Lab technique

IMPORTANCE & INTERPRETATION OF KIDNEY FUNCTION TESTS (KFTs) IN DOG & CAT

April 26, 2023

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IMPORTANCE & INTERPRETATION OF KIDNEY FUNCTION TESTS (KFTs) IN DOG & CAT

Kidneys play a critical role in eliminating metabolic body wastes and in maintaining the body’s overall water and electrolyte balance. The kidney function tests (KFT) help you understand whether your kidneys are working efficiently or not by examining how effectively your kidneys are removing wastes from your body. A 24-hour urine sample, a blood test, or both may be required for kidney function evaluation. Kidneys play a critical role in eliminating metabolic body wastes and in maintaining the body’s overall water and electrolyte balance. The kidney function tests (KFT) help you understand whether your kidneys are working efficiently or not by examining how effectively your kidneys are removing wastes from your body. A 24-hour urine sample, a blood test, or both may be required for kidney function evaluation.

What Do Your Kidneys Do?

Your genito-urinary system includes your kidneys, which are positioned just below your left rib cage at the back of your belly. Kidneys help your body in removing waste products through urine excretion. The additional functions of the kidneys include:

Production of blood pressure-maintaining hormones
Production of the oxygen-carrying red blood cells that circulate through your body
Production of vitamin D, which keeps your muscles and bones healthy

So, it is evident that taking care of your kidneys is of utmost importance. You can get a kidney function test (KFT) for a proper evaluation of your kidneys’ current health.

What Symptoms Can be Caused By Kidney Problems?

Symptoms that may indicate a problem with your kidneys include:

Frequent urges to urinate
Difficulty beginning urination
Pain during urination
High blood pressure
Blood in the urine
Swelling of the hands and feet

Your doctor will evaluate your symptoms, correlates it with the test findings, and arrive at a diagnosis. Also, a single symptom may not always mean something serious.

What Are Kidney Function Tests?

Bodily waste or toxic substances including excess fluid are usually removed from the bloodstream by healthy functioning kidneys. Blood specimens and urine tests indicate how properly your kidneys are working and how effectively wastes are excreted from the body. Urine tests can help detect whether the kidneys are leaking any amount of excessive protein, which is a sign of kidney damage.

Here we will discuss various tests that are used to understand kidney function. Most of these kidney function tests assess the glomerular filtration rate (GFR). GFR measures how well your kidneys remove waste from your system.

What are The Different Types of Kidney Function Tests (KFT)?

To diagnose the proper health of your kidneys your physician may prescribe different types of kidney function tests. The following are the different tests that are commonly included in KFT:

Serum Creatinine

The body’s metabolism produces creatinine as a waste product. The kidneys remove it from the blood and excrete it via urination. Generally, creatinine is almost eliminated from the body through the kidneys. In this way, our bodies maintain a low level of creatinine because this creatinine excretion is an ongoing process. Therefore, a high creatinine level could be a sign of kidney issues. In general,

For adult men, the normal range of serum creatinine lies between 0.74 to 1.35 mg/dL
For adult women, the normal range of serum creatinine lies between 0.59 to 1.04 mg/dL

Uric Acid

When our body chemically breaks down organic compounds metabolically, uric acid is created. The kidneys filter and eliminate this uric acid waste product through urine. Therefore, both higher or lower uric acid levels could indicate kidney damage.

In general, the normal range of uric acid lies between 3.7-9.2 mg/dL.

Blood Urea Nitrogen (BUN)

When the body breaks down proteins, it results in urea nitrogen production. Urea nitrogen is also a waste product that the kidneys eliminate through urine. Generally, the level of urea nitrogen in your blood should fall between specific limits. Other than kidney issues, some medications may also cause a rise in urea nitrogen levels in our blood. Therefore, if your BUN level is greater than usual, talk to your doctor for a detailed diagnosis and also inform him about your current medications.

In general, the normal range of BUN lies between 7-25 mg/dL.

Electrolytes

Electrolytes are minerals present in the blood and body fluids that facilitate several body functions. They can be positively or negatively charged. An electrolyte test helps monitor the body’s electrolyte levels and is a strong indicator of how well your kidneys are functioning. Sodium, potassium, and chloride are some of the main electrolytes. The normal range for each electrolyte can vary to some extent in men and women, and different age groups.

Estimated Glomerular Filtration Rate (eGFR)

The GFR calculates your kidneys’ ability to filter waste materials from the body. Your normal GFR results may not always correspond to your normal serum creatinine level. To evaluate your proper renal function, it is, therefore, preferable to estimate glomerular filtration rate or eGFR, also known as the “gold standard” measurement. Your serum creatinine, age, race, and sex, all will be used to estimate your GFR. Because this test result roughly equates to your remaining kidney function, this test helps you get easy-to-interpret information regarding the degree of your kidney damage.

