Renal Function Tests: GFR, Renal Plasma Flow, and serum maker

Let’s talk about renal function tests. Including GFR, renal plasma flow and the serum marker. Determination of GFR. By clearance of endogenous creatinine. Or by clearance of exogenous chemical substances. Such as inulin, iothalamate, technetium DTPA or chromium 51 EDTA. GFR by creatinine clearance. This is the formula that is similar to what we presented earlier and familiar to you. Here U is the drug concentration in the 24-hour urine. V is the urine volume. P is the creatinine concentration in the morning fasting plasma or serum. And the product of U and V actually represent excretion rate of creatinine. Now this formula may underestimate creatinine clearance. Because of the undercollection of urine particularly in the outpatient situation.

And the run around time or turn around time is about two days. Although you may order stat. GFR by technetium DTPA. This involves the intravenous injection of technetium DTPA. It’s a nucleotide of radionuclide of diethylenetriamine penta-acetic acid. They collect six to eight plasma samples for over a two-hour period. For the determination of area under the curve. We do need a gamma scintillation counter to measure technetium-99 radio activity. And the turn around time is about four to six hours. It is ideal for monitoring change of kidney function after renal transplantation for example. And therefore the clearance is measured by Dose divided by area under the curve. GFR by chromium 51 EDTA. It involves IV injection of a radioisotope, chromium 51-EDTA.

Collecting 6 to 8 plasma samples over a four-hour period. For the determination of area under the curve. And we need a mass spec to measure chromium 51. The turn around time is about 6 hours. And this method has been used to adjust dosing regimen in renal failure patient for drugs that are primary excreted unchanged in the urine. And here the clearance again is determined by dose divided by area under the curve. Renal plasma flow by PAH clearance is another way to look at renal function. And this involves the IV infusion of PAH. PAH is para-amino hippuric acid. PAH is an exogenous organic acid that is filtered and is secreted but not reabsorbed.

And therefore PAH presented to the kidneys is PAH cleared by the kidneys. Hence its clearance represent renal plasma flow. And the clearance is measured by Intravenous infusion rate divided by the steady-state concentration achieved after the intravenous infusion. Renal function by serum markers. There are several serum markers. Serum creatinine, blood urea nitrogen, blood urea nitrogen (BUN) divided by serum creatinine and the Cystatin-C. Let’s look at each and every one of them.