Problem Set Key Info

Question 1

Eqn for clearance

Answer 1

V*(Uosm/Posm)

Question 2

What is creatinine?

T

Answer 2

Creatinine is derived from phosphocreatine, a product of muscle metabolism. Creatinine production is quantitatively related to muscle mass and, in the absence of cellular injury or altered metabolism of muscle, remains relatively constant.

Question 3

If there has been no change in muscle mass (creatinine production), then a change in serum creatinine concentration is due to what?

Answer 3

A change in GFR.

This allows serial monitoring of GFR by measuring serum creatinine concentration and body weight/physical examination at clinic visits without the need for repeated 24-hour urine collections/creatinine clearance determinations.

Question 4

What conclusions concerning total body exchangeable sodium can be drawn from the serum sodium concentration alone?

Answer 4

none

Question 5

When is BUN increased?

Answer 5

Urea reabsorption by the renal tubules is increased in association with increased water reabsorption in volume depleted patients.

This increase in urea reabsorption reduces urea clearance, resulting in an increase in BUN.

Question 6

So how does BUN compare to creatinine in volume depletion?

Answer 6

Creatinine clearance is not altered in the same manner. Therefore, volume depletion is often associated with an increase in BUN which is disproportionate to the increase in serum creatinine concentration.