Blood Work for Senior Pets: What Every Test Checks

The vet says your 10-year-old dog or cat should have annual blood work done. You agree, they run the tests, and then you get a printout full of abbreviations and numbers with reference ranges. Most of it makes no sense.

Blood work for senior pets is one of the most valuable tools available for catching serious problems before they become emergencies. This guide explains exactly what each test checks, what abnormal results mean, and why certain conditions show up in blood before they show up as symptoms.
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Why Senior Pet Blood Work Matters

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Most organ diseases are silent until they are advanced. Kidney disease, for example, does not show clinical signs until 75% of kidney function is already lost. Blood work catches the early damage while treatment options are still meaningful.
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Annual results create a personal baseline. A single blood test tells you where your pet is today. A series of annual tests shows trends. A value that is still within the reference range but has doubled over three years is a more important finding than any single number.
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Hyperthyroidism in cats is caught almost exclusively through blood work. The clinical signs overlap with normal aging so completely that owners rarely identify it themselves. A T4 measurement takes seconds and confirms or rules it out immediately.
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Many medications used in senior pets require blood monitoring. NSAIDs for arthritis pain, for example, can affect kidney function over time. Regular blood work catches this early enough to adjust the treatment before damage becomes permanent.
Quick Answer: What Senior Pet Blood Work Checks
  1. CBC (Complete Blood Count): red cells, white cells, platelets, anaemia, infection, clotting
  2. BUN and creatinine: kidney function
  3. ALT and ALP: liver health
  4. Glucose: diabetes screening
  5. Total protein and albumin: nutrition and liver/gut health
  6. T4 (thyroxine): thyroid function, especially cats
  7. Electrolytes: sodium, potassium, calcium balance

Complete Blood Count: What It Checks

The CBC gives a snapshot of all three types of blood cells and what they are doing.

Red blood cells (RBC) and haematocrit: Red cells carry oxygen. A low count means anaemia. Senior pets develop anaemia for many reasons including kidney disease (kidneys produce the hormone that stimulates red cell production), chronic disease, blood loss, or bone marrow problems. Anaemia explains lethargy, weakness, and pale gums.

White blood cells (WBC): White cells are the immune system’s soldiers. Elevated counts suggest infection, inflammation, or stress. Very low counts suggest immune suppression or bone marrow disease. A moderately elevated count in a senior dog can indicate something as simple as excitement at the vet or something as serious as infection or cancer.

Platelets: Platelets allow blood to clot. Low platelet counts cause unusual bruising or slow-healing wounds. Some tick-borne diseases cause platelet drops in senior dogs who have spent time outdoors.

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Why This Matters for Older Pets

Anaemia in senior cats is far more common than most owners realise. A 2012 study found that approximately 30% of cats over 11 years of age had some degree of anaemia. Most showed minimal obvious symptoms because cats are extremely good at adapting to gradual changes. The blood test reveals what the cat’s behaviour conceals.

Biochemistry Panel: Organs and Metabolism

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BUN and Creatinine

Kidney waste products that build up in blood when kidneys fail. Elevation signals reduced kidney filtering capacity. SDMA is a newer, more sensitive marker that rises earlier than creatinine.

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ALT and ALP

Liver enzymes. ALT rises when liver cells are damaged. ALP rises with bile duct problems or steroid use. Both can be mildly elevated in senior pets for various reasons and must be interpreted in context.

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Glucose

Blood sugar. Persistently elevated glucose indicates diabetes. In cats, single readings can be elevated due to stress at the vet. Fructosamine is a better test for confirming actual diabetes rather than stress-related spikes.

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Total Protein and Albumin

Low albumin indicates problems with liver production, kidney protein loss, gut absorption, or inadequate nutrition. It affects how medications are distributed in the body, making it important for senior pets on multiple drugs.

Thyroid Test: Essential for Senior Cats

The T4 test measures thyroxine, the main thyroid hormone. In cats over 10, hyperthyroidism (too much T4) is the most common endocrine disease. Signs include weight loss despite good appetite, increased thirst and urination, hyperactivity in an older cat, and vomiting.

Hypothyroidism (too little T4) is far more common in dogs than cats and causes the opposite: weight gain, lethargy, coat changes, and slow heart rate. Both conditions are very manageable once identified.

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Ask for T4 testing specifically. Many standard senior wellness panels include it for cats but not always for dogs. If your senior dog shows unexplained weight gain, lethargy, or skin and coat changes, ask your vet to add thyroid testing to the blood panel even if it is not included by default.

Why Urine Is Tested Alongside Blood

Blood work and urinalysis together tell a more complete story than either alone. The kidneys’ job is to concentrate urine. If a dog or cat is drinking and urinating excessively, measuring the concentration of the urine shows how hard the kidneys are working to maintain it.

Protein in the urine alongside normal blood kidney values can be an early warning of kidney disease before creatinine rises. Glucose in the urine alongside normal blood glucose suggests the kidneys are not filtering correctly. Bacteria or white cells in urine indicate infection.

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How to Read Results at Home

What to Look for When You Get the Printout
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H and L flags: Values marked H (high) or L (low) are outside the reference range. Not every flagged value is a crisis, but all flagged values deserve a conversation with your vet.
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Trends over time: Keep your previous blood work results. A BUN of 28 this year versus 14 last year is significant even if both are within range.
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Reference ranges vary by lab. Different laboratories use slightly different normal ranges. Always compare your pet’s results to the ranges printed on that specific report.
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Ask for interpretation in plain language. You are entitled to a clear explanation of what each result means for your pet specifically. If the answer is unclear, ask again.
How often should a senior dog or cat have blood work?▼

Most vets recommend annually from age 7 in dogs and age 10 in cats, or sooner for breeds prone to specific conditions. For pets already diagnosed with kidney disease, liver disease, diabetes, or thyroid problems, testing every 3 to 6 months is standard. For pets on long-term NSAIDs, kidney values should be checked every 6 months. The frequency depends on what the results show and what conditions are being managed.

Does my pet need to fast before blood work?▼

For most comprehensive panels, yes. A 6 to 8 hour fast is standard. This prevents lipemia (fat in the blood from a recent meal) from interfering with test results, particularly liver enzymes and glucose. Water is fine right up to the appointment. Check with your vet when booking the appointment as specific requirements vary.

What if the blood work comes back normal but my pet still seems unwell?▼

Normal blood work is genuinely reassuring but does not rule out everything. Pain does not show up on a blood panel. Early arthritis, dental disease, and some cancers can be present with entirely normal blood values. If your pet seems unwell and blood work is normal, discuss what other diagnostics are appropriate, including X-rays, ultrasound, or specific disease testing.

The Bottom Line

Senior pet blood work is not just a formality. It is one of the most powerful tools available for catching serious conditions before they become emergencies.

Keep your previous results, ask for trend comparisons, and make sure you understand what each flagged value means for your specific pet. Annual blood work for a senior dog or cat costs less than most emergency vet visits and frequently prevents the need for them.