L3 Clearance

Question 1

Why measure GFR

Answer 1

Gives rough measure of number of function nephrons

Decreased GFR sign of renal disease

Reduction in GFR may indicate a disease progression or development of reversible problem

Grading of chronic renal insufficient and dosages of drugs secreted by kidneys are based on GFR..decrease dose if pt has limited ability to excrete

Question 2

Law of conservation of mass

Answer 2

If a material is neither reabsorbed or secreted by the nephron then what goes in must come out

Polysaccharide inulin is often used to measure GFR (inulin clearance = GFR)

GFR PX = UX V/PX

Question 3

Perfect GFR marker

Answer 3

Freely filtered but neither reabsorbed nor secreted

Inulin clearance is gold standard but it is not commonly used in clinical settings- have to radio label it and maintain stable plasma conc

Creatinine clearance and plasma creatinine conc are frequently used as indicators of GFR in clinical practice

Other techniques are available (ie clearance of EDTA, iothalamate, etc)

Question 4

Creatinine produced by

Answer 4

Skeletal muscle at relatively constant rate from breakdown of creatine phosphate

It’s freely filtered, not reabsorbed and is secreted only slightly

Is already at steady state conc in blood, if formation and secretion are arable

Blood creatinine levels can provide an estimate of GFR over the preceding hrs

Question 5

Creatinine to estimate GFR in two ways

Answer 5

CCr = (UCr)(V)/PCr ~GFR

or

Use PCr to estimate GFR using a standard predictive equation

Question 6

Don’t have values for UCr or V

Answer 6

Can use PCr by itself

GFR cut in 1/2, PCr doubles

GFR cut in 1/4, PCr quadruples

Plasma creatinine two times high? Maybe GFR is reduced by 1/2

Question 7

Algorithms for estimating GFR based on plasma creatinine

Answer 7

Cockcroft-gault equation
Age, weight, sex, and PCr

MDRD4
Age, sex, ethnicity, and PCr

CKD-EPI
Age, sex, ethnicity, and PCr

Question 8

Clearance represents

Answer 8

The volume of plasma that is cleared of solute x per minute

Question 9

Clearance of a secreted substance will be

Answer 9

Greater than CCr

Thus greater than GFR

Clearance > GFR implies secreted

Question 10

Clearance of reabsorbed substance will be

Answer 10

Less than CCr

And less than GFR

Clearance < GFR implies reabsorption

Ex: glucose, complete reabsorption, clearance is 0

Question 11

Clearance ratios

Answer 11

Compare clearance of any substance with inulin clearance ( can be creatinine too)

Clearance ratio= Cx/Cinulin

= 1 substance must also be a GFR marker (filtered but not secreted or reabsorbed)

<1 either the substance is not filtered or it is filtered and reabsorbed

> 1 substance is filtered and secreted

Question 12

Clearance of para-aminohippurate PAH

Answer 12

PAH in plasma 
Some PAH is filtered 
All of the remaining PAH is secreted from peritubular cap 
No PAH left in plasma 
All PAH is excreted

Since ALL blood is cleared of PAH, CPAH is an estimate of RPF

Question 13

Clearance of PAH and RPF

Answer 13

90% of PAH is cleared and represents effective RPF to nephrons

10% blood flow of kidney goes to tissue that don’t have nephrons and that PAH remains