Renal Function Test

Mr. Arvind Kumar

Introduction 

  • A Renal Function Test (RFT) is a series of blood and urine tests used to evaluate the performance of the kidneys.
  • These tests help assess the kidneys’ ability to filter waste products, balance electrolytes, and regulate fluid in the body.
  • The kidneys play a vital role in maintaining homeostasis by removing waste products, excess water, and regulating various functions like blood pressure and red blood cell production.
  • Renal function tests are commonly performed to diagnose kidney diseases such as chronic kidney disease (CKD), acute kidney injury (AKI), glomerulonephritis, and to monitor kidney health in individuals with conditions like diabetes, hypertension, or those on medications that may affect kidney function.
  • Key tests involved in RFT include:
    • Serum creatinine,
    • Blood urea nitrogen (BUN),
    • Glomerular filtration rate (GFR),
    • Electrolytes,
    • Urine albumin and protein
    • Urine analysis.

Specimen collection

Blood Sample Collection:

  • Inform the patient and check fasting requirements.

  • Perform venipuncture, collect blood, apply pressure to the site.

  • Label and send to the lab.

Urine Sample Collection:

  • Instruct the patient to clean the genital area.

  • Collect a midstream sample in a sterile container.

  • Label and send to the lab within 1–2 hours.

Blood Urea (BUN)

  • Blood Urea Nitrogen (BUN) is a waste product formed in the liver from the breakdown of protein.
  • It is carried in the blood to the kidneys, where it is filtered out and excreted in the urine.
  • Measuring BUN levels helps assess kidney function and the body’s ability to remove nitrogenous waste.

Normal Range:

  • Adult: 7 to 20 mg/dL

  • Children: 5 to 18 mg/dL

Test Method Name:

  • Enzymatic Method: This is the most common method, where urease is used to hydrolyze urea, and the resulting ammonia is measured.

  • Colorimetric Method: A reaction that produces a color change in the presence of urea, allowing its quantification.

Clinical Significance:

  • Increased BUN:

    • High levels can indicate kidney dysfunction

    • Acute kidney injury,

    • Chronic kidney,

    • Dehydration,

    • High protein intake.

  • Decreased BUN: Low levels can occur in cases:

    • Liver disease,

    • Malnutrition,

    • Overhydration.

Serum Creatinine

  • Serum creatinine is a waste product produced by muscles during normal metabolic processes.
  • It is filtered out of the blood by the kidneys.
  • Measuring serum creatinine levels is a key test in assessing kidney function, as it reflects the kidneys’ ability to filter out waste products.

Normal Range:

  • Adult male: 0.6 to 1.2 mg/dL

  • Adult female: 0.5 to 1.1 mg/dL

  • Children: 0.2 to 0.7 mg/dL (depending on age)

Test Method Name:

  • Jaffe Reaction: A colorimetric method where creatinine reacts with alkaline picrate, producing a red-orange color that is measured.

  • Enzymatic Methods: Uses enzymes to break down creatinine, measuring the resulting change in absorbance.

Clinical Significance:

  • Increased Creatinine:

    • High levels indicate impaired kidney function,

    • Acute kidney injury,

    • Chronic kidney disease,

    • Dehydration,

    • Muscle injury.

  • Decreased Creatinine:

    • Low levels may be seen in cases:

    • Reduced muscle mass,

    • Malnutrition,

    • Certain liver diseases.

Serum Sodium

  • Serum sodium is an essential electrolyte that plays a key role in maintaining fluid balance, blood pressure, and proper nerve and muscle function.
  • It is primarily regulated by the kidneys and is critical in maintaining the body’s osmotic balance and hydration status.

Normal Range:

  • Adult: 135 to 145 mEq/L

Test Method Name:

  • Ion-Selective Electrode (ISE) Method: This is the most common method, using electrodes that specifically measure sodium ion concentration in the blood.

  • Flame Photometry: An older method, where the intensity of light emitted by sodium ions is measured.

Clinical Significance:

  • Increased Sodium (Hypernatremia):

    • High serum sodium levels can indicate dehydration,

    • Excessive sodium intake,

    • Diabetes insipidus

    • Hyperaldosteronism.

  • Decreased Sodium (Hyponatremia):

    • Excessive fluid retention

    • Heart failure, 

    • Kidney disease,

    • Excessive sweating,

    • Hormonal imbalances

    • Adrenal insufficiency

Serum Potassium

  • Serum potassium is a crucial electrolyte that helps regulate nerve function, muscle contractions, and fluid balance.
  • It is primarily maintained by the kidneys and plays a vital role in maintaining the electrical charge of cells, especially in the heart and muscles.

