A diagnosis of chronic kidney disease can make a pet owner feel as if a pile of unfamiliar numbers has suddenly taken over the conversation. Creatinine, SDMA, urine protein, blood pressure, Stage 2, Stage 3, and sometimes a string of abbreviations can make a veterinary visit feel much harder to absorb than it should. The IRIS staging system gives those numbers a structure, but the stage is not a verdict and it is not a countdown clock.
IRIS stands for the International Renal Interest Society. Its staging system is designed for dogs and cats that have already been diagnosed with chronic kidney disease, or CKD, and whose kidney values are stable enough to classify. The current 2026 IRIS guidance uses blood creatinine and SDMA to assign a stage from 1 through 4, then adds two separate substages based on urine protein and systolic blood pressure.
Staging Comes After the Diagnosis
One of the easiest misunderstandings is to treat an IRIS stage as the test that diagnoses kidney disease. It is not. A veterinarian first has to decide that the pet truly has chronic kidney disease, using the history, physical examination, bloodwork, urinalysis, imaging when appropriate, and evidence that the abnormalities are persistent rather than temporary.
That distinction matters because dehydration, urinary obstruction, acute kidney injury, and other problems can raise kidney-related blood values without representing stable CKD. IRIS recommends staging a hydrated, stable patient, with creatinine and/or SDMA assessed on at least two occasions. In many cases, repeating testing after a few weeks helps show whether the values are stable enough for staging rather than changing because of a short-term problem.
For pet owners, that means a single elevated creatinine result does not always translate neatly into a permanent stage. The veterinarian may want repeat bloodwork, a urine sample, blood pressure, imaging, or other testing before attaching an IRIS label. That extra work is not indecision. It is part of separating chronic kidney disease from conditions that can temporarily alter kidney values.
What Creatinine and SDMA Are Telling Us
Creatinine is a waste product measured in the blood. Healthy kidneys normally remove it, so a rising creatinine concentration can be a sign that kidney filtration has declined. The limitation is that creatinine is affected by muscle mass, which can make interpretation harder in thin, elderly, or muscle-wasted animals.
SDMA, short for symmetric dimethylarginine, is another blood marker used as an estimate of kidney filtration. It is generally less affected by loss of lean body mass and may rise earlier than creatinine in some dogs and cats. IRIS now recommends considering both markers when available because they can give a fuller picture together than either number alone.
The two markers do not always agree. A thin senior cat, for example, may have a creatinine value that looks less concerning than the SDMA because reduced muscle mass can lower creatinine production. IRIS advises veterinarians to interpret disagreements in the context of the individual animal and, when uncertainty persists, to repeat testing and use the more advanced stage when appropriate so that relevant monitoring and treatment are not overlooked.
IRIS Stage 1: Kidney Disease Can Exist Before Creatinine Is High
Stage 1 is often the hardest stage for owners to understand because the blood creatinine may still be within the laboratory reference range. Under the 2026 IRIS system, Stage 1 creatinine is below 1.4 mg/dL in dogs and below 1.6 mg/dL in cats. SDMA is below 18 micrograms/dL, although a persistent SDMA above the usual reference limit of 14 can support a diagnosis of early CKD, and values of 15 to 17 fall into Stage 1 when CKD has been established.
A pet is not placed in Stage 1 simply because creatinine is low. There needs to be other evidence of kidney disease, such as persistent renal protein loss, abnormal kidney structure on imaging, a consistent upward trend in creatinine or SDMA, or loss of normal urine-concentrating ability when other causes have been excluded. In cats, inadequate urine concentration can be especially useful as an early clue, while interpretation in dogs is less straightforward.
Many Stage 1 pets feel completely normal. That can make the diagnosis seem almost abstract, yet this is exactly why routine senior bloodwork, urinalysis, and trend tracking can be so useful. The goal at this point is usually to understand the cause if possible, identify complications early, and watch the direction of the numbers over time rather than waiting for obvious illness.
IRIS Stage 2: Mild Loss of Kidney Function, Often With Few Signs
Stage 2 represents a clearer reduction in kidney filtration, but many pets still have mild signs or no obvious signs at all. In dogs, Stage 2 creatinine is 1.4 to 2.8 mg/dL, while in cats it is 1.6 to 2.8 mg/dL. The Stage 2 SDMA range is 18 to 35 micrograms/dL in dogs and 18 to 25 in cats.
This is often where owners first notice increased thirst or larger urine clumps in the litter box, although those changes can occur for many other reasons and should never be used to diagnose CKD at home. Some animals remain bright, active, and interested in food even while their laboratory results clearly place them in Stage 2. Merck Veterinary Manual similarly notes that clinical signs are often not yet evident at this stage.
Stage 2 is also where the value of trends becomes very clear. A cat that has stayed near the same creatinine and SDMA values for a long period may be in a very different situation from a cat whose values have been climbing steadily over several checks, even if both are technically in the same stage. The stage provides a category, while the rate of change helps the veterinarian understand the behavior of the disease.
