Senior cat blood tests: what your vet checks and why it matters every year

A cat can look completely fine at eleven years old and still be in the early stages of kidney disease. That is not an edge case, it is the rule. Feline kidneys lose function gradually, and cats are exceptionally good at compensating until the loss crosses a threshold the body can no longer hide. A blood test ordered before any symptom appears is the only way to catch that slide while there is still room to slow it.

A senior blood panel has two main parts: a complete blood count and a biochemistry profile, usually run together with a urine specific gravity on the same visit.

The complete blood count looks at red blood cells, white blood cells, and platelets. In a senior cat, the vet is watching for anaemia, not uncommon in cats with chronic kidney disease, because damaged kidneys produce less erythropoietin, the hormone that tells bone marrow to make red cells. A low white cell count or an abnormal differential can flag infection or, in older cats, early lymphoma.

The biochemistry profile is where most of the senior-specific information lives. Creatinine and BUN (blood urea nitrogen) are the traditional kidney markers. Both rise when the kidneys are no longer filtering efficiently. The problem is that creatinine does not rise above the normal range until roughly 75 percent of kidney function is already gone. That is why most vets now add SDMA, symmetric dimethylarginine, to the panel. SDMA climbs earlier, sometimes when only 25 to 40 percent of function has been lost, which gives the owner and the vet a meaningful head start.

ALT and ALP are liver enzymes. Elevated ALT usually points to liver cell damage; elevated ALP in a cat, unlike in a dog, where it can rise for many reasons, is taken more seriously and often prompts further investigation. Total bilirubin, albumin, and total protein round out the liver picture. Blood glucose is checked for diabetes, which becomes more common after middle age in cats, particularly in neutered males and those carrying extra weight. Phosphorus is read alongside the kidney markers, because cats with declining kidney function tend to retain phosphorus, which accelerates the damage.

Total T4, the thyroid hormone measurement, is not always bundled into a standard biochemistry panel, but most vets include it for cats over ten. Hyperthyroidism is the most common hormonal disorder in older cats. The thyroid gland overproduces hormone, driving up metabolism, heart rate, blood pressure, and appetite while the cat loses weight despite eating more.

The complication that makes T4 essential in a senior panel is this: hyperthyroidism can mask kidney disease. An overactive thyroid artificially raises the glomerular filtration rate, which keeps creatinine looking normal even when kidney function is declining underneath. Treat the thyroid and the creatinine climbs, revealing kidney disease that was always there. A vet who knows the T4 before treatment can prepare the owner for what the kidney numbers will show afterwards, and choose a treatment approach that manages both conditions rather than correcting one while uncovering another without warning.

The yearly interval exists because senior cat physiology can shift faster than an annual check captures. Many veterinary bodies, including the American Association of Feline Practitioners, recommend moving to every six months once a cat is in its mid-teens or once any abnormality has been found. The logic is straightforward: a creatinine that was borderline in January and is clearly elevated in July tells the vet the rate of decline, which shapes how aggressively to intervene. A single data point is a snapshot. Two points a year is a trend.

There is also a baseline argument. Running the first senior panel on a healthy ten-year-old cat, before anything is wrong, establishes that individual cat's normal. Cats vary. A creatinine of 1.8 might be high-normal for one cat and the same cat's personal baseline for years. Without an earlier result to compare, a vet reading 1.8 on a twelve-year-old has to decide whether it represents stability or decline without the context that would make the call clear.

An abnormal result is not a diagnosis. It is a direction. Elevated creatinine and SDMA together prompt a urine specific gravity test, a urine protein-to-creatinine ratio, and a blood pressure reading to stage the kidney disease and decide whether dietary phosphorus restriction is warranted yet. Elevated T4 opens a conversation about treatment options, medication, radioactive iodine, or diet, each with different implications for a cat that may also have kidney disease. A low packed cell volume with normal kidney markers sends the investigation toward bone marrow, diet, or immune-mediated causes.

The blood test does not end the conversation. It starts the right one.

If your senior cat is drinking more than usual, losing weight without eating less, vomiting regularly, or becoming noticeably quieter, do not wait for the next scheduled panel, those are signs that warrant a vet visit within days, not months.

Most senior blood panels require a fast of eight to twelve hours beforehand, because food affects glucose and some lipid values. Water is fine and should not be withheld. The blood draw itself takes under a minute from a jugular or cephalic vein and causes no more distress than a vaccination. Results from an in-house analyser are usually available within the hour; panels sent to an external laboratory take one to two days.

The cost varies by city and clinic, and panels differ in what they include, so it is worth asking the vet exactly which markers are covered before the appointment. Some clinics offer senior wellness packages that bundle the blood panel, urine test, blood pressure measurement, and a physical exam at a combined price.

A cat that clears every number cleanly still benefits from having those numbers on record. The clean panel from this year is the baseline that makes next year's results meaningful.