytes/
Merrill, L, 2012. Small Animal Internal Medicine for Veterinary Technicians and Nurses. Iowa: Wiley-Blackwell.
Sirois, M. 2020. Laboratory Procedures for Veterinary Technicians. 7th edition. Missouri: Elsevier.
