Kidney Function Tests (KFT) – Types, Who Should Get it

What is the normal function of kidneys?

Kidneys are bean-shaped organs and are the size of a fist. Two kidneys lie on either side of the spine, posterior to the abdomen and below the rib cage. 

Kidneys maintain the body’s level of water and various essential minerals. They filter waste materials and excess fluid from the blood and expel them from the body as urine. 

Additional functions of the kidneys include the production of:

• Vitamin D
• Red blood cells
• Hormones that regulate blood pressure

What are the symptoms of kidney problems?

Symptoms that may indicate a problem with kidneys include:

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

When would a physician recommend a KFT? 

• If the physician thinks kidneys may not be working based on risk factors, clinical signs, and symptoms
• To screen for and assess the cause for kidney dysfunction
• If conditions that can harm the kidneys, such as diabetes or high blood pressure, also coexist
• To monitor kidney disease progression and response to treatment

What are the types of KFTs?

The different methodologies to evaluate kidney function include: 

• Urinalysis: It screens for protein, blood, pus, bacteria, and sugar in the urine. It can help detect various kidney and urinary tract disorders, including chronic kidney disease, diabetes, bladder infections, and kidney stones. Since there are different reasons for protein in the urine, kidney disease may not always be a causal factor. The physician may repeat this test after a few weeks to see if the results are similar. Some urine tests require only a small volume, while others require a urine collection over the last twenty-four hours. A 24-hour urine test will demonstrate how much urine your kidneys produce and accurately measure how well your kidneys are working daily.

A container will be provided for urine sample collection and subsequently sent to a lab for analysis at the physician’s clinic or hospital. 

• Urinary Protein: The presence of protein in urine is known as proteinuria. You may have this measurement as part of urinalysis or a dipstick test. A positive dipstick test (≥1+) should be confirmed by:

• Checking for microalbuminuria: This is a more sensitive dipstick test that can detect a tiny amount of albumin (the most common type of urinary protein). Higher amounts of albumin in the urine may signal renal dysfunction. Individuals at risk for renal dysfunction (for example, those with diabetes or hypertension) should have this test or an albumin-to-creatinine ratio (ACR) if their standard dipstick test for proteinuria is negative.
• ACR: When we divide the amount of urinary albumin by urinary creatinine, we get the ACR. An ACR < 30 is considered normal. An ACR between 30-300 indicates moderate albuminuria. An ACR > 300 means severe albuminuria. 

• Creatinine Clearance: This assessment compares the creatinine level in a 24-hour sample of urine to the creatinine quantity in the blood to indicate how much waste products the kidneys are filtering out each minute.

• Blood samples: The two tests which require a blood sample taken at the physician’s clinic or hospital setting include BUN and serum creatinine tests. The technician first ties an elastic band around the upper arm, which makes the veins in the bend of the arm more prominent. The technician cleans the area over the vein, following which they slip a hollow needle through the skin and into the vein, making blood flow into a test tube. After the test, the technician will place gauze and a bandage over the puncture site. The technician will send the tube to a lab for analysis.

• Serum creatinine test: Creatinine is a normal breakdown product of muscles’ physiological wear and tear, which is usually completely filtered from the blood by the kidneys. According to the National Kidney Foundation, a creatinine level > 1.2 milligrams/deciliter (mg/dL) for women and> 1.4 mg/dL for men indicates kidney dysfunction. With progressive renal dysfunction, the level of creatinine in the blood rises. The serum creatinine test is a vital assessment of renal function and determines elevation in serum creatinine.

• Blood urea nitrogen (BUN): The BUN test helps determine the amount of urea nitrogen, a breakdown product of dietary protein, in the blood. Along with kidney damage, elevated BUN may also arise from the intake of certain medications, including aspirin and some types of antibiotics. Individuals need to notify their physicians regarding their drug and supplement intake history. If this test is required, the individual will need to stop these medications for a few days before the test. A typical BUN level ranges from 7 and 20 mg/dL. As renal function progressively declines, the BUN level rises, and a higher value could indicate health issues, including kidney dysfunction.

• Glomerular Filtration Rate (GFR): This test estimates how well the kidneys filter waste and excess fluid from the blood. We can determine the rate by considering various clinical parameters:
  • Serum creatinine level
  •  Age
  •  Gender
  •  Race
  •  Height
  •  Weight

This test measures how well the kidneys remove wastes and excess fluid from the blood. Normal GFR can vary according to age (can decrease with age). We can calculate the GFR as millilitres/minute/1.73m². The standard value for GFR is 90 or above. A GFR below 60 signifies that some form of renal dysfunction has set in. If someone’s GFR falls below 15, they are at high risk of requiring dialysis or a transplant. 

Imaging Tests that can assist with consolidating diagnoses along with KFTs

• Ultrasound and CT: They may be used to look for abnormalities in kidney size or position or obstructions such as stones or tumours.
• We may need a kidney biopsy in the following circumstances:
  • To identify disease onset and progression and determine potential response to treatment
  • To evaluate the extent of renal damage 
  • To find out causes for kidney transplant rejections

What are the treatment approaches for early kidney disease?

If KFTs indicate underlying kidney disease, the physician’s management intervention will target the causal pathology. In addition to educating the individual regarding lifestyle and dietary modifications, physicians may prescribe medications to control hypertension and diabetes. A referral to a cardiologist or endocrinologist may be required depending on test results and the overall clinical picture. Repeat KFTs will be needed to monitor progression and response to treatment.