Renal Function Tests Overview

Study Notes

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.

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

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

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

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).

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

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