Why do vets seem to always take bloods?

Examining a patient from the outside can be really useful – we can pick up a lot of information. Even more useful is what we call “obtaining a history” – finding out what they’re doing or not doing!) when they’re at home, what their symptoms are, when they started, and if there are any obvious triggers. However, the body of a dog, cat, rabbit or indeed any mammal is incredibly complicated.

For example, a simple symptom like vomiting has the following major (and fairly probable) causes: adverse food reactions, food allergies, eating a foreign object, eating something rotten (“dietary indiscretion”), sudden diet change, dehydration, drug reactions, drug side effects (especially painkillers), viral gut infections, bacterial gut infections, stomach ulcers, gut intussusception (where a loop of intestines turns inside out and gets stuck in itself), twisted gut, intestinal tumours, gut stasis, constipation, kidney failure, liver failure, septicaemia, systemic infections (e.g. with a fever), acidosis (where the blood is too acidic), low blood potassium, high- or low blood calcium, high- or low blood sodium, pancreatitis (inflammation of the pancreas), peritonitis (inflammation or infection of the abdomen), pyometra (a wound infection), Addison’s disease, diabetes, vestibular syndrome, meningitis, head trauma and concussion, worms, Giardiasis (another intestinal parasite), lead poisoning, antifreeze poisoning, mould poisoning, anaphylactic shock, heat stroke, motion sickness and pain.

And that’s only the common causes of acute, or sudden onset vomiting – the list of possible causes for chronic (ongoing) vomiting is even longer!

As a result, often the only way to find out exactly what’s going on inside the body is to look at the blood. Blood is a fascinating organ – it fights infection, maintains hydration, and has a host of other functions, but most importantly, it carries things around the body (nutrients, oxygen, wastes). As a result, it contains traces of chemicals released by every other organ – which means we can look for those traces, to tell us how well those organ systems are working.

Of course, there are so many thousands of different chemicals in a 1ml sample of blood that there’s no way we can test them all, so we concentrate on certain key “markers” – chemicals or other blood components that have been proven to demonstrate how well particular systems are working. In practice, what we’re usually running is a “screening” test – looking for common problems that might let us narrow down our search for a cause to your pet’s illness.

We can generally divide our blood test into two components:


This is the study of the different blood cells; and nowadays we usually use a machine to count them, then use a good old-fashioned microscope to examine individual cells for signs of damage or disease.

By far the most common cell type is the red blood cell, or erythrocyte. Counting these will tell us if the patient is anaemic (low) or dehydrated or possibly polycythaemic (high). Looking at them and measuring them will also tell us a lot – different changes in blood cell shape may suggest immune anaemia(immune mediated haemolytic anaemia), certain blood-parasites such as Feline Infectious Anaemia or Babesia canis, some types of poisoning (e.g. onion poisoning), or iron deficiency. It can also tell us if any anaemia is regenerative (i.e. the body is able to make new red blood cells) or non-regenerative (if it can’t for some reason).

We can also count the platelets (usually by eye with a microscope – the machines aren’t very good at it!). These are the clotting cells – too few mean the body cannot clot properly.

That said, we’re often more interested in the white blood cells. Different ratios of the various types can tell us a lot –  very high neutrophil counts suggest an acute infection; low numbers of neutrophils suggests either that the patient is immunocompromised or that they are fighting a severe infection. Levels of lymphocytes may indicate infection, or sometimes leukaemia, while eosinophils suggest either an allergy or a parasitic infestation.


…is the study of the chemical traces in the blood – often, these are salts, nutrients or waste products, but some are proteins released from damaged cells, or parts of the immune system that give us an inkling as to what is going on at the cellular level in our patients. Our standard screening test is a “Chem-12”, which looks at 12 key substances:

  • Albumin (ALB) – This is the main transport protein in the bloodstream; high albumin suggests dehydration, whereas this is only low following severe blood or plasma loss, occasionally really severe gut or kidney disease (protein losing enteropathy or protein losing nephropathy), or liver failure. Usually, this is pretty stable – anything dramatic enough that it changes the levels of albumin in the blood is likely to be fairly severe.
  • Globulin (GLOB) – This is essentially all the other proteins in the body. A low globulin is really rare; a high globulin count is suggestive of inflammationinfectionimmune disease, or a tumour.
  • Total Protein (TP) – This is just the levels of the albumin plus the globulin added together.
  • Blood Glucose (BG or GLUC) – Animals need glucose in their blood for fuel – low glucose suggests insulin overdose or, rarely, septicaemia; high glucose may mean stress, Cushing’s disease or (most likely) diabetes.
  • Blood Urea Nitrogen (BUN) – Urea is the main waste product from protein metabolism. A low urea almost always means liver disease, whereas a high level generally indicates kidney failure, a blocked bladder or dehydration.
  • Creatinine (CREA) – Another marker for kidney function, creatinine is most commonly raised only in kidney failure.
  • Alkaline Phosphatase (ALKP) – this is an enzyme that is released from damaged cells – mostly liver cells and a raised level often means cholestasis(where the bile stops flowing). However, it can also indicate liver disease, treatment with some drugs (e.g. phenobarbitone), Cushing’s disease and even some bone disorders.
  • Alanine Aminotransferase (ALT) – This is another liver enzyme, but is only released from damaged liver cells. High levels indicate liver damage.
  • Total Bilirubin (TBIL) – Bilirubin is released from the breakdown of red blood cells; high levels cause jaundice. This may be prehepatic (caused by haemolysis, abnormal red cell breakdown), intrahepatic (caused by liver disease) or posthepatic (due to a bile duct blockage).
  • Calcium (CA) – if blood calcium levels change much, the muscles, brain and heart stop working – so it’s really important to monitor it closely! Alterations may indicate cancerexcessive milk productionkidney disease or bone disease, among other possibilities.
  • Sodium (NA or SOD) – Sodium is a key salt in the blood – it is very tightly regulated by the body. In general, it will only change in severe dehydrationor salt poisoning.
  • Potassium (K or POT) – Potassium is a salt usually removed by the kidneys; however, if the levels get too low, muscles stop working; if they get too high, it can stop the heart. Possible causes for altered potassium include kidney diseasesevere vomiting, and tissue death (e.g. in a torsion).
  • Cholesterol (CHOL) – In humans, doctors really worry about cholesterol counts. In animals, it isn’t such a big deal – we tend to use it primarily as a marker of how recently the animal ate, although in cats especially it can also be a sign of fatty liver disease.

Other specific tests do, of course, exist, for hormones (e.g. thyroid levels), signs of pancreatitis, EPI, certain infections (e.g. FIV, FeLV, Parvovirus), and many other possible diseases.

However, there’s nothing else out there that gives us as much information in a hurry as a simple Haematology and Biochem – a test which we can do in house in about 20 minutes!