How to read kidney values on a cat blood test — creatinine, BUN, SDMA, phosphorus
You pick up the lab report and it is a wall of numbers. At the clinic you heard "the kidney values are a little high," and by the time you get home the paper means nothing to you.
This article is here to make that paper readable on your own. For the kidneys, four numbers carry most of the meaning: creatinine, SDMA, BUN and phosphorus.
The four values to look for first
| Value | On the report | What it tells you |
|---|---|---|
| Creatinine | CRE, CREA, Creatinine | Waste the kidneys filter out — the anchor for staging |
| SDMA | SDMA | Less affected by muscle mass — signals earlier |
| BUN | BUN, Urea | Protein waste — also moves with dehydration and diet |
| Phosphorus | P, PHOS, Phosphorus | The number you actively manage to protect the kidneys |
Your report may not have all four. SDMA is often an add-on test, while phosphorus is included in most standard chemistry panels.
1. Creatinine (CRE) — the anchor, but it moves
Creatinine is a waste product made in muscle and cleared by the kidneys. The less the kidneys filter, the more builds up in the blood, which is why the International Renal Interest Society (IRIS) uses it as the primary staging value.
| IRIS reference stage | Creatinine (mg/dL) | SDMA (μg/dL) |
|---|---|---|
| Stage 1 | < 1.6 | < 18 |
| Stage 2 | 1.6 – 2.8 | 18 – 25 |
| Stage 3 | 2.9 – 5.0 | 26 – 38 |
| Stage 4 | > 5.0 | > 38 |
Why a thin senior cat can look "better" than they are
Creatinine comes from muscle. So when muscle mass drops, the number drops with it. An older cat who has lost condition can show a lower value even though kidney function has not improved.
"Creatinine hasn't gone up since last year" can feel reassuring when what actually happened is muscle loss. This is why IRIS recommends reading SDMA alongside creatinine in cats with visible muscle wasting.
2. SDMA — it raises its hand first
SDMA (symmetric dimethylarginine) is a relatively recent addition to lab panels. It has two advantages.
- Less affected by muscle mass — more reliable than creatinine in thin or senior cats
- Rises earlier — creatinine only moves once a large share of kidney function is gone, while SDMA responds sooner
That is why you can see normal creatinine with an elevated SDMA. It is not necessarily a lab error; SDMA may simply have signalled first. One elevated result is not a conclusion — it is repeated a few weeks later to see whether it persists.
3. BUN — it says nothing on its own
BUN (blood urea nitrogen) is a waste product of protein breakdown. It rises with kidney disease, but it also moves for reasons that have nothing to do with the kidneys.
- Dehydration — concentrated blood raises it
- High-protein diet — more protein in, more waste out
- Gastrointestinal bleeding — blood absorbed from the gut raises it
- Conversely, a low-protein diet or reduced liver function — pushes it lower
For this reason IRIS does not use BUN for staging. Creatinine and SDMA set the stage; BUN adds context beside them.
If BUN has jumped, it is worth asking "has water intake dropped?", "did we change food?", "has there been vomiting?" before assuming the kidneys have worsened.
4. Phosphorus — the one number with a target
The first three values report a state. Phosphorus is the one you can actively lower. High phosphorus wears down the remaining kidney faster, which makes it the most practical target in kidney care.
What matters is that the target changes with the stage. A value inside the lab's normal range may still be too high for a cat in a later stage.
| IRIS stage | Blood phosphorus target (mg/dL) |
|---|---|
| Stage 1 – 2 | 2.7 – 4.5 |
| Stage 3 | 2.7 – 5.0 |
| Stage 4 | 2.7 – 6.0 |
Lower is not automatically better. Dropping below the floor (2.7) causes its own problems, so if a phosphate binder is in use the value should be rechecked.
The order of treatment is ① phosphorus-restricted diet, then ② a binder. The binder is added when diet alone does not reach the target.
5. How these numbers become a "stage"
IRIS staging combines these values. When creatinine and SDMA point to different stages, the conservative approach is to go with the worse of the two.
Two further measurements refine the stage:
- Proteinuria (UPC) — how much protein leaks into the urine (under 0.2 / 0.2–0.4 / over 0.4)
- Systolic blood pressure — under 140 / 140–159 / 160–179 / 180 and above
6. Why every lab has a different "normal range"
If the range printed on your report does not match a table you found online, neither one is necessarily wrong. Reference ranges differ from lab to lab.
- Different methods — enzymatic and Jaffe assays give different creatinine results
- Different reference populations — each lab builds its range from its own healthy cats
- Different units — some labs report creatinine in μmol/L (mg/dL × 88.4 ≈ μmol/L)
So when you judge a value, use the range printed on that report, not a table from the internet. For the same reason, numbers can look slightly different after changing clinics — when following a trend, compare results from the same lab where you can.
7. What to ask at the next visit
Once you can read the report, the next appointment gets much easier. These four questions are enough.
- "What stage would you put them at right now — based on creatinine, or SDMA?"
- "Is the phosphorus inside the target for that stage?"
- "Could we add SDMA and UPC to the next panel?"
- "When should we check blood pressure?"