What is SDMA, Creatinine, and BUN? A Deep Dive into Kidney Blood Work

When your vet hands you a printout of your cat's blood work, it can look like a confusing receipt of acronyms and numbers. However, when it comes to kidney health, three specific values tell the story: SDMA, Creatinine, and BUN.

Together, these markers form a triad that answers two critical questions:

  1. How well are the kidneys filtering toxins?

  2. How does the cat physically feel?

Here is a detailed breakdown of each marker, how they work, and the external factors that can skew the results.

1. Creatinine: The Old Reliable Standard

For decades, Creatinine was the gold standard for diagnosing Chronic Kidney Disease (CKD). While still vital, we now know it has significant limitations.

What is it? Creatinine is a waste product generated from the normal wear and tear of muscles. Your cat's muscles use a fuel called creatine; when it breaks down, it becomes creatinine. Since muscle mass stays relatively constant, the amount of creatinine produced each day is steady.

How it works: Creatinine is filtered out of the blood by the kidneys and dumped into the urine. It is not reabsorbed by the body. Therefore, if creatinine levels in the blood rise, it means the kidney filters (glomeruli) are clogged or damaged and not clearing it out.

The Blind Spot: The biggest flaw of creatinine is that it is a late indicator. The kidneys are so efficient at compensating that creatinine often remains normal until 75% of kidney function is lost. By the time creatinine spikes, the disease is already advanced.

Factors That Influence Creatinine:

  • Muscle Mass: This is the most critical factor. A muscular cat will have naturally higher creatinine. A skinny, elderly cat with muscle wasting (sarcopenia) will have very low creatinine production. Their blood work might look "normal" even if their kidneys are failing, simply because they don't have enough muscle to produce the waste to measure.

  • Hydration: Severe dehydration concentrates the blood, artificially raising creatinine.

  • Hyperthyroidism: High thyroid levels increase blood flow to the kidneys, artificially lowering creatinine and masking kidney disease.

2. SDMA (Symmetric Dimethylarginine): The Early Warning System

SDMA is a newer test that has revolutionized how we diagnose CKD. It solves the blind spot problem of creatinine.

What is it? SDMA is a methylated form of the amino acid arginine. It is produced by every cell in the body, not just muscles, and is released into the blood at a constant rate.

How it works: Like creatinine, SDMA is filtered almost exclusively by the kidneys. However, it is much more sensitive to minor changes in filtration. SDMA typically rises when 25% to 40% of kidney function is lost.

Why it is superior for cats:

  • Early Detection: It flags kidney stress months or even years before creatinine rises.

  • Muscle Independence: Unlike creatinine, SDMA is not affected by muscle mass. This makes it the most accurate test for skinny, older cats who have lost weight, which is the exact demographic most likely to have CKD.

Factors That Influence SDMA:

  • Few External Factors: SDMA is remarkably stable. It is not impacted by diet or gut health. However, severe dehydration can mildly elevate it, as with any kidney marker.

3. BUN (Blood Urea Nitrogen): The "Feeling Sick" Marker

BUN is often the highest number on the page, but it is also the "noisiest" and hardest to interpret alone.

What is it? BUN measures the amount of nitrogen in the blood that comes from the waste product urea. Urea is made in the liver when the body breaks down protein (from food or body tissues).

How it works: The kidneys filter urea out, but unlike creatinine, they also reabsorb some of it back into the blood. When kidneys fail, urea builds up quickly.

The Clinical Connection: BUN is the marker most closely linked to how your cat feels. High levels of urea (Uremia) cause nausea, stomach ulcers, bad breath, and lethargy. If BUN is high, your cat likely feels like they have a severe hangover or the flu.

Factors That Influence BUN (The Noise): Because urea comes from protein breakdown, BUN can skyrocket for reasons having nothing to do with kidneys:

  • High Protein Diet: A cat eating a raw meat diet will have naturally higher BUN than a cat on low protein kibble.

  • GI Bleeding: If a cat has a bleeding ulcer or fleas (swallowed blood), they are digesting their own blood. Blood is a protein, so digesting it causes a massive spike in BUN.

  • Dehydration: Dehydration causes the body to reabsorb more urea, spiking BUN much faster than creatinine.

  • Liver Disease: Since the liver makes urea, a failing liver will result in unnaturally low BUN.

Putting It All Together: The BUN/Creatinine Ratio

Vets rarely look at one number in isolation. They look at the relationship between them.

  • High BUN + Normal Creatinine: Usually suggests dehydration or a GI bleed, not necessarily kidney failure.

  • High SDMA + Normal Creatinine: Suggests very early kidney decline (Stage 1). This is the time to start dietary changes and hydration support.

  • High BUN + High Creatinine + High SDMA: Confirms established Chronic Kidney Disease.

The Takeaway

Understanding these values empowers you to ask better questions.

  • If your cat is skinny and has normal creatinine but high SDMA, you know not to be lulled into a false sense of security.

  • If BUN is high but creatinine is stable, you might look for sources of bleeding or check hydration rather than assuming the kidneys are getting worse.

Blood work is a puzzle, and these three markers are the corner pieces that help you and your vet build the picture of your cat's health.

Cited Research

  • 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.

  • Braff, J., et al. (2014). Relationship between serum symmetric dimethylarginine concentration and glomerular filtration rate in cats. Journal of Veterinary Internal Medicine, 28(6), 1699 1701.

  • 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.

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

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