Acute-on-Chronic Kidney Disease in Cats and Dogs: How Vets Diagnose It

By Dr. Rachael Kuhn-Siegel, DVM, owner of Prairie Animal Hospital in Brimfield, Illinois, serving pets and families in the Peoria area for 12 years.

If your cat or dog already has chronic kidney disease (CKD) and suddenly stops eating, starts vomiting, or seems much weaker, they may have a new acute kidney injury layered on top of their existing disease. Vets call this acute-on-chronic kidney disease. It is a separate problem from slow CKD progression, and it is often at least partly reversible if the cause is found and treated quickly. Diagnosis relies on comparing new bloodwork to your pet’s previous results, checking urine, measuring blood pressure, and usually imaging the kidneys to look for a treatable trigger.

What is acute-on-chronic kidney disease?

Chronic kidney disease is a slow, long-term loss of kidney function. It is especially common in older cats, and it also affects many senior dogs. Most pets with CKD stay stable for months or years with diet, hydration support, and regular rechecks.

An acute kidney injury (AKI) is damage that happens over hours to days. When an AKI hits kidneys that are already weakened by CKD, the result is acute-on-chronic disease. Because damaged kidneys have less reserve, even a modest new insult (a few days of not drinking, a urinary infection, a new medication) can cause a sharp drop in function.

The distinction matters because CKD itself cannot be reversed, but the acute part often can. Finding the trigger is what gives your pet the best chance of getting back toward their previous baseline.

Signs your pet’s kidney disease may have suddenly worsened

Contact your veterinarian promptly if a pet with known CKD shows:

• Refusing food, or eating much less than usual
• Vomiting or diarrhea
• Lethargy, hiding, or weakness
• A big change in drinking or urination, including producing very little urine
• Bad breath with an ammonia smell, or mouth ulcers
• Pain when picked up or touched around the back or belly
• Straining to urinate, or blood in the urine
• Sudden blindness, wobbliness, or seizures (possible signs of very high blood pressure)

These signs are not specific to kidney disease, which is exactly why testing is needed.

How vets diagnose acute-on-chronic kidney disease

1. History and physical exam

Your vet will ask about recent changes: new medications or supplements, any possible toxin exposure, appetite and water intake, vomiting, and how much your pet is urinating. Bring a list of everything your pet takes, including over-the-counter products.

Important toxin and medication questions include:

• Cats: any contact with lilies (every part of the plant, including pollen and vase water, is dangerous to cats)
• Dogs: grapes, raisins, or currants
• Both: antifreeze (ethylene glycol), human pain relievers such as ibuprofen or naproxen, and veterinary NSAIDs

On the exam, the vet checks hydration, body temperature, kidney size and shape (where they can be felt), signs of pain, and the mouth.

2. Bloodwork, compared with previous results

Blood tests measure the waste products that healthy kidneys filter out:

• Creatinine and blood urea nitrogen (BUN)
• SDMA (symmetric dimethylarginine), a newer marker that also reflects kidney filtration
• Electrolytes, especially potassium and phosphorus. Very high potassium can affect the heart and needs urgent treatment.
• A complete blood count (CBC) to look for infection, inflammation, or anemia

A single high creatinine value does not tell your vet whether the problem is new or old. The key question is how much the value has changed from your pet’s own stable baseline. The International Renal Interest Society (IRIS), whose guidelines vets widely use, recognizes that even a small rise in creatinine over a short period can signal an active kidney injury. This is one of the strongest reasons to keep up with routine CKD rechecks: those old numbers become a reference point in an emergency.

3. Urinalysis and urine culture

A urine sample tells your vet a great deal:

• Urine concentration (specific gravity) helps show how well the kidneys are working and whether dehydration is part of the picture.
• Sediment exam can reveal casts (a sign of active kidney tubule damage), crystals, white blood cells, or bacteria.
• Glucose in the urine with normal blood sugar can point to kidney tubule injury, which is seen with some toxins and with leptospirosis.
• Urine culture checks for bacterial infection. A kidney infection (pyelonephritis) is a common and treatable trigger in pets with CKD.
• Urine protein:creatinine ratio (UPC), usually rechecked once your pet is stable, measures protein loss through the kidneys.

Urine is often collected by cystocentesis, a quick needle sample taken directly from the bladder, so the culture result isn’t contaminated.

4. Blood pressure

High blood pressure is common in pets with kidney disease, especially cats. It can both result from and worsen kidney damage, and it can harm the eyes, brain, and heart. Measuring blood pressure is a routine part of the workup.

5. Imaging: ultrasound and X-rays

Abdominal ultrasound is one of the most valuable tests in acute-on-chronic disease because it can find causes that need specific treatment:

• Ureteral stones or blockages, a well-recognized cause of sudden kidney decline, particularly in cats. One kidney may be blocked while the other is already damaged by CKD.
• Dilated kidney pelvis, which can suggest obstruction or infection
• Changes in kidney size, shape, and structure that help separate long-standing disease from new injury
• Masses or other abnormalities

X-rays can show many mineral stones in the kidneys, ureters, and bladder, and help with overall assessment.