In general,

The normal level of GFR is considered at 60 or above
A below 60 GFR test result may indicate kidney-related diseases
A below 15 GFR test results may indicate severe kidney failure

Additionally, your doctor may also conduct 24-hour urine testing, such as:

Microalbuminuria: This test searches for a specific protein known as albumin in your urine. In general, a urine albumin level below 30 is considered normal, whereas an albumin level above 30 indicates that you may have kidney problems.

Urinalysis: This test examines your urine and looks for blood and proteins.

Purpose of Kidney Function Tests

Conditions such as high sugar levels or diabetes and high blood pressure or hypertension are two major conditions that have an impact on how well the kidneys work. If someone has either of these conditions, their doctor may prescribe KFT or kidney function tests to monitor their progress.

Similarly, if one is presented with symptoms that point to kidney problems, the person may also require a kidney function test. Among these symptoms are:

increasing blood pressure
blood present in the urine
frequent urination
difficulty urinating
pain during urination
buildup of fluids in the body leading to swelling of the hands and feet

Are There any Preparations Needed Before the Kidney Function Test (KFT)?

No, you do not need to take any preparation before your KFT test. You can either visit your nearest laboratory for KFT tests or can have your blood drawn for the KFT test at your home by trained professionals.

Procedure of Kidney Function Tests

As mentioned in the article earlier, 24-hour urine samples and blood tests are usually key requirements for kidney function tests.

Urine Test After 24 Hours

A 24-hour urine sample culture is often used to determine creatinine clearance tests. It helps to identify the level or amount of creatinine a patient’s body removes in a single day. Doctors provide a special container to store urination. During the collection process, it is advised to keep the container sealed and refrigerated. Make sure to properly mark the container and explain the reason to store it in the refrigerator to other family members.

Blood Samples

BUN and serum creatinine tests both require blood specimens to be taken in a lab or health clinic. Before drawing blood, the technician ties a band around the upper arm. This highlights the veins from which blood needs to be drawn. After that, the technician cleans the area and inserts a hollow needle into the patient’s vein through the skin. The blood will be collected with the help of a test tube and sent to a lab for further analysis.

When Should You Go for a Kidney Function Test?

KFT helps you detect any kidney-related health conditions that you might have. However if you have any of the following health conditions, or have certain lifestyle habits, kidney function tests are more crucial for you:

Any kidney-related health conditions
Any symptoms related to kidney damage
Any kidney-damaging health conditions, like high blood pressure or diabetes
Have a family history of kidney-damaging conditions
Aged above 60
A heavy smoker

If you think you have any of the above conditions, check your kidney health today. Book a kidney function test online and get tested from the comfort of your home by trained professionals.

What are The Different Types of Kidney Function Tests (KFT)?

To diagnose the proper health of your kidneys your physician may prescribe different types of kidney function tests. The following are the different tests that are commonly included in KFT:

Serum Creatinine

For adult men, the normal range of serum creatinine lies between 0.74 to 1.35 mg/dL
For adult women, the normal range of serum creatinine lies between 0.59 to 1.04 mg/dL

Uric Acid

In general, the normal range of uric acid lies between 3.7-9.2 mg/dL.

Blood Urea Nitrogen (BUN)

In general, the normal range of BUN lies between 7-25 mg/dL.

Electrolytes

Estimated Glomerular Filtration Rate (eGFR)

The GFR calculates your kidneys’ ability to filter waste materials from the body. Your normal GFR results may not always correspond to your normal serum creatinine level. To evaluate your proper renal function, it is, therefore, preferable to estimate glomerular filtration rate or eGFR, also known as the “gold standard” measurement. Your serum creatinine, age, race, and sex, all will be used to estimate your GFR. Because this test result roughly equates to your remaining kidney function, this test helps you get easy-to-interpret information regarding the degree of your kidney damage.

In general,

The normal level of GFR is considered at 60 or above
A below 60 GFR test result may indicate kidney-related diseases
A below 15 GFR test results may indicate severe kidney failure

Additionally, your doctor may also conduct 24-hour urine testing, such as:

Microalbuminuria: This test searches for a specific protein known as albumin in your urine. In general, a urine albumin level below 30 is considered normal, whereas an albumin level above 30 indicates that you may have kidney problems.

Urinalysis: This test examines your urine and looks for blood and proteins.

Purpose of Kidney Function Tests

Similarly, if one is presented with symptoms that point to kidney problems, the person may also require a kidney function test. Among these symptoms are:

increasing blood pressure
blood present in the urine
frequent urination
difficulty urinating
pain during urination
buildup of fluids in the body leading to swelling of the hands and feet

Are There any Preparations Needed Before the Kidney Function Test (KFT)?

