Cat Kidney Function: 3,000 mOsm/kg and SDMA Timing

A cat can concentrate its urine to about 3,000 mOsm/kg, roughly 2.5 times what a human kidney manages, and a normal specific gravity sits between 1.035 and 1.080. That power is the reason kidney disease is found late: by the time the cat drinks more and the urine gets watery, a large share of the nephrons is already gone.

Two numbers move earlier than thirst. Specific gravity under 1.035 in a cat that is not on wet food, and SDMA above the reference range, both appear while the cat still looks well.

2 kidneys, 3 layers, one job

A cat has 2 bean-shaped kidneys, each about 15 to 20 grams, sitting against the back wall under the 2nd to 4th lumbar vertebrae, the right one slightly further forward. Each is built in 3 layers: an outer cortex holding the glomeruli, a medulla of loops of Henle that do the concentrating, and a pelvis that drains into a ureter about 2 to 3 mm wide. Blood flow is enormous for the size, roughly 20 percent of cardiac output going to 0.5 percent of body weight.

The loop of Henle in a cat is long, and length is what buys the gradient. Water leaves the collecting duct only if the medulla is salty, and the saltiness is set by the loop. Cats evolved as desert hunters, took most of their water from prey, and kept the machinery: a cat can reabsorb 99 percent of the water it filters, which is why a 4 kg cat on dry food can produce as little as 100 mL of urine a day.

3,000 versus 1,200

Human urine tops out near 1,200 mOsm/kg; a cat reaches 3,000, and a cat in a hot, dry environment can briefly go higher. On the refractometer that reads as 1.035 to 1.080. Values between 1.020 and 1.035 mean the kidney is working but not concentrating hard, and under 1.017 the urine is close to plasma and the concentrating ability is gone.

Read the number against the diet. A cat on wet food, which is 75 to 80 percent water, will run a lower specific gravity than the same cat on dry food at 8 to 10 percent water. A specific gravity of 1.030 is unremarkable in a wet-fed cat and a warning in a dry-fed one that used to be 1.060.

Why SDMA moves before creatinine

Creatinine only climbs once roughly two thirds to three quarters of the nephrons have stopped working, because the remaining ones filter harder and keep the blood level flat. SDMA is different: it rises at about 40 percent loss, so it moves 1 to 2 stages earlier. That is why a cat can have a creatinine of 1.4 mg/dL, squarely inside the reference range, and an SDMA of 20 ug/dL that has been drifting up for 6 months.

SDMA is not perfect. Lean muscle mass affects creatinine more than SDMA, so a wasted old cat can hide disease in a normal creatinine for longer. Dehydration raises both, so a sample taken from a cat that has not drunk for 12 hours should be repeated after 2 to 3 weeks rather than acted on once.

The staging numbers

Chronic kidney disease in cats is staged on fasting creatinine, with SDMA and urine specific gravity filling in the gaps. Staging is done on a stable, well-hydrated cat, at least 2 samples 2 to 4 weeks apart.

IRIS stage	Creatinine, mg/dL	SDMA, ug/dL	Phosphorus target, mmol/L
1	under 1.6	under 18	0.9 to 1.5
2	1.6 to 2.8	18 to 35	0.9 to 1.5
3	2.9 to 5.0	36 to 54	0.9 to 1.5
4	over 5.0	over 54	under 1.9

Phosphorus is the lever

Phosphorus does more damage in a failing kidney than protein does. When filtration drops, phosphorus accumulates, the parathyroid gland responds, and calcium leaves the bone; the result is mineralization inside the kidney itself. Most vets aim to keep blood phosphorus between 0.9 and 1.5 mmol/L, which in a stage 2 or 3 cat usually means a renal diet at 0.5 to 0.7 percent phosphorus on a dry matter basis, plus a binder if diet alone will not hold it.

Protein is the second lever, and it is the one owners over-tighten. A moderate restriction, roughly 26 to 32 percent protein on a dry matter basis, is the usual compromise between muscle loss and nitrogen load. Pushing protein below 20 percent in an old cat costs muscle and appetite, and appetite is the thing that keeps a kidney cat alive.

Water: 40 to 60 mL per kg

Total water intake, food included, should land near 40 to 60 mL per kg per day; a 4 kg cat needs 160 to 240 mL. Over 100 mL per kg, or 400 mL in a 4 kg cat, is polyuria and a reason to test. To raise intake: 2 to 3 water bowls of 200 mL each in different rooms, a fountain, and a shift to wet food, which alone can double water intake without the cat drinking a drop more.

Urine output follows. A cat producing more than 30 mL per kg per day, 120 mL in a 4 kg cat, is losing concentrating ability, and one producing under 1 mL per kg per hour for 6 hours is a blocked or shut-down kidney: that cat is an emergency.

Protein in the urine, pressure in the vessels

Two tests finish the picture. A urine protein to creatinine ratio under 0.2 is normal, 0.2 to 0.4 is borderline, and over 0.4 is proteinuric and worth treating. Systolic blood pressure under 150 mmHg is fine, 160 to 179 needs a plan, and 180 or more needs treatment the same week. Both are checked at least once a year in any cat over 7, and every 3 to 6 months once a stage is assigned, as set out in early signs of kidney disease.

Testing urine at home

Non-absorbent litter beads let you catch a sample in a clean container; a 1 mL sample is enough for a dipstick, though the specific gravity needs a refractometer at the clinic. A cat that drinks more but whose urine stays at 1.050 is probably just on dry food in warm weather; one at 1.020 needs bloodwork. The collection routine is described in home urine testing.

4 steps once a number moves

Repeat the sample in 2 to 4 weeks before calling it a diagnosis, and take the cat's weight on the same scale each time. Ask for SDMA alongside creatinine, and for a urine specific gravity from the same visit. Switch to a renal diet at 0.5 to 0.7 percent phosphorus early rather than at stage 4. And use a cat age chart to set the interval: from 7 years, twice a year is the default, because a cat of 12 is roughly a human of 64 and loses kidney reserve faster than a cat of 4.

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