A healthy cat filters blood at 2-3 mL/kg/min, which for a 4 kg cat is 8-12 mL every minute, or roughly 14 liters a day. Kidney disease is silent until about two thirds of that capacity is gone, so the useful numbers are the ones that move early: SDMA, urine specific gravity, and a creatinine read against the cat's muscle mass. A creatinine of 1.5 mg/dL in a thin 12-year-old is not the same result as 1.5 mg/dL in a muscular 3-year-old.
Four numbers carry most of the diagnosis: GFR at 2-3 mL/kg/min, SDMA under 14 ug/dL, urine specific gravity above 1.035, and blood pressure under 160 mmHg.
What the kidney is doing with 2-3 mL/kg/min
Each kidney holds roughly 200,000 nephrons, and each nephron is a filter plus a recycling plant. Plasma is pushed through the glomerulus at a rate set by the pressure difference across the capillary wall, then 99% of the water and most of the sodium come back in the tubule. A 4 kg cat therefore handles 8-12 mL/min of filtrate and turns it into 100-200 mL of urine a day.
The kidney also takes about 25% of resting cardiac output. That is why a cat with a heart problem or a 10% dehydration can look like a kidney case for a day, and why one recheck after fluids matters more than one bad panel.
Why GFR cannot be read off a routine panel
GFR is the gold standard and the hardest number to get. In practice it is measured by clearance of a marker such as iohexol, over 4-6 hours, with 3-4 blood samples; few clinics do it. What you get instead is creatinine, which stays flat until roughly 75% of filtration is lost.
Useful thresholds: above 2 mL/kg/min is normal, 1.5-2 is a gray zone worth rechecking in 3 months, and under 1.5 mL/kg/min means most of the reserve is gone even if creatinine still reads 1.8 mg/dL.
Muscle mass: the correction that changes everything
Creatinine is a muscle breakdown product, so it tracks muscle, not just filtration. Reference ranges run 0.8-2.4 mg/dL in most labs, and a large muscular male can sit at 1.9 mg/dL with perfect kidneys while a wasted 14-year-old cat reads 1.3 mg/dL with GFR near 1.6 mL/kg/min.
Two checks take 30 seconds: the muscle score of the spine and hips, and a weight trend. A cat that lost 10% of body weight in 6 months but holds creatinine steady has probably lost muscle and gained kidney disease at the same time. In that cat, SDMA and a urine sample are worth more than repeating the chemistry.
SDMA at 14 ug/dL: the early flag
SDMA is produced by all nucleated cells at a fairly constant rate and cleared by the kidney, so it does not depend on muscle. The upper reference is 14 ug/dL. It typically rises when about 40% of function is gone, well before creatinine moves.
Read it in bands: under 14 is normal, 14-18 is a repeat in 2-4 weeks with a urine sample, 18-35 with a normal creatinine fits stage 1 or early stage 2, and above 35 needs staging the same week. Dehydration, a recent meal of cooked meat, and severe dental disease can all nudge it, so a single value never stands alone.
The staging table most vets use
| Stage | Creatinine mg/dL | SDMA ug/dL | Typical action |
|---|---|---|---|
| 1 | under 1.6 | 14-35 | Recheck 4 weeks, urine gravity, blood pressure |
| 2 | 1.6-2.8 | 18-35 | Repeat panel in 4 weeks, start kidney diet discussion |
| 3 | 2.9-5.0 | 36-45 | Panel every 8-12 weeks, phosphate and potassium control |
| 4 | above 5.0 | above 45 | Same-week review, hydration and quality of life plan |
Staging is not a diagnosis of cause. A cat in stage 2 at age 3 needs an ultrasound and a urine culture; the same numbers at age 15 usually mean chronic tubulointerstitial disease and nothing more.
Urine, protein and pressure: the three side numbers
Urine specific gravity of 1.035-1.060 means the kidney still concentrates. A reading of 1.015-1.025 in a dehydrated cat is a warning sign, not a normal result. Protein:creatinine ratio under 0.2 is normal, 0.2-0.4 is a repeat in 2 weeks, and above 0.4 needs treatment and a blood pressure check.
Blood pressure under 160 mmHg is the target; 160-179 is repeat in 2-4 weeks, and 180 or more with eye or brain signs is same-day work. About 20-30% of cats in stage 2-4 are hypertensive, and the retina pays first.
What shifts the numbers without changing the kidney
Dehydration of 5-8% raises creatinine by 0.2-0.5 mg/dL. A recent protein meal raises BUN more than creatinine. Hyperthyroidism hides kidney disease by raising GFR, so creatinine often climbs 2-6 weeks after treatment starts and the kidney case appears then. Glucocorticoids and some antibiotics alter the liver and muscle picture too.
Sample timing matters: draw before fluids, not after, and note the cat's weight and muscle score on the same form so the next panel is comparable.
Seven habits that keep the numbers readable
- Weigh monthly and write the number down.
- Ask for SDMA plus creatinine every 12 months from age 7, every 6 months from age 10.
- Bring a urine sample, or ask for cystocentesis, at every annual visit.
- Record blood pressure with the same cuff and position each time.
- Photograph the water bowl level at 8 am for 3 days.
- Note muscle score beside every creatinine value.
- Repeat any abnormal result once before changing the diet.
Go to the clinic the same day for not passing urine for 12 hours, vomiting more than 3 times in 24 hours, or a cat that stops eating for 24 hours. Book within 48 hours for a 200 g weight drop in a month, or a water intake above 100 mL per kg per day. Ask about the white cell count if fever or infection is in the picture, and use the breed quiz if you want to know whether your cat carries a breed risk worth earlier screening.
Terms mentioned in this article
Frequently asked
What GFR is normal in a cat?
About 2-3 mL/kg/min, which for a 4 kg cat works out to 8-12 mL of filtrate per minute and roughly 14 liters a day. Under 1.5 mL/kg/min means most of the reserve is gone, and creatinine often stays inside the reference range until about 75% of filtration capacity has been lost, which is why a normal chemistry panel does not rule out kidney disease.
Why is SDMA better than creatinine in a thin cat?
Creatinine comes from muscle, so a cat with muscle wasting reads falsely low while SDMA is produced at a steady rate by all cells and cleared by the kidney. Under 14 ug/dL is normal, 14-18 calls for a repeat in 2-4 weeks with a urine sample, and 18-35 with a normal creatinine fits stage 1 or early stage 2.
When should kidney screening start?
From age 7, once every 12 months with SDMA, creatinine, urine specific gravity and blood pressure on the same visit; from age 10, every 6 months. Any single abnormal value is repeated before a diet change, because 5-8% dehydration alone can lift creatinine by 0.2-0.5 mg/dL and look like progression.