What is IRIS Staging for CKD Cats? A Caretaker’s Roadmap

When your vet diagnoses Chronic Kidney Disease (CKD), you might hear them say, "Your cat is IRIS Stage 2." This isn't just a random number; it is the global standard for understanding how advanced the disease is and, most importantly, how to treat it.

The International Renal Interest Society (IRIS) created this system to help veterinarians and caretakers speak the same language. It moves the diagnosis from a vague "kidney failure" to a specific, actionable plan.

The Two Step Process

Staging is not just looking at one blood test. It is a two step process:

  1. Staging: Determining the severity of the disease based on blood markers (Creatinine and SDMA).

  2. Sub staging: checking for complications that make the disease worse, specifically Proteinuria (protein loss) and Hypertension (high blood pressure).

Step 1: The Four Stages of CKD

The primary stage is determined by the level of Creatinine and SDMA in the blood. These are waste products that healthy kidneys filter out. As kidneys fail, these numbers rise.

Stage 1: The Invisible Stage

  • Creatinine: < 1.6 mg/dL

  • SDMA: < 18 µg/dL

  • What it means: The kidneys are damaged, but they are still working hard enough to keep blood values normal. You might only know your cat has CKD because of a trend in blood work over years, an abnormal kidney shape on ultrasound, or consistently dilute urine.

  • Symptoms: Usually none. The cat acts 100% healthy.

  • Action: Focus on hydration (wet food) and avoiding toxins (like NSAIDs). No prescription diet is needed yet unless protein in the urine is high.

Stage 2: The Mild Stage (Early Diagnosis)

  • Creatinine: 1.6 to 2.8 mg/dL

  • SDMA: 18 to 25 µg/dL

  • What it means: The kidneys have lost enough function (roughly 66%) that waste is starting to build up. This is the most common time for diagnosis.

  • Symptoms: Subtle. You might notice the water bowl is empty more often (polydipsia) or larger clumps in the litter box. The cat may be slightly picky with food.

  • Action: This is the critical window for intervention. Start a renal therapeutic diet now. Studies show starting early slows progression significantly. Monitor blood pressure and urine protein.

Stage 3: The Moderate Stage (Clinical Signs)

  • Creatinine: 2.9 to 5.0 mg/dL

  • SDMA: 26 to 38 µg/dL

  • What it means: The kidneys are filtering less than 25% of what they should. Toxins are high enough to make the cat feel sick.

  • Symptoms: Obvious. Weight loss, unkempt coat, vomiting white foam, bad breath, and lethargy. The cat may be visibly dehydrated.

  • Action: Aggressive management. Subcutaneous fluids are often started here to flush toxins. Anti nausea meds and phosphorus binders are usually required. Blood pressure meds are common.

Stage 4: The Severe Stage (Crisis)

  • Creatinine: > 5.0 mg/dL

  • SDMA: > 38 µg/dL

  • What it means: The kidneys have failed. The cat is severely intoxicated by their own waste (uremia).

  • Symptoms: The cat is often critically ill, refusing all food, vomiting, and very weak.

  • Action: Focus shifts entirely to Quality of Life. Intensive support with fluids and medications is needed to keep them comfortable. Hospice care is the priority.

Step 2: The Sub Stages (The Complications)

Once the stage is set (e.g., Stage 2), the vet checks for two "modifiers" that change the prognosis.

1. Proteinuria (The UPC Ratio) Healthy kidneys keep protein in the blood. Damaged kidneys let it leak into the urine. This protein is toxic to the kidney tubules it passes through, acting like acid that causes more scarring.

  • Test: Urine Protein to Creatinine Ratio (UPC).

  • Goal: UPC < 0.2 (Non proteinuric).

  • Danger: UPC > 0.4 (Proteinuric).

  • Treatment: If high, meds like Telmisartan or Benazepril are prescribed to stop the leak.

2. Hypertension (Blood Pressure) Kidneys regulate blood pressure. Failing kidneys often cause systemic hypertension, which can cause blindness and strokes.

  • Goal: Systolic BP < 140 mmHg.

  • Risk: BP > 160 mmHg is dangerous. BP > 180 mmHg is a severe risk for retinal detachment.

  • Treatment: Amlodipine is the standard drug to lower pressure and protect the brain and eyes.

Why BUN Is Not Used for Staging

You will see BUN (Blood Urea Nitrogen) on the blood panel, but it is not used for IRIS staging.

  • Why: BUN is influenced by too many outside factors. It can rise if the cat is dehydrated, eats a high protein meal, or has GI bleeding from ulcers.

  • Creatinine is more stable because it comes from muscle breakdown, which happens at a constant rate.

  • SDMA is even more specific because it isn't affected by muscle loss like Creatinine is.

Is Urine Specific Gravity (USG) Used for Staging?

This is a very common point of confusion. The short answer is: No, USG is not used to determine the IRIS Stage number (1 - 4).

However, it is absolutely critical for the diagnosis that happens before staging begins.

Here is how it fits into the puzzle and why it should be mentioned in any conversation about staging.

The Gatekeeper of Diagnosis

You cannot officially diagnose CKD (and thus cannot Stage it) without checking the Urine Specific Gravity (USG) first.

  • The Role of USG: It proves the kidneys are the problem.

  • The Scenario: Imagine a cat has high Creatinine (e.g., 3.5 mg/dL). This could be Stage 3 CKD. But, it could also be a healthy cat who is severely dehydrated from vomiting.

  • The Test:

    • Dehydrated Healthy Cat: The kidneys will scream "save water!" and concentrate the urine. The USG will be high (>1.035). This cat does not have CKD; they have pre-renal azotemia.

    • CKD Cat: The kidneys are damaged and cannot concentrate. The USG will be low (<1.035), usually in the "isosthenuric" range (1.008–1.012). This confirms the high Creatinine is due to kidney failure, not just dehydration.

Summary: USG confirms the diagnosis of CKD. Once CKD is confirmed, Creatinine and SDMA take over to assign the Stage number.

Why USG Isn't Used for the Stage Number

You might wonder, "If low USG means bad kidneys, why don't we use it to rank the severity?"

The reason is that USG is an "all or nothing" marker. The ability to concentrate urine is one of the very first things to break. By the time a cat is in early Stage 2, their USG has usually already bottomed out at 1.008–1.012.

  • A cat in Stage 2 often has a USG of 1.010.

  • A cat in Stage 4 often has a USG of 1.010.

Since the number doesn't change much as the disease gets worse, it isn't useful for ranking severity (Staging). It is only useful for saying "Yes, the kidneys are not working."

Summary for the Caretaker

Knowing your cat is "Stage 2 with Proteinuria" is far more useful than just knowing they have "kidney failure."

  • Stage tells you how advanced the damage is.

  • Sub stage tells you how fast it might progress if untreated.

This system empowers you to ask the right questions: "Is her blood pressure under control?" "Is she losing protein?" "Is it time for fluids?" It turns a scary diagnosis into a managed plan.

Cited Research

  • International Renal Interest Society (IRIS). (2023). IRIS Staging of CKD Guidelines.

  • Sparkes, A. H., et al. (2016). ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease. Journal of Feline Medicine and Surgery, 18(3), 219 239.

  • Syme, H. M., et al. (2006). Survival of cats with naturally occurring chronic renal failure is related to severity of proteinuria. Journal of Veterinary Internal Medicine, 20(3), 528 535.

  • Jepson, R. E. (2011). Feline Systemic Hypertension: Classification and Pathogenesis. Journal of Feline Medicine and Surgery, 13(5), 311 318.