The International Renal Interest Society (IRIS) staging system is the global standard for classifying chronic kidney disease in cats and dogs. For feline practitioners, understanding how to apply IRIS staging — and recognizing its limitations — is essential for making informed treatment decisions and communicating prognosis to owners.
This guide provides a practical overview of the IRIS staging system as it applies to cats, including the 2023 updated guidelines.
Overview of IRIS Staging
IRIS staging is a two-step process:
- Stage the CKD based on fasting blood creatinine and/or SDMA
- Substage based on proteinuria (UPC) and blood pressure
Staging should only be performed on stable, well-hydrated patients. Acute kidney injury, dehydration, and post-renal obstruction must be ruled out or resolved before applying IRIS criteria.
IRIS Stages for Cats
Stage 1 — Nonazotemic
| Marker | Value |
|---|---|
| Creatinine | < 1.6 mg/dL |
| SDMA | < 18 µg/dL |
CKD is suspected based on other findings: persistent low USG, abnormal renal imaging, proteinuria, or rising trends in renal markers. This is the most challenging stage to diagnose because bloodwork may appear entirely normal.
Stage 2 — Mild Renal Azotemia
| Marker | Value |
|---|---|
| Creatinine | 1.6–2.8 mg/dL |
| SDMA | 18–25 µg/dL |
Cats may still appear clinically normal. Mild polyuria/polydipsia may be present. This is the stage where early intervention has the greatest impact on long-term outcomes.
Stage 3 — Moderate Renal Azotemia
| Marker | Value |
|---|---|
| Creatinine | 2.9–5.0 mg/dL |
| SDMA | 26–38 µg/dL |
Clinical signs are often apparent: weight loss, decreased appetite, vomiting, dehydration. Active management is required to slow progression and maintain quality of life.
Stage 4 — Severe Renal Azotemia
| Marker | Value |
|---|---|
| Creatinine | > 5.0 mg/dL |
| SDMA | > 38 µg/dL |
End-stage disease with significant clinical signs. Treatment is palliative, focused on comfort and quality of life. Prognosis is guarded to poor.
Substaging: Proteinuria
Proteinuria is assessed via the urine protein-to-creatinine ratio (UPC) and is an independent risk factor for CKD progression.
| UPC | Classification |
|---|---|
| < 0.2 | Non-proteinuric |
| 0.2–0.4 | Borderline proteinuric |
| > 0.4 | Proteinuric |
Persistent proteinuria (confirmed on at least two samples, 2 or more weeks apart) warrants investigation and potentially treatment with ACE inhibitors or ARBs.
Substaging: Blood Pressure
Hypertension is common in cats with CKD and contributes to target organ damage affecting the eyes, brain, heart, and kidneys.
| Systolic BP (mmHg) | Risk Category |
|---|---|
| < 140 | Normotensive |
| 140–159 | Prehypertensive |
| 160–179 | Hypertensive |
| ≥ 180 | Severely hypertensive |
Blood pressure should be measured using a standardized protocol in a calm environment. Treatment is typically initiated at systolic pressures consistently above 160 mmHg, or lower if target organ damage is present.
The Role of SDMA
Symmetric dimethylarginine (SDMA) was added to the IRIS guidelines as a complementary marker to creatinine. Its key advantages include:
- Earlier detection — SDMA may increase with as little as a 25 percent reduction in GFR, compared to 75 percent for creatinine
- Less affected by muscle mass — Creatinine is influenced by lean body mass, meaning cachectic or geriatric cats may have falsely low creatinine values that mask kidney disease. SDMA is not affected by muscle mass
- Confirmatory role — When creatinine and SDMA stage differently, the IRIS guidelines recommend staging based on the higher of the two markers
However, SDMA is not perfect. It can be affected by individual variation, and some healthy cats have SDMA values at the upper end of the reference range. As with all markers, trending over time is more valuable than any single measurement.
Practical Application
When to Stage
- Any cat over 7 with abnormal renal markers on wellness screening
- Any cat with clinical signs suggestive of CKD (PU/PD, weight loss, poor appetite)
- After resolution of an acute kidney injury episode
- During routine monitoring of known CKD patients
Staging Frequency
- Stage 1–2: Recheck every 3–6 months
- Stage 3: Recheck every 2–3 months
- Stage 4: Recheck every 1–2 months or as clinically indicated
Key Management Points by Stage
- Stage 1: Monitor, dietary discussion, baseline blood pressure and UPC
- Stage 2: Renal diet introduction, phosphorus management, blood pressure control
- Stage 3: Active fluid support, appetite stimulants, anti-nausea medications, phosphorus binders
- Stage 4: Quality of life focus, intensive supportive care, end-of-life planning discussions
Limitations of IRIS Staging
IRIS staging provides a useful framework, but it has limitations:
- It relies on stable, fasted samples — any deviation introduces error
- Stage 1 is inherently difficult to diagnose without imaging or urinalysis
- Staging does not account for the rate of progression, which is arguably the most important prognostic factor
- Individual variation in creatinine and SDMA means borderline values require clinical judgment
The most effective approach combines IRIS staging with serial monitoring, USG trending, and clinical assessment. No single number tells the full story.
Key Takeaways
- IRIS staging uses creatinine and SDMA to classify CKD severity in four stages
- Substaging with UPC and blood pressure adds prognostic and therapeutic information
- SDMA detects kidney disease earlier than creatinine, especially in cats with low muscle mass
- Stage 1 and early Stage 2 offer the best window for intervention
- Serial monitoring and trending are more valuable than any single staging assessment