Procedure of Kidney Function Tests

As mentioned in the article earlier, 24-hour urine samples and blood tests are usually key requirements for kidney function tests.

Urine Test After 24 Hours

Blood Samples

When Should You Go for a Kidney Function Test?

Any kidney-related health conditions
Any symptoms related to kidney damage
Any kidney-damaging health conditions, like high blood pressure or diabetes
Have a family history of kidney-damaging conditions
Aged above 60
A heavy smoker

A large number of older dogs and cats develop kidney disease or failure. With correct diagnostic testing, the problem can be caught early enough for more successful treatment.

Kidney disease and failure is one of the most common disorders I see in dogs and cats. It’s extremely prevalent as animals age, especially in those eight years of age and up. While it can be fatal, kidney disease can usually be treated and even cured if caught early. Let’s look at the testing that allows for the early diagnosis of this frequently-seen health problem in dogs and cats.

Kidney disease is the presence of functional or structural abnormalities in one or both kidneys. It is recognized by reduced kidney function or the presence of kidney damage. A decline in function is not essential to meet the definition of kidney disease. Kidney damage is defined as either: 1) microscopic or macroscopic renal pathology detected by kidney biopsy or direct visualization of the kidneys or 2) markers of renal damage detected by blood or urine tests or imaging studies (Table 1).³ Both the severity and clinical implications of kidney disease varies greatly depending on the magnitude of kidney involvement. Kidney disease is staged (described below) to reflect these variations. The term “kidney disease” may be applied to acute or chronic disease.

Table 1. Markers of kidney damage.*

Blood markers:

BUN concentration

Serum creatinine

Hyperphosphatemia

Hyperkalemia or hypokalemia

Metabolic acidosis

Hypoalbuminemia

Urine markers:

Impaired urine concentrating ability

Proteinuria

Cylindruria

Renal hematuria

Inappropriate urine pH

Inappropriate urine glucose concentration

Cystinuria

Imaging markers–abnormalities in kidney:

Size

Density

Shape

Number

Location

Mineralization

* Markers must be confirmed to be of renal origin to be evidence of kidney damage. For example, hypoalbuminemia due to urinary protein loss is evidence of kidney disease, while hypoalbuminemia due to hepatic failure is not.

Chronic kidney disease (CKD) is defined as: 1) kidney damage that has existed for at least three months, with or without decreased glomerular filtration rate (GFR), or 2) a reduction in GFR by more than 50% from normal persisting for at least three months. A duration of at least 3 months is used as the benchmark criterion for confirming the diagnosis of CKD based on the observation that renal compensatory hypertrophy and improvement in renal function may continue for up to three months following acute loss of nephrons.

Staging CKD

Patients with CKD can be categorized into stages along a continuum of progressive CKD. The value of staging CKD is to facilitate application of appropriate clinical practice guidelines for diagnosis, prognosis and treatment. The International Renal Interest Society (IRIS) has proposed a 4 tier system for staging CKD in dogs and cats (Tables 2 and 3). Although the specific values used to categorize patients with CKD into these stages are inherently arbitrary, staging is nonetheless useful for establishing prognosis and managing patients with CKD.

Table 2. Stages of CKD in dogs and cats.

*Stage*	*Serum creatinine values (mg/dl / μmol/L)*
*Stage*	*Dogs*	*Cats*
Stage 1	<1.4 / <125	<1.6 / <140
Stage 2	1.4-2.0 / 125-179	1.6-2.8 / 140-249
Stage 3	2.1-5.0 / 180-439	2.9-5.0/ 250-439
Stage 4	>5.0 / >440	>5.0 / > 440

Table 3. Classification of proteinuria by UP:C ratio.*

*Classification*	*Urine protein: Creatinine ratio*
*Classification*	*Dogs*	*Cats*
Proteinuric (P)	>0.5	>0.4
Borderline proteinuric (BP)	0.2-0.5	0.2-0.4
Non-proteinuric (NP)	<0.2	<0.2

* ACVIM Consensus Statement on Proteinuria (Lees, 2005)

The stage of CKD is assigned based on the level of kidney function. While not the only kidney function, the level of GFR is accepted as the best measure of overall kidney function in health and disease.³Ideally, two or more serum creatinine values obtained when the patient is fasted and well hydrated should be determined over several weeks to stage CKD. Further, variations between laboratories, patient-specific characteristics (e.g., breed, age, gender, body condition and lean body mass) and transient prerenal and postrenal events may influence serum creatinine values. Reduced muscle mass, a common manifestation of advanced CKD, may result in a substantial reduction in serum creatinine concentration relative to true GFR. Because of these variations, published reference ranges for serum creatinine are often exceedingly broad. Using the IRIS staging system, some patients classified as having CKD stage 2 may have serum creatinine values within published reference ranges. As a consequence, the patient’s overall clinical status should be considered when interpreting serum creatinine concentration and other laboratory tests and when planning patient management.

