Azotemia in cats - Types, Symptoms and Treatment

Azotemia is defined as the increase in non-protein nitrogenous waste products in the blood, with urea and creatinine being the most commonly measured. Although urea is the general term for any high levels of nitrogen-containing compounds, it is most commonly referring to high levels of urea, creatinine or both concurrently.

What is urea?

Urea is a small molecule organic compound which is the end product of protein metabolism in the liver. This series of biochemical reactions is known as the urea cycle. This substance is filtered by the glomerulus of the kidney and reabsorbed into the renal tubule and collecting ducts of the kidney.

What is creatinine?

Creatinine is a compound that is formed through the breakdown of creatine, an important nutrient for muscles. Creatinine is the waste product created in normal muscle metabolism and is produced at a constant rate, depending on the feline's muscle mass. It is also filtered in the glomerulus of the kidney, but it is not reabsorbed afterwards, being excreted in the urine.

There are three types of azotemia in cats. In all three, there is a decrease in renal glomerular filtration rate (GFR), i.e. the flow rate of filtered fluid through the kidney. Due to this decrease, there is a consequent increase in creatinine and urea.

Feline prerenal azotemia

Prerenal azotemia develops as a consequence of reduced kidney perfusion due to impaired blood flow. This impairment of blood flow could be due to conditions such as hypovolemia (low extra-cellular body fluid), inadequate cardiac rate, marked vasodilation or dehydration. However, it is not related to inherent kidney disease.

When renal perfusion decreases, the glomerular filtration rate also decreases. This results in a slower elimination of urea and creatinine, leading to higher concentrations in the blood. Urea is reabsorbed at a faster rate, so it is registered more quickly in urinalysis due to the slower transit in the tubules and ducts. Creatinine will increase more slowly as it is not reabsorbed.

As the nephrons remain intact, without damage or alteration in their functionality, when perfusion is restored, renal function returns to normal.

Feline renal azotemia

As its name suggests, renal azotemia in cats occurs when there has been damage to the kidney. A reduction in renal function between 66-75% leads to an increase in blood urea and creatinine, with insufficient urine density (1.008-1.012).

A density between 1.013 and 1.034 indicates that part of the concentration capacity of the urine is intact, but it is insufficient to compensate for the losses. In addition, cats with chronic kidney disease retain the ability to concentrate urine for longer periods than dogs. A density greater than 1.020 can be expected, but it will still be inadequate to prevent azotemia.

Postrenal azotemia

In postrenal azotemia, renal function and the glomerular filtration rate are completely normal and efficient. However, the impurities do not leave the body through the urine due to a blockage of the flow of urine to the kidneys.

The increase in creatinine and urea can occur in various situations, so it will also depend on the type of azotemia in question.

Causes of feline prerenal azotemia

Prerenal azotemia occurs when there is no kidney damage or obstruction of renal flow and develops as a consequence of a reduction in kidney perfusion due to an alteration in blood flow. This reduction can be caused by:

• Hypovolemia
• Inadequate cardiac output
• Prominent vasodilation
• Dehydration

Causes of Renal Azotemia in Cats

Renal azotemia occurs when there is damage to the kidneys themselves. Therefore, azotemia in these cases is produced by:

• Acute kidney disease: sudden and intense onset with reduced glomerular filtration rate. Sometimes it can be reversible. The most common causes in cats are nephrotoxins (drugs, ethylene glycol, heavy metals, lilies and iodinated contrast agents), hypercalcemia, hypophosphatemia, disorders that cause poor renal perfusion (hypovolemia, thrombosis, infarction, polycythemia, or hyperviscosity), or renal parenchymal disease (pyelonephritis, glomerulonephritis, urinary tract obstruction).
  
  
• Chronic kidney disease: progressive reduction in glomerular filtration rate and kidney function, which gives time for compensatory mechanisms to activate. It is common not to find an original cause in cats. It can derive from some cause of acute kidney disease such as urinary tract infections, non-steroidal anti-inflammatory drugs or hypovolemia. It can also be caused by hypertension in cats.

Causes of Postrenal Azotemia in Cats

Postrenal azotemia occurs when the flow of urine is blocked by extrarenal causes. The causes can be:

• Obstruction of the urethra
• Obstruction, rupture or ligation in the ureters
• Bladder leak or rupture

Other causes of azotemia in cats

High urea in cats without increased creatinine can occur after eating a high-protein diet when there is intestinal bleeding. Elevated urea and normal creatinine may also occur in cats when protein catabolism is increased secondary to pyrexia or corticosteroid use.

High creatinine in cats may also simply be due to the fact the cat has a lot of muscle mass. The more muscle mass a cat has, the higher the normal concentration of creatinine. Learn more with our article on high creatinine levels in cats.

As with the differing causes of azotemia in cats, the symptoms of feline azotemia will depend on the type which is produced.

