Calculate renal clearance rates and relate them to kidney function

Use the unit conversion method to solve these problems.

1. Ms. B came into the hospital after losing a lot of blood in a motorcycle accident. After her condition was stabilized she was given a creatinine clearance test to estimate her glomerular filtration rate. Creatinine is a substance that is filtered into the urine but neither reabsorbed nor secreted, so you can tell how much blood was filtered by how much creatinine goes out in the urine.

Plasma creatinine levels = 0.1 mg creatinine/mL blood

Urine creatinine levels = 16 mg creatinine/mL urine

Urine production = 0.5 mL urine/ min

What was her creatinine clearance rate (mL blood /min )? You should be able to solve this in one step by lining up the conversion factors to cancel out all except the units you want.

Are her kidneys working properly? Explain how you reached your conclusion.

2. Ms. B was given several blood transfusions and the next day the results of her creatinine test were:

Plasma creatinine levels = 0.2 mg creatinine/mL blood

Urine creatinine levels = 1.2 mg creatinine/mL urine

Urine production = 5 mL urine/ min

What was her creatinine clearance rate (mL blood /min )?

Are her kidneys working properly? Explain how you reached your conclusion.

You can calculate clearance rates of any substance, if you measure how much of it is in the blood and the urine. We usually don’t bother to do this, because most substances are not simply filtered into the urine. If a substance is secreted into the urine, its clearance rate will look as if more blood was filtered than the kidney actually filtered; if the substance is reabsorbed, its clearance rate will look as if very little blood was filtered.

3. At the same time as questions 2, Ms. B’s urine urea and glucose levels were taken. Calculate her urea and glucose clearance rates, in mL blood/min.

Plasma urea levels = 0.2 mg urea/mL blood

Urine urea levels = 2 mg urea/mL urine

Plasma glucose levels = 80 mg glucose/100 mL blood

Urine glucose levels = 0.4 mg glucose/mL urine

Urine production = 5 mL urine/ min

Which compound is being secreted into the urine? Which is being reabsorbed from the urine? Explain how you know.

4. Ms. B had some fluctuations in blood pressure during her stay in the hospital. Two sets of measurements were:

	Day 6	Day 8
Blood pressure	90/50	125/80
Plasma creatinine (mg/mL)	0.2	0.1
Urine creatinine (mg/mL)	24	5
Urine production (mL/min)	0.5	3
Plasma K+ (mEq/mL)	3 x 10^-3	3 x 10^-3
Urine K+ (mEq/mL)	1.44	0.1

What was her glomerular filtration rate on each day? Explain why it might have been different on the two days.

What was her K+ clearance on each day? On which day was she secreting more K+?

What do you think her aldosterone levels were like on each day? Why?