Proteinuria is a term used to describe an abnormally high amount of protein in the urine due to possible illness. Normal urine contains very little protein when measured by conventional methods, such as urine dipsticks. When larger amounts are found, this is considered abnormal and clinically important.

Proteinuria can be further classified based on the origin of the excess protein entering the urine: pre-renal, renal, or post-renal. Pre-renal proteinuria occurs when the excess of protein in the blood overflows into the urine. The kidneys act as a filter, preventing proteins from being filtered out of the body and into the urine. When the kidneys are damaged or overwhelmed, more protein can leak into the urine, a condition known as renal proteinuria.

Protein in the urine is often used as an indicator of kidney disease or other health problems. Post-renal proteinuria occurs when protein enters the lower urinary tract. All of these categories can result in an increase of protein in the urine, but to determine the most appropriate treatment, your veterinarian must localize the cause to one of these three categories.

Proteins are one of the four major macromolecules in the body and are essential for almost every bodily function. Protein is an umbrella term that includes a variety of types with various functions: transport of nutrients, immune function, hormone signaling for cell growth and function, etc. When the body lacks protein, your pet may become sick, but the symptoms depend on why and how it is losing protein.

Albumin is a tiny protein produced in the liver that helps keep fluid inside blood vessels, preventing it from leaking into surrounding tissues. This is one of the many functions that proteins have in the body. Albumin is the primary protein used to monitor kidney function and disease progression because it is the first protein to leak into the urine.

Diagnosis

Your veterinarian will collect and examine a urine sample. A urinalysis can be run in-house or sent to an outside lab. When a routine urinalysis is performed, such as during an annual wellness appointment, urine is collected in a container as your pet squats to urinate. This is called the free catch method. In cats, or when sample contamination is a concern, a needle is used to collect the urine directly from the bladder (cystocentesis).

After collection, the protein level is first checked with a dipstick test. A small drop of urine is added to a test strip, and the change in color of the test pad indicates the amount of protein, specifically albumin, in the urine. Results can range from negative or trace amounts in healthy animals to 4+, indicating a lot of protein. This semi-quantitative test can be used to rule out proteinuria.

When protein levels are 1+ or higher on repeated tests, your veterinarian may recommend a UPCR (urine protein–to-creatinine ratio). This test helps the veterinarian determine whether the kidneys are truly damaged, detect early kidney disease, monitor disease progression, and track how well treatment is working. This test provides more definitive results because, unlike the urine dipstick, the UPCR quantifies urine protein based on its ratio to creatinine. 

Additional diagnostic imaging, such as ultrasound and blood work, can be performed to provide supportive information on any underlying conditions that need to be treated.

Treatment

A treatment plan for proteinuria is created by your veterinarian based on the underlying cause. A common goal of all treatments is to protect the kidneys and slow the progression of damage by reducing the amount of protein that leaks into the urine. ACE inhibitors (Angiotensin-converting enzyme) such as Enalapril and Benzapril are commonly prescribed to reduce the force of blood flow to the kidneys, so less protein is filtered into the urine. Special, low-protein diets can also be fed to limit the amount of kidney-challenging protein in your pet's body. Additional medication can be prescribed depending on the underlying cause.

Prognosis

A patient-specific prognosis is based on the underlying cause, but its severity and the timing of proteinuria detection (as measured by UPCR) must also be considered. Additional bloodwork is important to assess kidney function. Protein in the urine is a diagnostic indicator of kidney damage that should be monitored long-term. Although proteinuria can be reversed with appropriate treatment, kidney damage can be permanent and progressive in some cases. When found and treated early, many dogs can live comfortably for years. If proteinuria is left untreated, further kidney damage is likely to occur.