Renal Function Tests Overview

Renal Function Tests (RFT)

• RFT are a group of tests used to evaluate kidney function
• Clinical significance of RFT includes assessing kidney function, diagnosing kidney diseases, predicting disease progression and severity, monitoring treatment response, managing medication, and early detection of kidney impairment.
• RFT should be assessed in conditions such as family history of chronic kidney disease, diabetes mellitus, hypertension, advanced age, autoimmune diseases, systemic infections, urinary tract infections, drug toxicity, and kidney stones.

Renal Function Tests - Components

• Urine analysis is performed
• Glomerular function tests are conducted
• Tubular function tests are included

Glomerular Function Tests

• Blood urea and BUN
• Serum creatinine
• BUN:creatinine ratio
• Serum uric acid
• Clearance measurements are included

Clearance Measurements

• Clearance is the volume of plasma from which a substance can be completely eliminated into the urine per unit of time (ml/min)
• Substances used should be synthesized at a constant rate, completely filtered by the glomeruli, neither reabsorbed nor secreted by the renal tubules, and easy to measure, accessible, and inexpensive
• Endogenous markers like creatinine and cystatin C are used
• Exogenous markers like inulin are also utilized

Markers (Substances)

• Endogenous markers: creatinine and cystatin C
• Exogenous marker: inulin

Clearance Measurements (Continued)

• Measurement of clearance estimates the Glomerular Filtration Rate (GFR).
• GFR is the volume of plasma filtered by glomerular capillaries per unit of time.

Creatinine Clearance

• Calculation of creatinine clearance is a standard laboratory method to determine GFR.
• Formula for Creatinine Clearance is: CCr = UCr (mg/dL) × VUr (mL) ×1.73 / PCr(mg/dL) × 1440 (min)
• Where:
• CCr = creatinine clearance
• UCr = urine creatinine
• VUr = urine volume excreted in 24 hours
• PCr = serum creatinine concentration
• 1.73 = generally accepted average body surface area
• A = actual body surface area of the individual

Reference Ranges

• Female: 88 – 128 ml/min
• Male: 97 – 137 ml/min
• Decreased GFR results due to age related physiological decreases by about 6.5 mL/min for each decade, or due to pathological decrease (renal failure).

Serum creatinine & Creatinine Clearance

• Serum creatinine is inversely proportional to creatinine clearance.

Estimated Glomerular Filtration Rate (eGFR)

• Cockcroft-Gault formula for eGFR is (140 - Age)×Weight(kg) / 72 × SCr(mg/dL) x (0.85 if female)

Urine Microalbumin & Albumin: Creatinine Ratio

• Microalbuminuria is the presence of small amounts of albumin in urine.
• Important in managing patients with diabetes at risk of nephropathy.
• Quantitative albumin-specific immunoassays, often using nephelometry or immunoturbidimetry, are widely used.
• Normal urinary excretion of albumin is <30 mg/day.
• Albumin concentrations of 50-200 mg/day are predictive of diabetic nephropathy.
• An albumin/creatinine ratio of 20-30 mg/g is indicative of microalbuminuria

Tests of Renal Tubular Function

• Electrolyte measurements (sodium, potassium, calcium, phosphate)
• Osmolality measurements
• Glycosuria and aminoaciduria
• Water deprivation test