Symptoms of feline prerenal azotemia

The symptoms in this case are those related to low perfusion due to alteration of normal blood flow. In these cases, the feline may manifest:

• Anemia
• Pale mucous membranes
• Weak pulse
• Increased skinfold
• Dryness of the mucous membranes
• Low hematocrit levels
• Decreased blood pressure
• Alterations in heart and respiratory rhythm

Symptoms of renal azotemia in cats

Renal azotemia from acute kidney disease can produce symptoms such as:

• Oliguria (reduced volume of urine)
• Anuria (not urinating)
• Arched back due to kidney pain
• Tachypnea (rapid breathing)
• Arrhythmias
• Temperature increase
• Depression
• Vomiting and/or diarrhea
• Normal or enlarged kidneys

Renal azotemia due to chronic kidney disease can produce symptoms such as:

• Oral ulcers
• Halitosis
• Dehydration
• Anemia of chronic disease
• Gastrointestinal signs
• Polyuria-polydipsia
• Reduced kidney size
• Lack of appetite with weight loss
• Vomiting
• Acute blindness

Symptoms of post-renal azotemia

Blockage of urine flow due to obstruction of the urethra by stones or mucous plugs in FLUTD (feline lower urinary tract disease), damage to the ureters or bladder rupture can produce symptoms such as:

• Dysuria (painful urination)
• Strangury (painful drop-by-drop urination)
• Frequency (urination small amounts many times a day)
• Hematuria (urinating blood)
• Licking of the urogenital area
• Urination outside the litter box
• Hyperkalemia (increased potassium)

Learn more about some of these symptoms of azotemia in cats with our article on why a cat is urinating blood and why my cat is having trouble peeing.

To detect azotemia, a blood sample should be taken. The purpose of this is to determine the concentration of urea in the serum and plasma. It will then be necessary to see if the azotemia is prerenal, renal or postrenal.

Diagnosis of prerenal azotemia

Dehydration in cats can be determined by performing the following tests:

• Skinfold test
• Check for dryness of the mucous membranes
• Check the state of the eyeball
• Blood work to check for an increase in hematocrit and protein

A thorough physical examination should be performed to detect hypovolemia.

Diagnosis of renal azotemia

In kidney disease, the glomerular filtration rate is reduced and the creatinine concentration has been considered as an indirect indicator of the glomerular filtration rate. However, symmetric dimethylarginine (SDMA) levels more accurately reflect this rate and help diagnoses kidney disease earlier than creatinine. SDMA increases when at least 25% of kidney function has occurred and creatinine does not increase until this loss is at least 75%.

Creatinine levels also depend on the muscle mass of a cat. For this reason, we may be given false results in a very muscular or very thin cat, as well as cats with hyperthyroidism. To diagnose the stage of kidney disease, a series of measurements and parameters such as SDMA, creatinine, UPC (urine protein/creatinine ratio) and systolic blood pressure must be made.

A medical history should be taken to find out if they have been in contact with a nephrotoxic drug or substance, as well as any sign of urinary tract infection, hypertension or low renal perfusion, It will also determine the concentrations of phosphorus and calcium to find the cause of kidney disease.

An ultrasound of the kidney should also be performed to assess its size and shape, as well as imaging the rest of the structures of the urinary system.

Diagnosis of post-renal azotemia

To diagnose a urethral or ureteral obstruction or bladder rupture, the following tests should be performed:

• Blood biochemistry to detect azotemia, hyperkalemia, hyperphosphatemia and metabolic acidosis.
• Imaging techniques to detect fluid in the abdomen (due to uroabdomen or ruptured bladder) and sometimes even an obstruction can be seen. Analysis of the liquid after extraction should be carried out to know if it is urine.
• Urinalysis to detect crystals, mucous plugs or blood.

Learn more with our article on why a cat has crystals in their urine.

When prerenal azotemia has been diagnosed, fluid need to be immediately replenished. This will be carried out with fluid therapy. In some cases, a blood transfusion may be required.

In cases of renal azotemia, the cause of acute kidney disease must be treated. Dehydration and electrolyte imbalance must also be corrected. It is important to treat concomitant diseases if they exist (diabetes, hyperthyroidism, heart disease, tumor, etc.). Specific treatment for kidney disease consists of:

• Dehydration treatment with fluid therapy
• Hypertension treatment with amlodipine
• Proteinuria treatment with ACE inhibitors (e.g. benazepril)
• If Hyperphosphatemia (elevated phosphate levels) is present, start with a diet for cats with kidney problems and provide a phosphorus binder if phosphate is still high
• Appetite stimulants such as mirtazapine
• Antiemetics such as maropitant or metoclopramide
• Omeprazole for cats or ranitidine if there is a gastric ulcer
• Feeding tube if they cannot eat
• Reduction of protein, phosphorus, sodium and increase of potassium, fat and B vitamins in diet
• Erythropoietin if there is anemia with hematocrit less than 20%
• Antibiotics if there is a urinary tract infection

In post-renal azotemia, the blockage must be removed, damage repaired, urinary stones removed with diet (struvite) or surgery (calcium oxalate). Surgery is also required in cases of bladder rupture to repair damage.

This article is purely informative. AnimalWised does not have the authority to prescribe any veterinary treatment or create a diagnosis. We invite you to take your pet to the veterinarian if they are suffering from any condition or pain.

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