Why Your Vet Recommends Blood Tests
Blood tests are one of the most useful diagnostic tools available in veterinary medicine, yet they can feel opaque and overwhelming when you are handed a results sheet covered in abbreviations and numbers. Understanding what is actually being measured — and why it matters — helps you have more informed conversations with your vet and make better decisions for your dog.
The Two Main Components of a Blood Panel
Most standard blood panels in dogs consist of two sections: the complete blood count and the biochemistry profile. These measure very different things and together give your vet a broad picture of your dog's internal health.
The Complete Blood Count
The complete blood count, usually abbreviated to CBC, examines the cellular components of blood. It looks at three main cell types: red blood cells, white blood cells, and platelets.
Red blood cells carry oxygen around the body. The CBC measures their number, size, and haemoglobin content. Low values indicate anaemia, which can result from blood loss, destruction of red cells by the immune system, or a failure in production — often pointing to bone marrow or chronic disease issues. Unusually high values are less common but can suggest dehydration.
White blood cells are the immune system's foot soldiers. The total count matters, but so does the breakdown of cell types, called the differential. Neutrophils are the first responders to infection or inflammation. Lymphocytes are involved in longer-term immune responses. Eosinophils often rise in response to parasites or allergic conditions. A high total white cell count typically suggests infection, inflammation, or, in some cases, cancer. A low count can mean the immune system is suppressed, which may be a side effect of medication or a sign of viral disease.
Platelets are essential for blood clotting. A low platelet count, called thrombocytopaenia, can cause abnormal bleeding and bruising. It is seen in conditions ranging from tick-borne diseases to immune-mediated disorders.
The Biochemistry Profile
The biochemistry profile measures substances dissolved in the liquid part of blood, the serum. It tells your vet how well your dog's internal organs are functioning.
Liver values are among the most scrutinised. Alanine aminotransferase (ALT) is released from liver cells when they are damaged, so elevated ALT suggests liver cell injury. Alkaline phosphatase (ALP) can rise with liver disease, bone issues, or steroid use. These enzymes do not tell your vet exactly what is wrong with the liver — they simply flag that something is happening and that further investigation may be needed.
Kidney function is assessed primarily through two values: creatinine and urea (sometimes listed as BUN — blood urea nitrogen). These waste products normally filtered by the kidneys accumulate in the blood when kidney function is reduced. Importantly, these values only rise noticeably once a significant proportion of kidney function is already lost, which is why vets increasingly use an additional marker called SDMA, which can detect kidney decline earlier.
Blood glucose levels indicate how well the body is regulating sugar. Elevated glucose in a fasted dog raises suspicion of diabetes mellitus. Low glucose can cause collapse and seizures and may point to insulinoma, a type of pancreatic tumour, or other serious conditions.
Total protein and albumin levels reflect nutritional status, liver function, and protein loss through the kidneys or gut. Low albumin in particular can cause fluid to leak from blood vessels into body cavities.
Electrolytes and Other Values
Many panels also include electrolytes: sodium, potassium, and chloride. These are tightly regulated by the body and can be disrupted by vomiting, diarrhoea, kidney disease, and hormonal conditions such as Addison's disease, where potassium rises and sodium falls in a characteristic pattern.
Calcium levels matter too. High calcium can be associated with certain cancers, parathyroid gland problems, or vitamin D toxicity. Low calcium causes muscle tremors and can occur after whelping in nursing dogs.
Pre-Anaesthetic Bloods: A Special Case
You will often be offered blood tests specifically before your dog undergoes a general anaesthetic. These are not simply a money-making exercise; they serve a real purpose. Anaesthetic drugs are processed by the liver and kidneys, and if either organ is compromised, drug dosing may need to be adjusted or additional monitoring put in place. Finding an unexpected problem before surgery gives the team time to plan, whereas discovering it mid-procedure does not. Even in young, apparently healthy dogs, occasional pre-anaesthetic bloods turn up abnormalities that change the approach.
Understanding Reference Ranges
Every value on a blood test comes with a reference range. It is important to understand that these ranges are established from a population of healthy dogs and are not absolute thresholds for disease. A result just outside the range in an otherwise healthy, normal dog may be insignificant. A result within the range in a dog with suspicious symptoms may still prompt investigation. Your vet interprets results in the context of your dog's clinical presentation — the numbers alone rarely tell the whole story.
Some values also vary between breeds. Greyhounds, for example, have naturally higher red blood cell counts than most breeds. Certain large breeds have different normal ranges for ALP. These breed-specific variations are well-documented and a good vet takes them into account.
When Results Come Back Abnormal
A single abnormal value does not automatically mean a serious diagnosis. Stress alone can elevate blood glucose and white blood cell counts. Strenuous exercise can temporarily alter muscle enzyme values. Your vet may recommend repeating a borderline test in a few weeks, or may proceed to more specific diagnostics such as urine testing, imaging, or specialist blood panels, depending on what the values suggest and how your dog is presenting clinically.
Blood testing is a snapshot in time. Used wisely — at appropriate life stages, before anaesthesia, and during illness — it gives your vet a powerful window into your dog's internal world that no amount of external examination can replicate.