Chronic kidney disease in cats is staged with 4 stages, and the stage comes from 2 values: serum creatinine and symmetric dimethylarginine, or SDMA. Stage is a snapshot of where the kidney is now, not a prediction of when it will fail, and it should only be assigned from 2 samples taken at least 2 weeks apart in a hydrated cat. A stable cat is rechecked every 3 months, because the plan changes on trends rather than on any single result. The stage tells you what to monitor; the trend tells you when to act.
The 4 stages, with both indicators
Cutoffs below follow the IRIS creatinine and SDMA ranges for cats, with creatinine in both units because laboratories in China report either one. A cat is staged by the higher of the 2 indicators when they disagree, which is why SDMA catches some cats a stage earlier.
| Stage | Creatinine, mg/dL | Creatinine, µmol/L | SDMA, µg/dL | Recheck interval |
|---|---|---|---|---|
| 1 | Below 1.6 | Below 140 | Below 18 | 3 months |
| 2 | 1.6 to 2.8 | 140 to 250 | 18 to 35 | 3 months |
| 3 | 2.9 to 5.0 | 251 to 440 | 36 to 54 | 2 to 3 months |
| 4 | Above 5.0 | Above 440 | Above 54 | 1 to 2 months |
Stage 1 is a diagnosis, not a number
A cat in stage 1 has a creatinine under 1.6 mg/dL and an SDMA under 18, so the blood values look normal and the stage is made from something else: a urine specific gravity under 1.035 despite good hydration, protein in the urine, or an abnormal kidney shape on ultrasound. This is the stage most often missed, because a 10-year-old cat with a creatinine of 1.4 mg/dL gets told it is fine. Ask for a urine specific gravity on every senior panel; it costs a few yuan and it is the earliest cheap signal available.
Why 2 indicators beat 1
Creatinine depends on muscle mass, and an 11-year-old cat that has lost 15% of its muscle in 18 months will read low, sometimes by 0.4 mg/dL, which can hide a full stage of disease. SDMA is far less muscle-dependent, rises with roughly 40% loss of kidney function, and often moves 3 to 6 months before creatinine does. The trade-off is that SDMA is less available and can rise with dehydration, so it is used alongside creatinine, never instead of it. When the 2 disagree, stage on the higher value and repeat in 2 to 4 weeks before changing the plan.
Two substages that change the plan
Proteinuria is substaged by urine protein-to-creatinine ratio: under 0.2 is non-proteinuric, 0.2 to 0.4 is borderline, and above 0.4 is proteinuric. Blood pressure is substaged as under 150 mmHg normal, 150 to 159 borderline, 160 to 179 hypertensive, and 180 mmHg or above severely hypertensive. Repeat a UPC on 2 samples before treating, because a single free-catch sample can read 0.2 high from stress or a full bladder alone. Both substages matter more than the stage number in some cats, because a stage 2 cat with a UPC of 0.8 and a systolic of 170 needs a different plan from a stage 2 cat with a UPC of 0.1 and a systolic of 130.
What the 3-month recheck contains
Six items, and skipping any one of them makes the visit half-useful: body weight and body condition score, blood creatinine and SDMA, phosphorus and potassium, urine specific gravity, UPC on a free-catch sample, and systolic blood pressure measured 3 to 5 times and averaged. Weight is the cheapest trend in the file, so weigh monthly at home on a baby scale and bring the log; a loss of 5% in 3 months on a stage 2 cat often precedes a rise in creatinine by a full quarter. For context on how fast weight should move, see the weight trend chart.
What changes at each stage
Stage 1 needs no diet change, only monitoring and blood pressure every 3 months. Stage 2 usually brings a phosphorus target under 0.5% to 0.6% on a dry matter basis, or roughly 250 mg per 100 kcal, plus a switch toward wet food to raise water intake. Stage 3 adds potassium monitoring, because low potassium causes the hind-leg weakness that owners report as arthritis, and often an appetite stimulant under veterinary direction. Feeding to a fixed gram target instead of free choice makes the weight trend readable, since intake and weight move together within 2 weeks. Stage 4 needs a feeding plan built around 45 to 60 kcal/kg, anti-nausea support, and a discussion about fluids, all inside a schedule set by the clinic rather than by the owner.
Numbers that mean call the clinic today
A creatinine jump of 0.5 mg/dL or more within 2 weeks, a potassium under 3.0 mmol/L, a systolic pressure of 180 mmHg or more, a UPC above 0.4 with a rising trend, or 48 hours of not eating are all reasons for a same-day call, because acute-on-chronic kidney injury is treatable when caught in the first 48 to 72 hours and much less so at day 7. The same urgency applies to a cat that stops producing urine for 12 hours, which is an emergency rather than a monitoring question. Read any new food or supplement against the toxicity list before it enters the bowl, and put the recheck schedule into the CKD follow-up plan so the 3-month cycle does not slip.
4 things to set up this month
1. Ask for creatinine, SDMA, phosphorus, potassium, urine specific gravity, UPC, and blood pressure in one visit, and keep a copy of the numbers. 2. Weigh weekly on a baby scale and write it down; a 100 g change is meaningful at 4 kg. 3. Photograph the water bowl level each morning at the same time for 5 days. 4. Book the next appointment at 3 months before leaving the clinic, and set 2 reminders at 2 weeks and 3 days before it.
Terms mentioned in this article
Frequently asked
My cat is stage 2 but still eats well. Why recheck every 3 months?
Because the plan is driven by trends, not by the stage label. A 5% weight loss in 3 months, a phosphorus rise of 0.3 mg/dL, a UPC crossing 0.4, or a systolic crossing 160 all change what you do, and each is visible 3 to 6 months before a cat stops eating. Rechecking at 3 months also keeps the 2 measurements needed to confirm any change from being taken 9 months apart.
Is SDMA better than creatinine?
It is earlier, not better. SDMA rises at roughly 40% loss of kidney function and is far less affected by muscle mass, so it often moves 3 to 6 months before creatinine, which matters in an older cat that has lost muscle. It is also less available and rises with dehydration, so the rule is to stage on whichever value is higher and repeat both in 2 to 4 weeks.
What does stage 1 actually mean if the blood is normal?
It means kidney disease is present without azotemia, proven by a urine specific gravity under 1.035 in a well-hydrated cat, protein in the urine, or an abnormal kidney on imaging. That cat needs monitoring and blood pressure checks every 3 months, not a kidney diet, and it is the stage where a cheap urine test earns the most.