6. Infectious disease testing (especially in dogs)

Leptospirosis is a bacterial infection spread through the urine of wildlife and other animals, often via contaminated water or soil. It can cause acute kidney injury in dogs, and it can also infect people. If your vet suspects it, they may run blood and urine tests for leptospirosis. Depending on where you live and your pet’s history, testing for tick-borne diseases may also be recommended.

Common causes of a sudden decline in pets with CKD

Trigger	What it is	How it’s usually found
Dehydration	Not drinking enough, or losing fluid through vomiting or diarrhea	History, exam, bloodwork, urine concentration
Kidney infection (pyelonephritis)	Bacteria travel up into the kidney	Urine culture, ultrasound, bloodwork
Ureteral obstruction	A stone or debris blocks the tube from kidney to bladder	Ultrasound, X-rays
Toxins	Lilies (cats), grapes/raisins (dogs), antifreeze	History, urinalysis, bloodwork
Medications	NSAIDs and some other drugs can stress the kidneys	Medication history
Leptospirosis (dogs)	Bacterial infection	Blood and urine testing
Uncontrolled high blood pressure	Ongoing damage to kidney blood vessels	Blood pressure measurement
Low blood pressure or poor blood flow	From anesthesia, heart disease, or severe illness	History, exam, monitoring

Sometimes no single cause is found. Treatment then focuses on supporting the kidneys while they recover.

Why staging waits until your pet is stable

Vets use the IRIS staging system to describe how advanced CKD is, and they use a separate IRIS grading system for acute kidney injury. CKD staging should only be done once a pet is stable, because creatinine and SDMA values measured during an acute crisis overstate how much permanent damage is present. After treatment, your vet will recheck bloodwork over the following weeks to see where your pet’s “new normal” settles. That value guides long-term diet, medication, and monitoring.

What happens after diagnosis

Treatment depends on the cause and how sick your pet is. It may include:

• Hospitalization with IV fluids to correct dehydration and support kidney blood flow, with careful monitoring of urine output and body weight
• Antibiotics for kidney infection or leptospirosis
• Anti-nausea medication, appetite support, and stomach protectants
• Treatment for high potassium or high blood pressure
• Procedures to relieve a ureteral blockage, sometimes at a specialty or referral hospital
• Dialysis, available at some referral centers, for pets that don’t respond to standard treatment

The outlook varies. Some pets bounce back close to their previous baseline. Others recover partially and live comfortably with more advanced CKD. Your vet’s best guide to prognosis is how your pet’s bloodwork and urine output respond over the first several days of treatment.

How to protect a pet with CKD from acute flare-ups

• Keep up with recheck visits. Regular bloodwork, urine tests, and blood pressure checks catch problems early and provide the baseline your vet needs in an emergency.
• Support hydration. Water fountains, multiple bowls, wet food, or vet-prescribed fluids at home can all help.
• Ask before giving any new medication, including over-the-counter pain relievers and supplements.
• Pet-proof against toxins. Keep lilies out of homes with cats, and keep grapes and raisins away from dogs.
• Ask about leptospirosis vaccination for dogs with outdoor or wildlife exposure.
• Don’t wait out a bad day. In a pet with CKD, a day or two of not eating or vomiting is worth a call to your vet.

Frequently asked questions

Can acute-on-chronic kidney disease be reversed?

The acute injury often can be partly or fully reversed if the cause is found and treated quickly. The underlying chronic kidney disease cannot be reversed, but it can usually be managed.

Is a sudden rise in creatinine always an emergency?

A sudden rise in a pet with CKD should always be taken seriously and assessed promptly, especially if your pet is unwell. Your vet will compare it to previous values and look for a cause.

What is the difference between SDMA and creatinine?

Both are blood markers of how well the kidneys filter. SDMA is less affected by muscle mass than creatinine, which can be helpful in thin or older pets. Vets often look at both together.

Why does my vet want an ultrasound if they already know my pet has kidney disease?

Ultrasound can find treatable causes of sudden decline, such as a blocked ureter or signs of infection. These problems look the same as CKD progression on bloodwork alone.

Will my pet need to stay in the hospital?

Many pets with acute-on-chronic disease benefit from a hospital stay for IV fluids and monitoring. Mild cases are sometimes managed as outpatients. Your vet will recommend the right level of care based on the bloodwork and how your pet is feeling.

Are cats or dogs more likely to have this problem?

CKD is more common in older cats, so acute-on-chronic episodes are frequently seen in cats. Dogs are affected too, and leptospirosis and grape or raisin toxicity are dog-specific risks to keep in mind.

Is your pet with kidney disease acting differently? Call us or request an appointment. We’ll compare today’s results with your pet’s baseline and find out what’s changed.