IRIS Stage 3: Moderate Azotemia and a Wider Range of Clinical Signs
Stage 3 is defined by creatinine from 2.9 to 5.0 mg/dL in both dogs and cats. SDMA ranges from 36 to 54 micrograms/dL in dogs and from 26 to 38 in cats. IRIS describes this stage as moderate renal azotemia, meaning that nitrogen-containing waste products are accumulating in the blood because the kidneys are no longer clearing them as efficiently.
The day-to-day picture becomes more variable in Stage 3. Some pets may still have relatively few outward signs, while others may have decreased appetite, weight loss, nausea, vomiting, dehydration, reduced energy, or more pronounced changes in drinking and urination. IRIS even recognizes the practical difference between an early Stage 3 animal with few systemic signs and a later Stage 3 animal with more marked illness.
This is one reason the stage should never replace observation. In our own years caring for cats with kidney disease, the lab category helped organize the medical discussion, but the daily record of eating, drinking, body weight, litter box habits, activity, and medication response often supplied the context that numbers alone could not. A veterinarian can do much more with a clear trend than with a vague report that a pet has “seemed off lately.”
IRIS Stage 4: Severe Loss of Kidney Function and Greater Risk of Uremic Illness
Stage 4 is the most advanced IRIS category. Creatinine is above 5.0 mg/dL in both dogs and cats, while SDMA is above 54 micrograms/dL in dogs and above 38 in cats. IRIS associates this stage with an increasing risk of systemic illness and uremic crises, which occur when retained waste products and related metabolic disturbances make the animal significantly ill.
Pets in Stage 4 may have poor appetite, weight loss, vomiting, dehydration, weakness, diarrhea, or other signs related to advanced kidney dysfunction, although the exact picture varies. A Stage 4 designation does not tell an owner how many days or months a pet has, and it should not be interpreted as a fixed life-expectancy label. Quality of life, response to supportive care, other diseases, hydration, nutritional status, blood pressure, anemia, phosphorus balance, and the pace of change all matter to the clinical picture.
Repeated vomiting, inability to keep water down, refusal of food, severe weakness, collapse, sudden blindness, seizures, or a marked change in responsiveness deserves prompt veterinary attention. Those signs can occur with advanced kidney disease, severe hypertension, dehydration, or other serious problems, and an article cannot determine the cause from home observations alone.
The Stage Number Is Only Part of the IRIS Classification
After assigning Stage 1 through 4, IRIS recommends substaging CKD according to protein loss in the urine and systolic blood pressure. These are not minor footnotes. A Stage 2 cat with significant proteinuria or hypertension may need a different management plan from another Stage 2 cat without those findings.
Proteinuria means excess protein is being lost into the urine. IRIS uses the urine protein-to-creatinine ratio, often written UP/C or UPC, after other causes of protein in the urine have been considered. Dogs are considered nonproteinuric below 0.2, borderline proteinuric from 0.2 to 0.5, and proteinuric above 0.5. Cats are nonproteinuric below 0.2, borderline from 0.2 to 0.4, and proteinuric above 0.4. IRIS recommends confirming the pattern with repeat urine testing rather than treating one isolated result as the whole story.
Blood pressure receives its own substage because kidney disease and hypertension can affect each other, and high blood pressure can injure organs such as the eyes, brain, heart, and kidneys. For most dogs and all cats, IRIS classifies systolic pressure below 140 mmHg as normotensive, 140 to 159 as prehypertensive, 160 to 179 as hypertensive, and 180 or higher as severely hypertensive. Multiple readings are preferred because stress and measurement conditions can influence blood pressure, although evidence of target-organ injury can make immediate action necessary.
A Stage Can Change, and Stability Matters
An IRIS stage is not permanently stamped onto a pet’s medical record. Kidney disease can progress, values can stabilize, and the assigned stage or substages can be revised as new information appears. IRIS specifically recommends reclassification when creatinine, SDMA, urine protein, or blood pressure changes enough to alter the category.
Some older cats remain in the same stage for long periods with little progression, while others move through stages more quickly. That difference is one reason it is risky to compare one pet’s Stage 2 or Stage 3 diagnosis with another animal’s experience on a forum or social media group. The same stage can include animals with different underlying causes, different complications, different body condition, and very different rates of progression.
What to Track Between Veterinary Visits
For a pet owner, the most useful role is not to calculate the stage independently but to help build a reliable history between appointments. Record body weight when you can, appetite changes, water intake if it is practical to measure, vomiting or nausea-like behavior, stool changes, urination patterns, accidents outside the litter box or house, energy, mobility, and any difficulty taking prescribed food or medication. A simple dated notebook or spreadsheet can make patterns easier to see.
Keep copies of bloodwork and urinalysis results so you can compare creatinine, SDMA, phosphorus, potassium, hematocrit, urine specific gravity, UPC, and blood pressure over time. Ask your veterinarian which values matter most for your particular dog or cat, how often they want rechecks, and whether they are following IRIS treatment recommendations for the current stage and substages. Those conversations are far more useful than concentrating on one number in isolation.
CKD staging works best as a shared language between the veterinary team and the person living with the animal every day. The veterinarian has the training to interpret the laboratory data and choose medical care, while the owner sees the changes that happen at home between appointments. Used together, the IRIS stage, the substages, the laboratory trends, and the pet’s actual day-to-day condition give a much clearer picture of what kidney disease means for that individual dog or cat.