Why Blood Tests Matter for Dogs
A dog cannot tell you what hurts or how long they have felt unwell. Blood tests give your veterinarian a detailed, objective picture of what is happening inside your pet's body — often identifying problems before clinical signs are apparent. Both the BSAVA (British Small Animal Veterinary Association) and the WSAVA (World Small Animal Veterinary Association) recommend blood testing as part of routine wellness care for older dogs and as a standard pre-anaesthetic assessment before any elective surgery.
Understanding what the numbers on a blood test report mean allows you to ask better questions and engage more meaningfully with your vet's findings. This guide explains the most common measurements in plain terms.
The Complete Blood Count (CBC)
The CBC — also called a haematology panel — analyses the cellular components of the blood. It tells your vet about red blood cells, white blood cells, and platelets.
Red Blood Cells (RBC) and Haematocrit
Red blood cells carry oxygen around the body. The haematocrit (also called packed cell volume or PCV) measures what percentage of blood is made up of red cells. A low haematocrit indicates anaemia — the dog's blood is not carrying enough oxygen. This can be caused by blood loss, destruction of red blood cells (haemolytic anaemia), or insufficient production (often linked to bone marrow problems or chronic disease). A high haematocrit may indicate dehydration or, rarely, a condition called polycythaemia.
White Blood Cells (WBC)
White blood cells are the immune system's frontline defenders. An elevated white cell count (leucocytosis) typically indicates infection, inflammation, or stress. A low count (leucopenia) can suggest viral illness, bone marrow suppression, or immune-mediated disease. The differential count breaks WBCs down into subtypes — neutrophils, lymphocytes, monocytes, eosinophils, and basophils — each giving additional diagnostic clues.
Platelets
Platelets are tiny cells responsible for blood clotting. Low platelet counts (thrombocytopenia) can result in bruising or bleeding and may be caused by tick-borne diseases (particularly relevant in dogs in southern Europe and the Mediterranean region), immune-mediated destruction, or toxin exposure.
The Biochemistry Panel
The biochemistry (or chemistry) panel assesses organ function and metabolic status by measuring proteins, enzymes, and waste products in the blood.
Liver Enzymes: ALT and ALP
ALT (alanine aminotransferase, also called SGPT) is the most liver-specific enzyme in dogs. Elevated ALT indicates damage to liver cells — hepatocytes — and can signal hepatitis, toxin exposure, liver tumours, or a range of other hepatic conditions. The degree of elevation gives a rough guide to severity, though it does not indicate the cause.
ALP (alkaline phosphatase) is less specific than ALT. It is found in the liver, bone, intestinal lining, and placenta. Elevated ALP in dogs can indicate liver disease, Cushing's disease (hyperadrenocorticism — a common condition in middle-aged and older dogs), corticosteroid treatment, or bone remodelling. In young dogs, mildly elevated ALP from bone growth is normal and should not cause alarm.
Kidney Markers: BUN and Creatinine
Blood urea nitrogen (BUN) and creatinine are waste products filtered out of the blood by the kidneys. When kidney function declines, these substances accumulate. However, traditional kidney markers such as BUN and creatinine do not become elevated until approximately 75% of kidney function has been lost — meaning significant damage may be present before conventional tests detect it.
BUN is also influenced by diet (high protein intake raises BUN), hydration status, and intestinal bleeding, making it less specific for kidney disease when interpreted in isolation. Creatinine is more reliable as a kidney marker, though it is influenced by muscle mass — a very muscular dog may have a higher baseline creatinine that is still normal for them.
SDMA: The Early Kidney Marker
Symmetric dimethylarginine (SDMA) is a newer kidney biomarker now routinely included in many veterinary blood panels across Europe. Crucially, SDMA becomes elevated when only around 25–40% of kidney function has been lost — significantly earlier than BUN or creatinine. WSAVA and IRIS (International Renal Interest Society) guidelines now recommend SDMA as part of chronic kidney disease staging. Early detection means earlier intervention, which can meaningfully slow progression and improve quality of life.
