Your cat's bloodwork, translated

What you're looking at

Find the printout your vet gave you. It'll have a column of names, a column of numbers, and a reference range beside each one so you can see what's high and what's low.

Here's what each of them is actually telling you, in the order I'd read them.

Two things worth knowing before you start. Numbers move around, and one bad result on one day isn't a verdict. And the trend across several visits matters far more than any single value, which is why your vet keeps asking you to come back rather than just reading you today's sheet.

Creatinine

A waste product that comes from normal muscle activity. Healthy kidneys filter it out. As kidney tissue is lost, it builds up in the blood.

This is the number your cat's IRIS stage is based on, so it's the one that gets quoted most.

Two things skew it, and both matter. A dehydrated cat's creatinine reads higher than her true kidney function, which is why the number often improves after a few days of fluids without anything having actually changed. And a very thin cat with little muscle can have a deceptively low creatinine even with poor kidneys. If she's lost a lot of weight, her creatinine may be flattering her.

SDMA

Another waste product, and a more sensitive one. It starts rising earlier in the disease than creatinine does, sometimes a year or more earlier.

SDMA is usually the value that catches kidney disease in the first place, often on a routine senior panel in a cat who seemed perfectly well. If that's how you got here, that's genuinely good news even though it doesn't feel like it. Earlier is better.

BUN

Blood urea nitrogen, another waste product. It's on every panel and it's the least useful of the three for judging kidney function on its own.

The reason is that BUN is easily shifted by things that have nothing to do with kidneys. A high-protein meal raises it. Dehydration raises it. Bleeding into the stomach or intestines raises it, sometimes a lot. Your vet looks at it alongside creatinine rather than on its own.

Phosphorus

This is the one I work hardest to control, and if you only take one value away from this page, take this one.

Failing kidneys can't dump phosphorus properly, so it accumulates. High phosphorus makes cats feel genuinely unwell. Nauseated, flat, off their food. It also speeds the disease along, so it isn't only a comfort issue.

Unlike creatinine, we have real control here. Lowering phosphorus is what a kidney diet is mostly for, and what a phosphorus binder does directly.

Potassium

Potassium drives muscle strength, and kidney cats can run into trouble with it in both directions. This is the part that's usually explained badly, so read it carefully.

Low potassium is the common problem, in the earlier and middle stages. Damaged kidneys leak potassium into the urine. A low cat is weak and wobbly, sometimes walking as though the floor is uneven, and classically holding her head and neck oddly low. That's a real sign with a real name, and it corrects with a supplement.

High potassium happens in late-stage disease, and it's dangerous. When the kidneys are failing badly enough that she's producing little urine, potassium builds up instead of being lost. High potassium affects the heart.

The reason this matters to you: a supplement that was exactly right for her in stage 2 can become actively harmful in late stage 4. Potassium is decided on a blood value, rechecked, not on symptoms and not on last year's prescription. Never start, stop, or change a potassium supplement on your own.

Urine protein, the value that's often missing

This one isn't on the blood panel, and plenty of cats have never had it done.

A UPC, urine protein to creatinine ratio, measures whether the kidney's filters are leaking protein into the urine. It matters for two reasons. Protein loss makes kidney disease progress faster, and it's treatable with a specific medication that your vet won't prescribe without the number.

Along with blood pressure, UPC is what IRIS calls substaging. A stage on its own is an incomplete picture. If your cat has a stage but has never had a UPC run, that's a reasonable thing to ask about.

Blood pressure

Also not on the blood panel, also frequently skipped, and the one I'd least want you to skip.

Roughly one in five to one in three cats with kidney disease has high blood pressure. It usually causes no symptoms at all until it causes a sudden and permanent one, most often blindness from a detached retina.

It's measured with a small cuff on a leg or the tail. It takes a few minutes and your cat mostly finds it boring. It's treated with a daily tablet.

If blood pressure isn't on the list at her recheck, ask for it.

Red blood cells, later on

Further down the sheet you'll find hematocrit or PCV, which is the proportion of her blood made up of red cells.

Healthy kidneys make a hormone that tells the bone marrow to produce red cells. As kidney tissue is lost, that signal fades and cats become anemic. It's a later-stage problem, it's treatable, and treating it can transform how an advanced cat feels.

What to actually do with all this

Write down four numbers after every recheck: creatinine, phosphorus, potassium, and her weight. Put them in the notes app on your phone with the date.

That's it. Six months from now, when you're trying to work out whether things have changed, you'll have a straight answer instead of a guess. It's the single most useful thing I ask owners to do and almost nobody does it.