SDMA is the earliest routine blood marker of chronic kidney disease (CKD) in cats. A result above 14 µg/dL, repeated 2 to 4 weeks later, moves a cat into investigation 6 to 18 months before creatinine does. After 10 years, test every 6 months, keep the numbers in one place, and read the slope rather than the single value. Paired with a weekly weight, SDMA turns a vague worry into a dated trend.
What SDMA measures
Symmetric dimethylarginine is a small protein breakdown product that the kidney filters out. Because it comes from protein turnover rather than muscle bulk, it is far less sensitive to muscle mass than creatinine is, which matters in a 12-year-old that has lost 10 percent of its lean tissue and therefore looks better on creatinine than it should. Most references put the rise at roughly 40 percent loss of glomerular filtration rate, while creatinine typically waits for 70 to 75 percent. That gap of 6 to 18 months is the whole reason to run the test.
The 14 line, and the 15 to 17 gray zone
Most laboratories report a reference interval of 0 to 14 µg/dL, and a few use 0 to 18. Intervals differ by analyzer and by lab, so treat 14 as a common ceiling rather than a law; different laboratories vary by 1 to 2 µg/dL on the same sample. A single value of 15 to 17 is not a diagnosis. Repeat it in 2 to 4 weeks, check hydration, and look at urine specific gravity. A persistent result of 18 µg/dL or more with a creatinine still under 1.6 mg/dL (140 µmol/L) is what most vets file as IRIS stage 1, and it earns a urine protein to creatinine ratio plus a blood pressure reading.
Six to eighteen months ahead of creatinine
Creatinine is a late marker because it is buffered by muscle mass and by the kidney's reserve. A cat can lose two thirds of its filtration and still print a creatinine of 1.4 mg/dL, inside the 0.8 to 2.4 mg/dL interval. In that same cat SDMA may already be 19. The practical consequence is simple: if you only measure creatinine once a year, you will meet the disease when it is stage 2, with 3 to 4 kg of body weight already gone. If you measure SDMA every 6 months from age 10, you meet it at stage 1, when the useful work is diet, water, blood pressure, and phosphate, and none of it is urgent yet.
A 6 month panel after 10 years
One blood draw covers it. Ask for the numbers, not just the words normal or borderline, because the trend is the asset.
| Test | Common reference interval | Unit | Recheck every (months) | Signal in the trend |
|---|---|---|---|---|
| SDMA | 0 to 14 | µg/dL | 6 | +2 in 6 months, or 18 or more |
| Creatinine | 0.8 to 2.4 | mg/dL | 6 | +0.3 in 6 months |
| BUN | 15 to 36 | mg/dL | 6 | +10 in 6 months |
| Urine specific gravity | 1.035 to 1.060 | ratio | 6 | under 1.030 on 2 samples |
| Urine protein to creatinine | under 0.2 | ratio | 12 | 0.2 to 0.4, or above 0.4 |
| Phosphorus | 3.1 to 7.5 | mg/dL | 6 | above 4.6 at stage 2 |
Intervals are typical rather than universal: different laboratories run different analyzers and shift the ceiling by 1 to 2 units, so always compare a cat against the interval printed on its own report, not against a chart. If a lab changes, keep 1 old sample result beside the new one.
Read the slope, not the snapshot
One number tells you where the cat is; 2 numbers 6 months apart tell you where it is going. A rise of 2 to 3 µg/dL in 6 months with a stable weight is worth repeating in 8 weeks. A rise of 5 or more in 6 months, or any jump past 18, deserves a urine test and a blood pressure reading within 30 days. A fall of 3 or more after treatment usually means the cat was dehydrated at the first draw, which is common after 12 hours without water before a visit. Write every result with its date and unit in one notebook; 4 results across 2 years is a curve you can act on.
Pair the number with weight and water
SDMA alone is easy to over-read; SDMA plus 5 percent weight loss in 6 months is hard to argue with. Weigh weekly to 10 g, and record water intake for 3 days before each visit. A 4.5 kg cat normally drinks 180 to 270 ml a day, that is 40 to 60 ml per kg. Intake above 100 ml per kg a day, or 3 or more urine clumps a day at double the old size, is a reason to move the appointment forward, not to wait for the next 6 month panel. The intake side is covered in water fountains for senior cats, and the weighing routine in weight monitoring for senior cats.
What a stage 1 result does not mean
It does not mean a renal diet tomorrow. Protein restriction before staging costs muscle in a cat that is already losing it; most vets keep protein at 40 to 45 percent on a dry matter basis until creatinine is above 1.6 mg/dL, then step down. It also does not mean starting phosphate binders with a normal phosphorus: 4.6 mg/dL is the usual trigger at stage 2 to 3. What stage 1 does mean is 3 things done early and cheaply: blood pressure under 160 mmHg, water intake protected, and weight logged. A cat that holds weight for 24 months after a stage 1 result is a common and real outcome, and the reason is that the work started 18 months before the creatinine moved.
Cost and a 12 month plan
- Month 0: SDMA, creatinine, BUN, urine specific gravity, UPC, phosphorus, T4, blood pressure.
- Month 1: start the weekly weight and the 3 day water log.
- Month 6: repeat the panel, compare the slope, not the single value.
- Month 12: repeat, add a retinal and thyroid check, and review the diet with numbers.
- Any time: 5 percent weight loss in 6 months, or drinking above 100 ml per kg, pulls the next panel forward.
A panel plus a urine test and a blood pressure reading usually sits in the range shown on the cost tables, and doing it every 6 months for 2 years still costs less than 1 emergency visit for a blocked or dehydrated cat. The expensive version of this disease is the one found at stage 3, after 6 months of vomiting that everyone called hairballs.
Terms mentioned in this article
Frequently asked
Is an SDMA of 16 always kidney disease?
No. 15 to 17 µg/dL sits in a gray zone and needs a repeat in 2 to 4 weeks, plus a hydration check and a urine specific gravity. A persistent value of 18 or more with a creatinine under 1.6 mg/dL is what most vets call IRIS stage 1. Values move 1 to 2 µg/dL between laboratories, so compare against your own report.
Why test every 6 months if the cat looks fine?
Because creatinine lags by 6 to 18 months. A cat can lose two thirds of its filtration and still read 1.4 mg/dL, inside the 0.8 to 2.4 interval. Two SDMA results 6 months apart show the slope, and a rise of 2 to 3 µg/dL already justifies a repeat in 8 weeks.
Should a stage 1 cat start a kidney diet?
Usually not yet. Renal diets run 26 to 28 percent protein on a dry matter basis, and cutting protein early costs muscle in a cat that is already losing lean mass. Most vets keep 40 to 45 percent until creatinine passes 1.6 mg/dL, and hold phosphate binders until phosphorus is above 4.6 mg/dL.