Introduction
The kidneys are vital organs responsible for maintaining the body’s internal balance by excreting waste products, regulating fluid and electrolyte balance, and producing essential hormones. Kidney Function Tests (KFTs) are a group of biochemical tests used to assess how efficiently the kidneys perform these tasks. They help detect renal damage, monitor disease progression, and evaluate treatment response.
Role of the Kidney
- The kidney is an essential organ with multiple functions.
- The nephron is the structural and functional unit of the kidney, consisting of the glomerulus and renal tubules.
- It maintains water, electrolyte, and acid-base balance through filtration, reabsorption, and secretion.
- It removes non-protein nitrogenous waste products such as urea, uric acid, and creatinine from the blood.
- It also synthesizes hormones like erythropoietin (stimulates RBC formation), calcitriol (active vitamin D), renin (regulates blood pressure), and prostaglandins.
Since both glomeruli and tubules are vital to kidney function, most tests evaluate their performance in filtration or reabsorption efficiency.
Laboratory Tests for Kidney Function
Laboratory investigations play a critical role in diagnosing and assessing renal disorders, as clinical symptoms may be mild or absent in early stages. These tests help detect functional impairments in:
- Glomerular Filtration – measured by creatinine clearance or GFR tests.
- Tubular Function – assessed through urine concentration and dilution tests.
Because the nephron functions as an integrated unit, damage to one part (glomerulus or tubule) often affects the entire nephron. Fortunately, the kidneys have a reserve capacity, so significant damage must occur before lab tests show abnormalities.
Classification of Renal Dysfunction
Renal impairment can be classified into three broad categories based on the origin of dysfunction:
1. Pre-Renal Causes
These conditions affect kidney function indirectly by reducing blood flow or pressure to the kidneys.
- Hemorrhage
- Shock or dehydration
- Cardiac failure
- Prolonged diarrhea or vomiting
- Intestinal obstruction
These are usually reversible once blood flow or fluid balance is restored.
2. Renal Causes
These involve direct structural or functional damage to the kidney itself. Common causes include:
- Glomerular diseases (e.g., glomerulonephritis)
- Tubular diseases (e.g., acute tubular necrosis)
- Vascular diseases (e.g., nephrosclerosis)
3. Post-Renal Causes
These result from obstructions to urine flow such as:
- Kidney stones (calculi)
- Tumors compressing the urinary tract
- Enlarged prostate (benign prostatic hypertrophy)
Obstruction increases pressure in the nephrons and decreases filtration.
Common Kidney Diseases and Their Effects
1. Glomerulonephritis
A diffuse inflammation of the glomeruli caused mainly by Group A β-hemolytic Streptococcal infection. It often presents with hematuria, proteinuria, edema, and hypertension. In chronic cases, progressive renal failure may occur.
2. Nephrotic Syndrome
Characterized by massive proteinuria, hypoalbuminemia, edema, and hyperlipidemia. The glomerular membrane becomes leaky, allowing proteins to pass into urine. It may be associated with systemic lupus erythematosus, amyloidosis, or syphilis. Corticosteroid therapy often helps.
3. Pyelonephritis
An infection of the kidney tissue caused by ascending bacteria. Chronic cases cause fibrosis and scarring of the renal parenchyma, leading to reduced renal function.
4. Arteriolar Nephrosclerosis
Results from thickening and narrowing of small renal arteries due to long-term hypertension. It causes ischemic damage, proteinuria, and progressive renal failure. The benign form progresses slowly, whereas malignant hypertension leads to rapid kidney failure.
5. Renal Tubular Acidosis (RTA)
A group of disorders in which the renal tubules fail to excrete acids adequately, leading to metabolic acidosis. Types include:
- Distal RTA: Defective H⁺ secretion; urine cannot acidify below pH 5.5.
- Proximal RTA: Defective HCO₃⁻ reabsorption; leads to bicarbonate loss.
RTA is often hereditary and may present with hypokalemia, hyperchloremia, and growth retardation in children.
Key Biochemical Parameters in Kidney Function Tests
1. Serum Creatinine
Creatinine is a waste product of muscle metabolism. It is filtered freely by the glomerulus and not reabsorbed by the tubules, making it a reliable marker of glomerular filtration rate (GFR).
- Normal range: 0.6 – 1.3 mg/dL
- Increased levels: Indicate reduced GFR or renal impairment.
2. Blood Urea Nitrogen (BUN)
Urea is the end product of protein metabolism, synthesized in the liver and excreted by the kidney.
- Normal range: 8 – 18 mg/dL
- Increased BUN: Seen in renal failure, dehydration, or high-protein diet.
- Decreased BUN: Seen in liver failure or malnutrition.
3. Creatinine Clearance
Estimates the GFR and overall filtration capacity of the kidneys. It is calculated from serum and urine creatinine values and urine volume collected over a specific time period.
Creatinine Clearance (mL/min) = (Urine Creatinine × Urine Volume) / Plasma Creatinine
4. Urinalysis
Provides insight into both glomerular integrity and tubular function. Tests include measurements of specific gravity, pH, protein, glucose, and detection of blood, bilirubin, and ketones using dipsticks.
Summary Table: Major Kidney Function Tests
| Test | Parameter Assessed | Clinical Significance |
|---|---|---|
| Serum Creatinine | Glomerular Filtration | Increased in renal failure |
| Blood Urea Nitrogen (BUN) | Protein metabolism & renal excretion | Increased in renal or pre-renal conditions |
| Creatinine Clearance | GFR estimation | Decreased in renal disease |
| Urinalysis | Tubular and glomerular integrity | Abnormal findings indicate infection or nephropathy |
| Uric Acid | Purine metabolism | Increased in gout, renal dysfunction |
Detailed Notes:
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