Tuesday, July 28, 2009

MARKERS OF KIDNEY FUNCTION

GFR is the speed of flow of the fluid that has been filtered through the kidney.

Creatinine Clearance is the volume of blood that is cleared of creatinine over a set time period.

To calculate both, you need to measure how much creatinine is in blood and urine at the same time.

The actual calculations are:

Creatinine clearance (CCr) can be calculated if values for creatinine's urine concentration (UCr), urine flow rate (V), and creatinine's plasma concentration (PCr) are known. Since the product of urine concentration and urine flow rate yields creatinine's excretion rate, creatinine clearance is also said to be its excretion rate (UCr×V) divided by its plasma concentration. This is commonly represented mathematically as

C_{Cr} = \frac { U_{Cr} \times V }{ P_{Cr} }

Example: A person has a plasma creatinine concentration of 0.01 mg/ml and in 1 hour produces 60ml of urine with a creatinine concentration of 1.25 mg/mL.

C_{Cr} = \frac {1.25 mg/mL \times \frac{60ml}{60min}}{0.01 mg/ml} = \frac { {1.25 mg/mL} \times {1 mL/min}}{0.01 mg/mL} = \frac {1.25 mg/min}{0.01 mg/mL} = {125 mL/min}

Commonly a 24 hour urine collection is undertaken, from empty-bladder one morning to the contents of the bladder the following morning, with a comparative blood test then taken. The urinary flow rate is still calculated per minute, hence:

C_{Cr} = \frac { U_{Cr} \ \times \ \mbox{24-hour volume} }{P_{Cr} \ \times \ 24 \times 60 mins}

To allow comparison of results between people of different sizes, the CCr is often corrected for the body surface area (BSA) and expressed compared to the average sized man as mL/min/1.73 m2. While most adults have a BSA that approaches 1.7 (1.6-1.9), extremely obese or slim patients should have their CCr corrected for their actual BSA.

C_{Cr-corrected} = \frac{{C_{Cr}} \ \times \ {1.73}} {BSA}
BSA can be calculated on the basis of weight and height.
Posted by at

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)