BUN-CREA-BUA

Kidney Function Tests

  • Structure of the Kidneys:

    • Bean-shaped organs in the retroperitoneal space.

    • Divided into two regions: cortex (outer) and medulla (inner).

    • Nephrons are the functional units comprising glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

  • Functions of the Kidneys:

    1. Elimination of waste products.

    2. Maintenance of blood volume.

    3. Regulation of electrolyte balance.

    4. Maintenance of acid-base balance.

    5. Endocrine functions, including erythropoietin secretion.

Tests for Glomerular Filtration Rate (GFR)

  • GFR Defined:

    • Measure of clearance of molecules freely filtered by glomeruli.

    • Ideal indicator of kidney function, with normal filtration rate around 150 liters per day.

    • Declines by 1.0 mL/minute/year post age 20.

  • Clearance:

    • Represents the volume of plasma cleared of a solute per minute.

    • Inversely related to plasma concentration.

    • Clearance formula:[ \text{Clearance (mL/min)} = \frac{A \times u \times \text{Volume (mL)}}{P \times 1.73} ]

      • A: Body surface area

      • U: Urine concentration of analyte

      • P: Plasma concentration of analyte

Methods of Assessing Renal Function

  • Inulin Clearance:

    • Reference method, requires IV infusion and timed collections.

    • Values: Male: 127 mL/min, Female: 118 mL/min.

  • Creatinine Clearance:

    • Estimates plasma flow through glomeruli.

    • Normal values: Male: 85-125 mL/min; Female: 75-112 mL/min.

  • Urea Clearance:

    • Reflects response to therapy and progression of renal disease.

  • Cystatin C:

    • Constantly produced and filtered; increases in GFR impairment.

    • Reference value: 0.5-1.0 mg/L.

  • β-Trace Protein:

    • Indicator of renal disease; correlates with GFR.

Blood Urea Nitrogen (BUN)

  • Characteristics:

    • Main product of protein metabolism; synthesized in the liver.

    • Elevated in renal dysfunction; normal range: 8-23 mg/dL.

  • Interference and Methods:

    • Urea measured using enzymatic methods and colorimetric methods.

Creatinine

  • Overview:

    • Byproduct of muscle metabolism; constant production linked to muscle mass.

    • Reference values: Male: 0.9-1.3 mg/dL; Female: 0.6-1.1 mg/dL.

    • Major methods include Direct Jaffe and Enzymatic methods.

Disease Correlations

  • Types of Azotemia:

    • Pre-renal: Decreased blood flow.

    • Renal: Kidney damage.

    • Post-renal: Urinary tract obstruction.

  • Uremia: Clinical syndrome marked by high plasma urea levels and electrolyte imbalance.

Blood Uric Acid Tests

  • Function and Normal Values:

    • Product of purine metabolism; normal excretion ~1g/day.

    • Reference values: Male: 3.5-7.2 mg/dL; Female: 2.6-6.0 mg/dL.

  • Hyperuricemia Causes:

    • Gout, increased metabolism (cancers), chronic renal disease.

  • Testing Methods:

    • Enzymatic and chemical methods to determine uric acid levels.

Measuring Tubular Function

  • Excretion Tests: Assess renal plasma flow and tubular mass.

  • Concentration Tests: Measure urine solute concentration, assessing renal damage.

    • Specific Gravity and Osmolality: Key indicators of renal concentrating ability, reflecting solute particles in urine.

    • Reference values for urine osmolality: 300-900 mOsm/kg.