Glucose
Blood glucose measures the concentration of sugar in the bloodstream. Elevated glucose in a non-stressed dog suggests diabetes mellitus, which is relatively common in middle-aged and older dogs. Low glucose (hypoglycaemia) can indicate insulinoma (a pancreatic tumour), prolonged fasting, sepsis, or Addison's disease. Stress can cause transient glucose elevation in healthy dogs, so mildly elevated results should be interpreted in context.
Total Protein and Albumin
Total protein measures the overall protein content of the blood. Albumin is the most abundant individual protein and plays a key role in maintaining fluid balance. Low albumin can indicate liver disease (the liver produces albumin), protein-losing enteropathy (intestinal disease causing protein loss), protein-losing nephropathy (kidney disease causing protein leakage into the urine), or chronic malnutrition. Very low albumin is associated with fluid accumulation in the abdomen or chest.
Electrolytes
Electrolytes are minerals dissolved in the blood that regulate fluid balance, nerve conduction, and muscle function. Standard panels measure sodium, potassium, and chloride.
- Sodium: Abnormalities indicate problems with water balance. Low sodium (hyponatraemia) is a classic finding in Addison's disease (adrenal insufficiency), a condition more common in certain breeds including the Labrador, Standard Poodle, and Leonberger.
- Potassium: High potassium (hyperkalaemia) is also a hallmark of Addison's disease. Low potassium (hypokalaemia) can cause muscle weakness and is seen in vomiting, diarrhoea, and some diuretic medications.
- Chloride: Usually interpreted alongside sodium. Significant derangements indicate acid-base disturbances and are particularly important in vomiting dogs or those on fluid therapy.
Thyroid Function: T4
Total T4 (thyroxine) measures thyroid hormone levels. Unlike cats, who commonly develop hyperthyroidism (overactive thyroid), dogs are far more prone to hypothyroidism — an underactive thyroid gland. Hypothyroidism is one of the most commonly diagnosed hormonal conditions in dogs, particularly in Golden Retrievers, Dobermanns, Cocker Spaniels, and other mid-to-large breeds.
Clinical signs of hypothyroidism — weight gain, lethargy, coat changes, intolerance to cold, and recurrent skin infections — are often gradual and easily attributed to ageing. A T4 measurement, ideally alongside TSH (thyroid-stimulating hormone), helps confirm or rule out the diagnosis. Treatment with daily oral thyroxine supplementation is highly effective and generally lifelong.
When Should Blood Tests Be Done?
WSAVA and BSAVA guidelines recommend considering blood testing in the following situations:
- Before any anaesthesia: Pre-anaesthetic blood tests allow the vet to identify risk factors and adjust the anaesthetic protocol accordingly. Most European vets recommend this routinely for all dogs undergoing elective surgery.
- Annual wellness testing in older dogs: Dogs aged seven and above benefit from annual blood screening to catch age-related conditions early. Many practices now offer senior health packages that include bloods as standard.
- When a dog is unwell: Vomiting, weight loss, excessive thirst, lethargy, and changes in urination are all indications for blood testing.
- Monitoring dogs on long-term medication: Dogs taking anti-epileptic drugs, corticosteroids, non-steroidal anti-inflammatory drugs (NSAIDs), or certain antibiotics long-term should have periodic blood checks to monitor liver and kidney function.
Interpreting Results: What Abnormal Really Means
Reference ranges on blood test reports represent the values found in a statistically normal population of healthy dogs. Two important caveats apply: first, reference ranges vary between laboratories and analysers, so the same result may appear flagged on one report and within range on another. Second, a mildly out-of-range result is not always clinically significant — your vet will interpret results in the context of your dog's symptoms, breed, age, and other findings.
A single abnormal value rarely tells the whole story. Blood results should always be interpreted alongside a clinical examination, history, and often in conjunction with urinalysis and further diagnostics. If you have questions about your dog's results, ask your vet to walk through them with you — good communication about blood test findings is a cornerstone of the veterinary relationship the RCVS and BSAVA both emphasise in their professional standards.
Article reviewed by Sarah Bennett, veterinary health writer. References: WSAVA Global Veterinary Community guidelines; BSAVA Manual of Canine and Feline Clinical Pathology; IRIS (International Renal Interest Society) CKD staging guidelines; RCVS Code of Professional Conduct.