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Let’s start our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.
Play USMLE Audio MP3 59 03 Clearance of Inulin Below
Begin 59 03 Clearance of Inulin Transcription
The renal clearance of what nontoxic substance is used to measure the glomerular filtration rate?
What is inulin?
- Inulin is a fructose polysaccharide.
Alphabetically, list the three main reasons for using inulin to measure the GFR.
First, freely filtered. Second, not reabsorbed. Third, not secreted.
What are two more reasons for using inulin?
- It is biologically inert and not metabolized.
Using the same equation we used before, how is the clearance of inulin calculated?
- Inulin clearance equals the concentration of inulin in the urine times the volume of urine passed per unit time and then that product divided by the concentration of inulin in the plasma.
And what does the clearance of inulin equal?
- The clearance of inulin equals the glomerular filtration rate. Clinically the 24 hour clearance of another substance is used to estimate the GFR.
What is it?
- Creatinine. Two reasons for using creatinine clearance to estimate the GFR have to do with reabsorption and secretion in the renal tubules.
What are these two reasons?
- Creatinine isn’t reabsorbed and is secreted only in small amounts.
And what’s another good reason for using creatinine rather than inulin?
- Creatinine occurs naturally in the body.
How does creatinine clearance compare to the GFR?
- It is slightly larger than the GFR.
- Because of the secretion.
Is serum creatinine concentration also a good estimate of the GFR and renal function?
- No, it’s not.
What can the use of serum creatinine concentration to estimate the GFR lead to?
- Drug overdose.
What can the renal clearance of para-aminohippuric acid, PAH, be used to estimate?
- The clearance of PAH is almost equal to renal plasma flow.
By what percent does PAH clearance underestimate renal plasma flow?
- 10 to 15%.
- Because 10 to 15% of total renal plasma flow perfuses nontubular portions of the kidneys.
Student doctor, please pause the tape and summarize what we’ve discussed about inulin, creatinine, and para-aminohippuric acid clearance.
The clearance of the nontoxic fructo-polysaccharide inulin is used to measure the GFR. The main reasons that inulin is used to measure the GFR are because it is freely filtered, not reabsorbed, and not secreted by the renal tubules. Two other reasons for using inulin are that it is biologically inert and not metabolized. Inulin clearance equals the concentration of inulin in the urine times the volume of urine passed per unit time and then that product divided by the concentration of inulin in plasma. The clearance of inulin equals the GFR. Clinically, creatinine clearance is used to estimate the GFR because it occurs naturally in the body. It is not reabsorbed and it is secreted only in small amounts. Creatinine clearance is a little larger than the GFR because of the secretion. Using creatinine serum concentration to estimate the GFR can lead to drug overdose. The renal clearance of para-aminohippuric acid, PAH, can be used to estimate renal plasma flow, although it underestimates renal plasma flow by 10 to 15%. This is because that portion of total renal plasma flow perfuses nontubular parts of the kidneys.
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