Early CKD in Senior Cats: SDMA, USG, IRIS Stage 1

Kidney disease is the most common organ failure in cats past 10 years of age, and it stays quiet for most of its run: a cat can lose two thirds of its working nephrons before creatinine climbs out of range. The numbers that catch it earlier are small, and they only mean anything read together: an SDMA of 15 µg/dL, a urine specific gravity of 1.030, a phosphorus of 1.5 mmol/L, collected on a schedule from a cat whose weight and water intake you already track at home.

Why Stage 1 Is the Stage Worth Catching

IRIS Stage 1 means creatinine under 1.6 mg/dL (140 µmol/L) with SDMA under 18 µg/dL, plus at least one other abnormality: urine specific gravity under 1.035, protein in the urine, a rising SDMA trend across 2 samples, or small irregular kidneys on palpation. Function at that point is usually 60 to 75 percent of normal, which leaves room to slow the decline. Cats found at Stage 1 commonly hold the stage for 1 to 3 years on a renal diet with blood pressure controlled; cats first found at Stage 3 often reach Stage 4 within 6 to 18 months.

The IRIS Table, Read in One Minute

Most clinics write these 4 lines on the cage card. Creatinine is in mg/dL, SDMA in µg/dL, phosphorus in mmol/L, and the last column is the recheck interval for a stable cat with no vomiting, no infection and no hypertension.

IRIS stage	Creatinine	SDMA	Phosphorus target	Recheck
1	under 1.6	under 18	0.9–1.45	every 3 months
2	1.6–2.8	18–35	0.9–1.45	every 3 months
3	2.9–5.0	36–54	0.9–1.6	every 4–8 weeks
4	over 5.0	over 54	0.9–1.9	every 2–4 weeks

Stage is set by creatinine, not by SDMA. The SDMA column is a cross-check, and a cat with an SDMA of 20 and a creatinine of 1.2 mg/dL is still Stage 1. Filing that cat as Stage 2 is a common error and leads to over-treatment, including phosphorus binders it does not need yet.

SDMA at or Above 14 µg/dL

SDMA starts to rise when about 40 percent of kidney mass is gone, against 75 percent for creatinine, which is why 15 µg/dL beside a creatinine of 1.4 mg/dL is a real signal rather than noise. Most laboratories put the top of the reference range at 14 µg/dL. A single reading of 15 or 16 deserves a repeat in 2 to 4 weeks, because dehydration, a meal within 3 hours and normal assay variation each shift the value by 2 to 3 µg/dL. Two results above 14 taken 3 weeks apart, with a urine specific gravity under 1.035, is enough for most internists to label early CKD and start a renal diet trial.

Urine Specific Gravity Under 1.035

A cat on dry food should concentrate urine to 1.045 or better, and many reach 1.060 after 12 hours without water. A gravity of 1.030 to 1.035 in a cat that has not drunk for 6 hours means the medullary gradient is failing; under 1.020 the loss of concentrating ability is substantial. Take the sample before fluids, never after: 100 mL of subcutaneous fluid drops specific gravity from 1.050 to about 1.025 for 6 to 10 hours and makes a healthy cat look sick. A sample left on the bench for more than 2 hours reads 0.002 to 0.004 higher, so hand it to the technician on arrival.

Phosphorus: Hold the 1.45 Line

After proteinuria, phosphate predicts progression better than anything else on the panel. Stage 1 and Stage 2 cats are held at 0.9 to 1.45 mmol/L (2.7 to 4.5 mg/dL), Stage 3 allows 0.9 to 1.6, and Stage 4 up to 1.9. A Stage 1 cat sitting at 1.6 mmol/L is roughly 2 to 3 times more likely to move up a stage within 12 months than one held at 1.2, so a value in the 1.45 to 1.6 band is a reason to change food, not to wait 3 months. Renal diets carry 0.5 to 0.9 percent phosphorus on a dry matter basis against 1.0 to 1.4 percent in adult maintenance food. Phosphate binders (aluminum hydroxide, calcium carbonate, lanthanum carbonate) are prescription drugs, added only when diet alone cannot hold phosphorus under 1.6 mmol/L. They never replace the diet change.

Protein in the Urine: UPC and the 0.4 Line

Urine protein-to-creatinine ratio (UPC) under 0.2 is normal, 0.2 to 0.4 is borderline and needs a repeat in 2 to 4 weeks, and above 0.4 is proteinuria that changes treatment. Rule out a urinary infection first, because roughly 1 in 5 positive cultures in older cats shows no visible signs. A persistent UPC above 0.4 in a Stage 1 cat is a reason to discuss an ACE inhibitor or telmisartan, both prescription-only, and to recheck UPC and creatinine 2 weeks after any change to watch for a creatinine rise above 30 percent.

The 3-Month Recheck Cycle

Stable Stage 1 and Stage 2 cats are rechecked every 3 months, and each visit needs the same 8 items so the trend stays readable: weight, body condition score, systolic blood pressure, creatinine, SDMA, phosphorus, potassium and urine specific gravity, with UPC every 6 months and T4 once a year. Blood pressure above 160 mmHg on 2 visits, or a systolic reading over 180 at any single visit, means treatment now; hypertension affects 20 to 65 percent of cats with CKD and can damage the retina within weeks. Potassium under 3.5 mmol/L is common in Stages 2 and 3 and causes the hind-leg weakness owners often describe as arthritis.

Home Monitoring That Actually Helps

Weigh the cat on the same scale every 2 weeks: a 5 percent drop over 6 months is a clinical finding, not a diet success. Measure drinking water across 24 hours, since 45 to 60 mL/kg per day is normal and a 4 kg cat emptying 300 mL is at 75 mL/kg, which deserves a call. Three changes justify moving the next visit forward: drinking more than usual for 7 straight days, vomiting more than twice in a week, or refusing food for 24 hours.

Five Steps for This Week

1. Ask for SDMA on the next chemistry panel. It is 1 extra line and usually 15 to 30 USD.
2. Collect a free-catch urine sample before any fluids, and request specific gravity plus UPC.
3. Write the current phosphorus value on the food container with the 1.45 mmol/L target beside it.
4. Book the 3-month recheck on the same weekday and put weight and blood pressure on the list.
5. Transition to a renal diet over 7 to 10 days at 25 percent per step, and drop salty treats and cheese.

When to Skip the Schedule and Go Now

Call the clinic the same day for vomiting that lasts past 24 hours, a cat that stops eating for 2 days, urine output that stops, or a sudden loss of vision. Advanced staging and the substaging rules for blood pressure and proteinuria are summarized in the IRIS staging table, and the first signs owners report are collected in early kidney disease signs. If a UPC comes back high, work through the proteinuria check before assuming progression.

Terms mentioned in this article

SDMAUPCCKDT4USG

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