Renal values in cats: what each line on the report actually measures

Seven lines that answer seven different questions

When a vet says the kidney values are up, that covers several measurements which do not move at the same time or for the same reasons. Knowing which question each one answers is what stops a report being read as a single verdict.

Why creatinine is late and SDMA is early

The kidney has far more filtering capacity than it needs. When units are lost the rest take up the work, so blood levels of the substances the kidney clears stay inside the printed range for a long time. Creatinine only climbs clearly once a large share of capacity has gone, conventionally put at around three quarters. That is the first reason a normal creatinine does not clear the kidney.

The second is muscle. Creatinine comes from muscle, so a cat that has lost condition produces less of it. In the exact population where chronic kidney disease is common, older and thinner cats, creatinine therefore understates the loss of function. SDMA was brought onto panels for that reason: it is cleared by the kidney as well, but it comes from normal protein turnover in every cell rather than from muscle, so it is much less affected by a cat getting thinner, and it tends to cross out of the range earlier.

Neither is a cause. Both say filtration was lower at the moment the blood was drawn, and a single sample cannot tell a lasting reduction from a temporary one. Dehydration, a recent anaesthetic, an infection, a blockage downstream and long standing damage can all produce the same raised line. The sample repeated later, on a hydrated and stable cat, is what separates them.

Why urine is read at the same time

The concentrating job and the filtering job are two different functions of the same organ, and urine is where the first one is visible. A cat that is still concentrating its urine well is doing something a badly damaged kidney cannot do, so the same creatinine means a different thing depending on what the urine shows. Practically, this is why a vet asks for a urine sample, and why it has to be collected before any fluids are given: a drip rewrites the specific gravity within hours and that information cannot be recovered afterwards.

The urine protein:creatinine ratio comes from the same sample. Protein leaking into the urine is measured as a ratio against urinary creatinine so that the figure is not distorted by how dilute the sample is. It is one of the lines that most often changes how closely a cat is followed.

Phosphorus and potassium

Phosphorus is normally cleared by the kidney, so it tends to rise as filtration falls, usually later than creatinine. It is watched closely because it is one of the values that most often changes what a vet proposes next. What is proposed is their decision, made for your cat, and nothing on this page substitutes for it.

Potassium goes both ways. Cats with long standing kidney disease often run low, which shows as weakness, a neck that is carried lower than usual and poor appetite. In a blocked or barely producing kidney it can instead run dangerously high, which affects the heart. A potassium outside the range in either direction is always read in context, never on its own.

What IRIS staging is, and what it is not

IRIS is an international group that published a common scale for chronic kidney disease, so that two vets describing the same cat describe it the same way. The scale has four stages based on blood creatinine, with SDMA used alongside it, and each stage is then sub-staged on two further axes: how much protein is leaking into the urine, and blood pressure.

Two conditions are built into it and they are the whole point. The animal must be stable and properly hydrated, and the value must be confirmed on at least two occasions, because a single reading taken on a dehydrated cat does not stage anything. Staging exists to set the rhythm of monitoring and to make the conversation precise. It is assigned by your vet, with numbers from your laboratory, and this page does not assign one.

Questions to ask your vet

1. Which of these values are outside the range printed by your laboratory, and by how much?
2. Was my cat hydrated when the sample was taken, and could dehydration explain part of it?
3. Was urine specific gravity measured on a sample collected before any fluids?
4. Do creatinine and SDMA agree, and could lost muscle be flattering the creatinine?
5. What is the urine protein:creatinine ratio, and has blood pressure been measured?
6. Are you able to place this on the IRIS scale yet, or does that need a second confirmed sample?
7. When would you want to repeat the panel, and at the same laboratory?
8. What should make me ring you before that recheck?

What are normal kidney values for cats?

There is no single set of normal figures, and any chart that offers one is misleading you. Each laboratory prints its own reference range next to each result, because the range follows the analyser and the method used. Units differ too: in the UK creatinine is usually reported in micromoles per litre, while other countries use milligrams per decilitre, so a figure copied from a foreign site may be off by a factor of ten or more. Read your value against the range on your own report and nothing else.

What does high SDMA mean in cats?

It means kidney filtration has fallen at the moment the sample was taken. SDMA is cleared almost entirely by the kidney and, unlike creatinine, is barely affected by how much muscle the cat carries, so it tends to leave the range earlier and is more honest in a thin older cat. One high SDMA does not say why, since when, or whether it will last. Dehydration on the day can do it. That is why a second sample on a properly hydrated cat is usually what a vet asks for.

Why does creatinine rise so late?

Because the relationship between filtration and concentration is not a straight line. The remaining filtering units take over the work, so the blood level stays inside the range while capacity is being lost, then climbs relatively quickly once roughly three quarters of function has gone. Muscle adds a second bias: a cat that has lost weight makes less creatinine, so its value can look reassuring while filtration is already reduced. Those two facts together are why creatinine is never read alone.

What is the difference between kidney disease and kidney failure in cats?

They are loosely used terms rather than two separate findings. Kidney disease describes damage to the kidney, which can be present long before any value leaves the range. Kidney failure is the older wording for the point at which the remaining function can no longer keep waste products down, which is when creatinine and urea rise. Vets now tend to use chronic kidney disease with an IRIS stage attached, because that says something specific rather than relying on a word. Which words apply to your cat is a question for your vet.

How do you lower SDMA in a cat?

SDMA is a reading, not a target to push down at home, and there is nothing sold over the counter that legitimately claims to move it. What changes the number is what is happening to filtration: hydration on the day of the sample, any treatable cause behind the rise, and whatever your vet decides to do about it. Diagnosis and treatment in the United Kingdom are reserved to veterinary surgeons registered with the RCVS under the Veterinary Surgeons Act 1966, so the honest answer to this question is that it belongs in the consulting room.