Question

3) What is the creatinine clearance test? How is it be used to estimate glomerular filtration rate?

Answer 1

Creatinine is an endogenous amine produced as a result of muscle catabolism. It is excreted unchanged in the urine by glomerular filtration only. An advantage of this test is that it can be correlated to the steady state concentration of creatinine in plasma and needs no collection of urine. The method involves determination of serum creatinine levels Since creatinine production varies with age, weight and gender, different formulae are used to calculate creatinine clearance from the serum creatinine values.

For Children (between I to 20 years).

.C1ct = 0.48 H [w/70]0.7

  Scr

for adults above 20 years :

Males,	(140- Age) W                   72 Scr
Females.	(140 — Age)W                   85Scr
	= 0.9.Clcr of Male
where. CLcr = creatinine clearance in ml/min. Scr= serum creatinine in mg%, H = height in ems, and W = weight in Kg.

Age is measured in years.

A direct method for determining creatinine clearance is determination of the amount of creatinine excreted in urine in 24 hours (to calculate the rate of creatinine excretion) and the mean of serum creatinine from blood samples taken just before and immediately after the urine collection period. Following formula is used:

ClR =Rate of creatinine excretion ÷ Serum creatinine in mg %

The normal creatinine clearance value is 120 to 130 ml/min. A value of 20 to 50 ml/min denotes moderate renal failure and values below 10 ml/min indicate severe renal impairment.

The renal function, RF calculated by equation :

RF = clearance of a patient ÷ creatinine clearance of a normal person