# Nephrology Question 6 Instructor

Calculate the creatinine clearance in a patient with a stable serum creatinine of 278 micromol/L and a 24 hour urine collection with 1.2 L and 9.4 mmol/L creatinine.

Creatinine clearance (CrCl) represents the volume of blood that is cleared of creatinine by the kidney per unit time. If we assumed that all creatinine in the urine got there by being filtered through the glomerulus, then the total volume of blood cleared of creatinine (CrCl) multiplied by the concentration of creatinine in the serum should equal the total amount of creatinine found in the urine.

CrCl * SCr = Total urine creatinine

Since urine creatinine can be calculated by multiplying the urine creatinine concentration by the total volume of urine, the formula can be converted to:

CrCl * SCr = UCr * V

Which converts to:

CrCl = [UCr * V] / SCr

This formula determines the total volume cleared in a given period of time, which by convention we measure over 24 hours but report as ml/min.

Please note that the term [UCr * V] / SCr solves for a volume. This must then be divided by the total time of 1 day or 24 hours or 1440 minutes.

In this case:
CrCl = [UCr * V] / SCr
CrCl = [(9.4 mmol/L * 1.2 L) / 0.278 mmol/L]/day
= 40.58 L/day
= 40.58 L/day * 1000 ml/L * 1/1440min/day
= 28.2 ml/min

Why does this likely represent an overestimation of the true glomerular filtration rate (GFR)?

This is an overestimation of true GFR because some of the urinary creatinine was secreted, in addition to majority that was filtered. Creatinine secretion accounts for a larger proportion of urinary creatinine at lower GFRs. This results in the urine creatinine (UCr) being higher than it should be. This makes the term:

CrCl = [UCr * V] / SCr

higher than it should be as the numerator is increased.

Therefore, creatinine clearance calculated based on a 24 hour urine collection tends to over-estimate true GFR, especially in patients with more advanced chronic kidney disease.

Review more details about renal function measurement:
Mitchell H. Rosner, W. Kline Bolton. Renal Function Testing. AJKD. Volume 47, Issue 1, Pages 174-183 (January 2006).

Rosner MH, Bolton WK. Renal function testing. Am J Kidney Dis. Jan 2006;47(1):174-83.