Decoding Kidney Function Markers

Decoding Kidney Function Markers

Learning to read your cat's blood work is one of the most empowering things you can do as a caretaker. Veterinarians are busy and may only give you the highlights ("Her kidneys look a bit worse"), but the lab report tells the full story.

To interpret these numbers correctly, you need to understand what they measure, what influences them, and how they interact with each other. A single number in isolation is rarely an answer; it is the trend over time that matters.

Here is your detailed guide to the primary markers of kidney function.

1. The Filtration Markers: How Well Are the Kidneys Cleaning?

These three values—Creatinine, SDMA, and BUN—are waste products. If the kidneys are working well, these numbers should be low. If they are rising, the filter is clogged.

Creatinine

• Normal Range: 0.8–2.4 mg/dL (70–212 µmol/L)

• What it is: A waste product from normal muscle metabolism. It is produced at a steady rate and filtered out by the kidneys.

• The Nuance: Creatinine is directly linked to muscle mass.

  • The Trap: A skinny, elderly cat with muscle wasting (sarcopenia) produces very little creatinine. Their blood work might show a "normal" creatinine level (e.g., 1.8 mg/dL) even though their kidney function is severely reduced.

  • The Tip: Always look at Creatinine alongside the cat's body condition. If your cat is losing weight but Creatinine is "stable," the disease may actually be progressing.

SDMA (Symmetric Dimethylarginine)

• Normal Range: <14 µg/dL

• What it is: A sensitive marker of Glomerular Filtration Rate (GFR). Unlike creatinine, it is produced by all cells, not just muscle.

• The Nuance: SDMA is the "early warning" system. It typically rises when 25–40% of kidney function is lost, whereas creatinine often waits until 75% is lost.

  • The Trap: While less affected by muscle, SDMA can be falsely elevated by severe dehydration or hyperthyroidism (though less so than creatinine).

  • The Tip: A high SDMA with a normal Creatinine is the definition of IRIS Stage 1 CKD. This is your window to start kidney-friendly hydration strategies before severe damage occurs.

BUN (Blood Urea Nitrogen)

• Normal Range: 16–36 mg/dL (5.7–12.9 mmol/L)

• What it is: Urea is a byproduct of protein breakdown in the liver. It is filtered by the kidneys.

• The Nuance: BUN is the "dirty" marker because so many things affect it.

  • The "Feel Good" Factor: High BUN (Uremia) correlates with nausea. If BUN is high, your cat likely feels sick (lethargic, no appetite).

  • Non-Kidney Causes:

    • Dehydration: Causes BUN to spike disproportionately high compared to Creatinine.

    • GI Bleed: Digested blood is a protein source, causing a massive BUN spike.

    • High Protein Diet: Raw fed cats often have slightly higher BUN baselines.

  • The Tip: If BUN is high but Creatinine/SDMA are stable, check for dehydration or stomach ulcers first.

2. The Electrolytes: The Delicate Balance

Kidneys regulate the salt and mineral balance in the blood. When they fail, these levels swing dangerously, causing physical symptoms.

Phosphorus

• Normal Range: 2.5–6.0 mg/dL (0.81–1.94 mmol/L)

• What it is: A mineral essential for energy and bones. It is excreted by the kidneys.

• The Danger: In CKD, kidneys cannot get rid of excess phosphorus. High phosphorus is toxic; it calcifies the kidney tissue, causing faster disease progression and making the cat feel terrible.

• Targets: In CKD, "normal" isn't good enough. We want:

  • Stage 2: < 4.6 mg/dL

  • Stage 3: < 5.0 mg/dL

  • Stage 4: < 6.0 mg/dL

• The Tip: If phosphorus is above these targets, you must switch to a lower phosphorus diet or use a binder.

Potassium (K⁺)

• Normal Range: 3.5–5.1 mEq/L (mmol/L)

• What it is: The "spark plug" for muscles and nerves.

• The Danger: CKD cats often lose too much potassium in their urine (Hypokalemia).

• Symptoms: Severe muscle weakness. You may see Cervical Ventroflexion—the cat cannot lift its head and tucks its chin to its chest. They may also walk with a stiff, stilted gait.

• The Tip: Potassium supplements (like Gluconate) are an easy fix that can dramatically improve a cat's energy.

Calcium (Total)

• Normal Range: 8.2–11.8 mg/dL (2.05–2.95 mmol/L)

• What it is: Vital for bone and heart health.

• The Danger: Calcium regulation goes haywire in CKD. It can be high (Hypercalcemia) or low.

• The Tip: Watch the Calcium-Phosphorus Product (Calcium × Phosphorus). If this number exceeds 70, soft tissue mineralization (turning tissue to stone) becomes a serious risk.

3. The Urine Markers: The Output Check

Blood tells us what is staying in; urine tells us what is leaving.

USG (Urine Specific Gravity)

• Normal Range: >1.035

• What it is: A measure of urine concentration.

• The Danger: Isosthenuria (1.008–1.015). This means the urine is as dilute as water. It confirms the kidneys have lost the ability to concentrate. This usually happens before blood values rise.

• The Tip: If USG is consistently low (<1.035), your cat has CKD even if their blood work looks "okay."

UPC (Urine Protein:Creatinine Ratio)

• Normal Range: <0.2

• What it is: Measures how much protein is leaking through the kidney filters.

• The Danger: Protein is toxic to the kidney tubules. Proteinuria (UPC >0.4) is associated with a much faster decline in survival time.

• The Tip: Always check a Urine Culture if UPC is high. Infections can cause temporary proteinuria. If no infection is present, meds like Telmisartan are needed to stop the leak.

Summary for the Caretaker

When you get your lab report, do not just look for the "H" (High) or "L" (Low) flags. Look at the relationships.

• Creatinine + Body Condition: Is she losing muscle?

• BUN + Hydration: Is she dehydrated?

• Phosphorus + Diet: Is her food keeping phosphorus in the safe zone?

References

• Reynolds, B. S., & Lefebvre, H. P. (2013). Feline CKD: Pathophysiology and risk factors. Journal of Feline Medicine and Surgery, 15(1, Suppl), 3–14.

• Hall, J. A., et al. (2014). Comparison of serum concentrations of symmetric dimethylarginine and creatinine as kidney function biomarkers in cats with chronic kidney disease. Journal of Veterinary Internal Medicine, 28(6), 1676–1683.

• Sparkes, A. H., et al. (2016). ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease. Journal of Feline Medicine and Surgery, 18(3), 219–239.

• Kidder, A. C., & Chew, D. J. (2009). Treatment options for hyperphosphatemia in feline CKD: what’s new? Journal of Feline Medicine and Surgery, 11(11), 913–924.

• International Renal Interest Society (IRIS). (2023). IRIS Staging of CKD Guidelines.