Two blood tests answer one question: how much filtering capacity is left. Symmetric dimethylarginine (SDMA) rises after roughly 40% of kidney function is gone. Creatinine usually waits until 66% to 75% is gone. That gap is why a 12-year-old cat can eat well, groom normally, and still sit in early chronic kidney disease (CKD) with a creatinine of 1.4 mg/dL. From age 10 the working schedule is a full panel every 6 months, and staging uses the 4 IRIS bands, not one number read in isolation.
What SDMA measures that creatinine cannot
SDMA is released by every nucleated cell when arginine is methylated, and it is cleared almost entirely by glomerular filtration. Muscle mass barely changes it. Creatinine is a breakdown product of muscle creatine, so the same filtration rate reads 0.3 to 0.6 mg/dL lower in a thin, sarcopenic 15-year-old than in a muscular 10-year-old. A cat that has lost 15% of body weight to muscle wasting can hold a creatinine of 1.5 mg/dL while its SDMA sits at 28 µg/dL. Most reference laboratories print SDMA as 0 to 14 µg/dL, while some use 0 to 18 µg/dL. Compare the result with that laboratory's own interval, never with a range from a different analyzer.
The 10-year rule: every 6 months
CKD is common enough after age 10 that twice-yearly sampling beats waiting for symptoms. A practical senior panel is 10 to 14 chemistry analytes plus SDMA, a complete blood count, total T4, and a urinalysis with specific gravity. That is 2 visits per year, 3 to 5 mL of blood at each, and roughly 0.5 mL of serum for the kidney markers alone. Fasting for 8 to 12 hours sharpens glucose and lipid values but changes SDMA very little. Budget roughly 120 to 250 USD for the bloodwork and 40 to 70 USD for the urine, depending on the region. Cats aged 7 to 9 usually need the same panel once a year.
Staging: the 4 IRIS bands
IRIS staging is a 4-step ladder built on creatinine, with SDMA as the earlier signal and urine and blood pressure as modifiers. The table below shows the cut points most laboratories and clinicians apply.
| IRIS stage | Creatinine (mg/dL) | Creatinine (µmol/L) | SDMA (µg/dL) | What else marks it |
|---|---|---|---|---|
| 1 | below 1.6 | below 140 | below 18 | non-azotemic, but urine specific gravity under 1.035, UPC above 0.4, or abnormal imaging |
| 2 | 1.6 to 2.8 | 140 to 250 | 18 to 35 | mild azotemia, often no clinical signs |
| 3 | 2.9 to 5.0 | 251 to 440 | 36 to 54 | weight loss, low appetite, rising phosphate |
| 4 | above 5.0 | above 440 | above 54 | marked azotemia, vomiting, dehydration |
When the two markers disagree, most clinicians record the higher stage and repeat the panel in 2 to 4 weeks rather than arguing with a single result. Stage 1 exists precisely because a normal creatinine does not exclude disease: it needs one supporting finding out of the urine, the protein ratio, or the imaging. The same ladder carries 2 substages. Proteinuria is graded on urine protein-to-creatinine ratio as under 0.2, 0.2 to 0.4, and above 0.4. Blood pressure is graded as under 150, 150 to 159, 160 to 179, and 180 mmHg or higher.
Biological noise versus a real trend
A single SDMA of 16 µg/dL can read 13 µg/dL next month without anything changing in the kidney. Analytical variation plus day-to-day hydration is enough to move a borderline value across the line. Confirm persistence: repeat in 2 to 4 weeks, same laboratory, ideally the same time of day. Dehydration and intravenous fluids shift creatinine more than SDMA, so a sample drawn during a vomiting episode can overstate the stage by 1 band. Hyperthyroidism runs the other way, raising glomerular filtration and pulling creatinine down; a sizeable share of hyperthyroid cats only reveal their kidney disease 3 to 6 months after the thyroid value normalizes. Age, breed, and body condition do not change the cut points, but they change how much you trust a creatinine within 0.2 mg/dL of a boundary.
Reading a trend: what to track and when
Stable stage 1 cats are usually rechecked every 3 to 6 months, stage 2 every 3 months, stage 3 every 4 to 8 weeks, and stage 4 as directed, often every 1 to 2 weeks. A rise in SDMA of 5 µg/dL or more between visits, or a creatinine rise of 0.3 mg/dL inside 12 months, is a progression signal worth acting on. Weigh the cat monthly at home: a 5% loss in 6 months is easy to miss on a scale you only see twice a year. Ask for urine specific gravity every visit; a value under 1.035 in an azotemic cat points to a renal cause, while a value of 1.045 or higher points to a prerenal one. Owners who track water intake will notice a jump from 180 to 300 mL per day long before the creatinine moves.
Five things to do before the next visit
- Book the panel at the 6-month mark from age 10, and write the date on the calendar rather than relying on a reminder call.
- Ask for SDMA and creatinine on the same tube, from the same laboratory, so the 2 numbers remain comparable.
- Bring a 3-day water and litter log; intake above 45 mL/kg/day is worth mentioning.
- Request a urine sample with specific gravity and protein ratio whenever the bloodwork is drawn, ideally by cystocentesis.
- Have blood pressure measured at each of those 2 yearly visits, since 160 mmHg or higher changes the plan.
Call the clinic sooner, not later, for any of these: vomiting more than 2 days, refusing food for 24 hours, urine output under 1 mL/kg/hour, sudden blindness, or a seizure. Those are emergency signs, not staging questions. For how the rest of the panel fits together, see the notes on white cell counts and on feline vision, or use the breed quiz to see which lines carry known kidney conditions.
Terms mentioned in this article
Frequently asked
Can a cat have kidney disease with a normal creatinine?
Yes. Creatinine only leaves the reference interval after roughly 66% to 75% of filtering capacity is gone, so a cat can sit at 1.4 mg/dL while its SDMA reads 28 µg/dL. That combination is IRIS stage 1, and it needs 1 supporting finding from the urine specific gravity, the protein ratio, or the imaging before anyone calls it a diagnosis rather than a suspicion.
How often should an older cat be tested?
From age 10 the usual interval is every 6 months, which means 2 panels per year with SDMA, creatinine, a complete blood count, total T4 and a urinalysis. Cats aged 7 to 9 usually need the same panel once a year, and any cat already staged needs 4 to 8 weeks between checks at stage 3.
Does a high SDMA alone prove chronic kidney disease?
No. A single value 4 to 6 µg/dL above the interval can reflect hydration, analytical variation, or a transient problem, so most clinicians repeat it in 2 to 4 weeks at the same laboratory. Persistent elevation with a urine specific gravity under 1.035 is a much stronger case than one number on one morning.