A man\'s leg was crushed between a car bumper and a wall. His physicians believe

Question

A man's leg was crushed between a car bumper and a wall. His physicians believe their patient has suffered kidney damage from myoglobin blocking glomerular pores. Tests showed the following results plasma creatinine: 30 mg/100 mL plasma 24 hour urine specimen: volume = 1 liter urine creatinine 30 mg/mL urine a. How many mg of creatinine are in the urine specimen? How much creatinine appears in the urine per hour? b. What is the patient's creatinine clearance in mL/min? c. What is the patient's GFR? d. Is this a normal GFR? Did the patient sustain kidney damage?

Explanation / Answer

a. Urine Creatinine = 30 mg/mL; volume of 24 hour specimen = 1 L (i.e. 1000 mL)

Therefore, creatinine present in specimen = 30 × 1000 = 30,000 mg

Creatinine appearing per hour = 30000/24 = 1250 mg

b. Clearance = [concentration in urine × rate of urine formation (mL/min)]/concentration in plasma

= [30 × 1000/(24×60)]/(30/100) = 69.44 mL/min

c. There are a number of formulae that relate GFR to creatinine clearance and one or more factors such as age, height, muscle mass, etc. Such information has not been provided in the question. In general, creatinine clearance tends to exceed GFR by 10-20%, i.e. 80-90% of creatinine clearance represents GFR. Hence, we shall calculate GFR (rather estimated GFR or eGFR) based on an average value of 85%.

GFR = creatinine clearance×85% = 69.44×85% = 59.02 mL/min

d. GFR above 60 is considered normal; hence, this (59.02) GFR is just about the threshold for renal damage. The patient may have sustained slight kidney damage.

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