It is important to understand that evidence of CKD should be sought beyond just the serum creatinine value. Just knowing that a dog has a serum creatinine of 2.6 mg/dl does not mean that the dog has Stage III CKD. Creatinine can be used as a marker of kidney disease as discussed previously. True normal values for serum creatinine are usually, but not invariably, quite low (less than 1.4 mg/dl). When creatinine increases above this value, it may be due to renal, prerenal or postrenal causes. It is essential that values above this standard be examined to ascertain which of these causes explains the increase in serum creatinine. Serum creatinine should never be interpreted without consideration of the clinical finding in the patient including examination of the urinalysis (with emphasis on the urine specific gravity). Even though some laboratories report a “normal range” that extends above 1.4 mg/dl, one should question values above 1.4 mg/dl by examining a concurrently collected urine specific gravity. For example, a serum creatinine value above 1.4 mg/dl (in a dog) that is associated with a urine specific gravity of 1.014 implies something entirely different that the same serum creatinine value with a urine specific gravity of 1.039. Values lying between 1.4 mg/dl and the upper limit for a given laboratory would be expected to be associated with relatively concentrated urine to be interpreted as true evidence of normal kidney function (i.e., specific gravity values above 1.03 in dogs and 1.035 in cats).

Stage 1 CKD includes dogs and cats with CKD that are not azotemic, while stage 2 CKD includes dogs and cats that are mildly azotemic (Table 2). Patients in these stages of CKD typically do not have clinical signs of kidney dysfunction with the exception of polyuria and polydipsia.

Occasionally cats with stage 2 CKD may have weight loss or selective appetites. However, patients may have clinical signs resulting from their kidney lesions (e.g., acute pyelonephritis, nephrolithiasis). Patients with marked proteinuria or systemic hypertension due to CKD may have clinical signs related to these aspects of CKD. Renal function is often stable or only very slowly progressive for an extended period in non-proteinuric, non-hypertensive dogs and cats with stages 1 and 2 CKD. However, when progression does occur in this group of patients, it may occur largely as a consequence of their primary CKD.Patients with stages 1 and 2 CKD should be evaluated with the goals of identifying and providing specific treatment for their primary CKD.

Patients with moderate azotemia are classified as stage 3 CKD. Patients in this stage may have clinical signs referable to their loss of kidney function; however, with appropriate treatment, they typically do not have clinical signs of overt uremia.

Patients with stage 3 CKD may progress due to inherent mechanisms of spontaneous progression as well as their underlying CKD. Therefore, in addition to identifying and treating primary CKD, therapy designed to modify factors promoting progression of renal disease may be of benefit to these patients.

Stage 4 CKD includes dogs and cats with severe azotemia (serum creatinine values greater than 5.0 mg/dl). This stage is also called chronic kidney failure and is frequently associated with clinical signs that occur as a consequence of loss of kidney function. Diagnostic and therapeutic initiatives in this stage include those appropriate for stage 3 patients as well as therapy designed to prevent or ameliorate signs of uremia.

It is useful therapeutically and prognostically to further subclassify patients according to their urine protein loss and systemic blood pressure. Proteinuria and hypertension may influence prognosis and may be amenable to therapeutic intervention. Classification of patients as proteinuric necessitates eliminating hemorrhage and/or inflammation as the cause for proteinuria and determination of the urine protein-to-creatinine ratio. Further, proteinuria should be shown to be persistent by reexamining the UPC ratio 2 to 3 times over at least one or two months. For both dogs and cats, patients are classified as proteinuric (P) when their protein-to-creatinine ratio exceeds 0.5 and 0.4, respectively (Table 4). Patients with borderline proteinuria should be re-evaluated after two months to reassess classification. In some patients, classification of proteinuria may change due to the natural course of their disease or in response to therapy.

Table 4. Blood pressure risk groups for dogs and cats.

*Risk level*	*(Designator)*	*Systolic blood pressure range*
Minimal	(0)	<150/95 mmHg
Low	(1)	150/95 to 159/99 mmHg
Moderate	(2)	160/100 to 179/119 mmHg
Severe	(3)	>180/120 mmHg

The lack of consensus as to what blood pressure values constitute hypertension in dogs and cats obfuscates any classification of patients as to their hypertension status. It is likely that “normal” blood pressure values for dogs and cats will change as more data becomes available. A 2003 ACVIM Hypertension Consensus Group has proposed the following hypertension classification system for dogs and cats.