Normal Range:

  • Adult: 3.5 to 5.0 mEq/L

Test Method Name:

  • Ion-Selective Electrode (ISE) Method: This is the most common technique, which directly measures the concentration of potassium ions in the blood.

  • Flame Photometry: An older method that measures the intensity of light emitted by potassium ions.

Clinical Significance:

  • Increased Potassium (Hyperkalemia):

    • High levels can indicate kidney dysfunction,

    • Adrenal insufficiency,

    • Acidosis, or

    • Excessive potassium intake

    • Dehydration

    • Trauma is particularly dangerous as it may lead to life-threatening heart arrhythmias.

  • Decreased Potassium (Hypokalemia):

    • Low levels may result from conditions such as excessive loss due to vomiting,

    • Diarrhea,

    • Diuretic use.

    • It can lead to muscle weakness,

    • Arrhythmias,

    • Other cardiac issues.

Glomerular Filtration Rate Test

  • The Glomerular Filtration Rate (GFR) is a key indicator of kidney function.
  • It measures how well the kidneys are filtering blood, specifically how much blood is being cleared of waste (such as creatinine) per minute.
  • GFR is considered the best estimate of overall kidney function and is used to diagnose and stage chronic kidney disease (CKD).

Normal Range:

  • Normal GFR: 90–120 mL/min/1.73 m²

  • GFR below 60 mL/min/1.73 m² for 3 months or more indicates CKD.

Test Method Name:

  • Estimated GFR (eGFR): Most commonly calculated using serum creatinine levels, age, gender, and ethnicity using equations like the CKD-EPI or MDRD formula.

  • 24-hour urine collection: A less commonly used method that measures the total volume of urine filtered by the kidneys over a 24-hour period.

Clinical Significance:

  • Increased GFR: A GFR higher than normal can occur temporarily, for example, due to increased fluid intake or in pregnancy, but is not generally a sign of concern.

  • Decreased GFR: A GFR lower than 60 mL/min/1.73 m² for 3 months or more may indicate kidney dysfunction. It is used to assess the stage of CKD:

    • Stage 1: GFR 90 or more (normal or high)

    • Stage 2: GFR 60–89 (mildly decreased)

    • Stage 3: GFR 30–59 (moderately decreased)

    • Stage 4: GFR 15–29 (severely decreased)

    • Stage 5: GFR below 15 (kidney failure, requiring dialysis)

Urine albumin and protein

  • Albuminuria and Proteinuria refer to the presence of abnormal amounts of proteins, particularly albumin, in the urine.
  • These conditions are often indicators of kidney damage or disease, as healthy kidneys usually do not allow significant amounts of protein to pass into the urine.

  • Albuminuria specifically refers to the presence of albumin (the most abundant protein in the blood) in the urine, while proteinuria refers to the presence of any type of protein in the urine, including albumin.

Normal Range:

  • Albuminuria:

    • Normal: Less than 30 mg of albumin per gram of creatinine in urine.

    • Microalbuminuria (early stage): 30-300 mg of albumin per gram of creatinine.

    • Macroalbuminuria (severe): Greater than 300 mg of albumin per gram of creatinine.

  • Proteinuria:

    • Normal: Less than 150 mg of protein per day in urine.

    • Persistent proteinuria (pathological): More than 150 mg of protein per day.

Test Method Name:

  • Albuminuria:

    • Urine Albumin-to-Creatinine Ratio (UACR): A urine test that measures the amount of albumin in a urine sample, normalized to the concentration of creatinine, to account for urine dilution.

    • Dipstick Urinalysis: A quick screening test that detects albumin in the urine.

  • Proteinuria:

    • 24-hour Urine Collection: Measures total protein excreted in the urine over 24 hours.

    • Urine Dipstick Test: A screening tool for detecting protein levels, though it is less accurate than the 24-hour collection.

    • Urine Protein-to-Creatinine Ratio: A test that provides an estimate of total protein in the urine.

Clinical Significance:

  • Albuminuria:

    • Increased Albuminuria is often the earliest sign of kidney damage, especially in conditions like diabetic nephropathy or hypertension. It is associated with a higher risk of kidney disease progression and cardiovascular problems.

    • Microalbuminuria can indicate early kidney damage and requires monitoring, as it may progress to overt kidney disease if not managed.

  • Proteinuria:

    • Increased Proteinuria is typically a sign of more significant kidney damage. It can occur in diseases like glomerulonephritis, diabetic nephropathy, hypertension, and nephrotic syndrome.

    • Persistent proteinuria is a strong indicator of kidney dysfunction and can worsen kidney function over time, leading to kidney failure if untreated.