Cat Health · Updated 2026-02-28

Cat Kidney Disease: The Early Signs, The Tests, And The Treatment That Buys Years

A comprehensive look at feline chronic kidney disease — early signs, the SDMA test, staging, and the interventions that meaningfully extend life.

The most common serious illness in older cats

Feline chronic kidney disease (CKD) affects an estimated 30–50% of cats over fifteen, and appears in meaningful numbers from age eight onward. It is the single most common reason for euthanasia in senior cats, yet it is also one of the most treatable chronic conditions when caught early. The gap between those two sentences is closed by one habit: annual screening bloodwork starting at age seven.

This guide covers the early signs, the tests worth running, the IRIS staging system, and the interventions that actually change trajectory.

Why cats hide it

By the time a cat shows classic symptoms of kidney disease — increased drinking, increased urination, weight loss — roughly two-thirds of kidney function is already gone. The kidneys have enormous reserve capacity, and the cat compensates so effectively that subtle changes fly under the radar. Owners report "slowing down" for months before anyone tests.

The subtle early signs

• A slow, incremental increase in water consumption — owners notice the water bowl empty more often before they notice the cat drinking
• Slightly larger urine clumps in the litter box
• A gradual decline in coat quality
• Mild weight loss, especially muscle mass over the spine
• Reduced appetite, sometimes with food aversion that looks like pickiness
• Intermittent vomiting that owners attribute to hairballs
• Occasional constipation
• Bad breath with a faintly ammonia character

Any combination of two or three of these in a cat over seven is a reason for a bloodwork and urinalysis visit, not an observational pause.

The tests, and why SDMA changed the game

Traditional CKD markers — BUN and creatinine — don't rise until roughly 65–75% of kidney function is lost. SDMA (symmetric dimethylarginine), widely available since 2015, rises earlier, at around 40% loss. For a senior cat, the workup for suspected CKD or annual screening includes:

• CBC and chemistry panel including BUN, creatinine, phosphorus, potassium
• SDMA
• Urinalysis including specific gravity (USG) and protein (UPC)
• Blood pressure — hypertension is common in CKD cats and is silent
• Total T4 — hyperthyroidism mimics and complicates CKD

A cat whose USG is persistently below 1.035 with an elevated SDMA — even with normal creatinine — is in IRIS stage 2 early CKD. That's the cat whose trajectory you can meaningfully change.

IRIS staging

The International Renal Interest Society's staging system guides treatment. In short:

• Stage 1: creatinine below 1.6, with other evidence of kidney damage. Often asymptomatic.
• Stage 2: creatinine 1.6–2.8. Often asymptomatic or with subtle signs.
• Stage 3: creatinine 2.9–5.0. Clinical signs usually present.
• Stage 4: creatinine above 5.0. Severe symptoms, complex management.

Sub-staging adds proteinuria (UPC) and blood pressure categories. The treatment paths differ meaningfully by stage.

Interventions that actually help

Diet

Prescription therapeutic renal diets — Hill's k/d, Royal Canin Renal, Purina NF — are the best-studied intervention in veterinary medicine for extending survival in CKD cats. Published data consistently shows months to years of added survival for stage 2 and 3 cats. The diets are phosphorus-restricted, modestly protein-adjusted, omega-3-enriched, and formulated for palatability because appetite is the recurring challenge. Transition takes 2–4 weeks with gradual mixing. If the cat refuses, a hydrolyzed or alternative renal diet is worth trying before abandoning the intervention.

Hydration

Water fountains, wet food, broth over kibble, multiple water stations in different rooms. For stage 3 and 4 cats, subcutaneous fluid administration at home is a routine and effective supportive intervention. Most owners learn to do it comfortably after one training session.

Phosphorus binders

When dietary phosphorus restriction alone is not enough, oral binders (aluminum hydroxide, lanthanum, calcium-based) lower absorption. The stage at which a binder becomes useful depends on the cat's blood phosphorus level relative to IRIS targets.

Blood pressure management

Amlodipine, commonly used in cats with hypertension. Proteinuria management may add an ACE inhibitor or telmisartan depending on the individual.

Anemia management

Late-stage cats often develop non-regenerative anemia. Erythropoietin analogues (darbepoetin) can help in select cases.

Symptom management

Anti-nausea medications (Cerenia/maropitant), appetite stimulants (mirtazapine, which also exists as a transdermal gel for cats — a meaningful quality-of-life addition), and acid reducers when uremic gastritis is present. A cat whose nausea is controlled is a cat whose appetite holds.

What doesn't help, despite the marketing

• Over-the-counter "kidney support" supplements — not evidence-supported
• Dramatic protein restriction in early CKD — the pendulum has swung; appropriate protein with phosphorus restriction is the current standard
• Homemade diets without formulation by a DACVN — nutritional imbalance can accelerate decline
• Delaying intervention because the cat "still eats" — the window for meaningful intervention is before appetite drops

What senior cat owners should do this month

If your cat is over seven and hasn't had a senior workup in the last twelve months, schedule it. Ask for the SDMA specifically; some clinics don't run it without being asked. Ask for a blood pressure reading. Bring a two-week water-and-food diary. That $200–$400 visit is the single highest-ROI spend in senior cat care.

Where to go next

Pair this with cat pain signs — arthritis frequently accompanies CKD in senior cats and both deserve attention. If your cat's GI symptoms have been chronic, cat vomiting covers related diagnostic territory.

The encouraging version

A stage 2 CKD cat caught at age nine and managed with diet, fluids, and regular monitoring often lives another five or six years. That's not a disease you prevent; it's a disease you manage — and managed CKD is one of the quiet success stories of modern feline medicine.