Nephrologists seem to love ratios. The urinary albumin-to-creatinine ratio (UACR) and the urinary protein-to-creatinine ratio (UPCR) are discussed in Chapter 4. And fractional excretions, discussed in Chapter 3, are essentially a ratio of ratios, that is, the ratio of the urine-to-plasma concentration ratio of the substance of interest to the urine-to-plasma creatinine concentration ratio.

What about the urine-to-plasma creatinine ratio (U/P Cr)? It is the denominator of the fractional excretion of sodium (FENa) formula, so why do we need it if we have FENa? The reason is that the U/P Cr is a measure of water reabsorption/excretion whereas FENa reflects sodium reabsorption/excretion. Also sometimes you may have only a urine creatinine (UCr) but no urine sodium (UNa), since someone has ordered a UACR or UPCR.

Most healthy individuals are in an antidiuretic state most of the time (so they are not spending too much time going to the bathroom!). A U/P Cr of 100/1 is thus typical. How is the kidney able to concentrate creatinine 100-fold? There are only two possible ways that the kidney could accomplish this task. Either it could be secreting creatinine against a huge concentration gradient (which does not happen!) or it is reabsorbing 99% of the filtered water (which does happen). Many trainees initially suggest the first mechanism and not the correct one when asked this question.

Here is another more colloquial way to teach the concept. Your grandmother asks you to make her a pot of tea. You do so, but after she pours herself a cup and takes one sip, she returns the pot to you and states,
“This tastes like water—make it 100 times stronger!” Now you could add 100 times more tea to the pot, but there is no more tea in the house. So instead you boil off 99% of the water, which makes grandmother happy again (albeit with a much smaller cup of tea).

Some like to convert the U/P Cr to fractional excretion of water (FEH₂O).

FEH₂O = H₂O excreted/H₂O filtered = Urine flow/GFR = V/GFR = V/[(UCr × V)/PCr] = P/U Cr × 100

So, if the U/P Cr is 100, the P/U Cr is 0.01 and FEH₂O is 1%, which is the same thing as saying that 99% of the water has been reabsorbed. If the U/P Cr is 40, the FEH₂O is 2.5% or 97.5% of the water has been reabsorbed.

Let us take the example of a healthy male medical student weighing 72 kg who excretes 20 mg/kg of creatinine in the urine in 24 hours. His 24-hour UCr excretion is thus 1,440 mg. If he makes 1,440 mL of urine a day, his UCr concentration will be 100 mg/dL. Assuming his serum creatinine is 1 mg/dL, he will have a U/P Cr of 100/1 or FEH₂O of 1%. Now let us assume that he has a big test coming up and has to study for 24 hours straight and has no time to eat or drink. He is now in a marked antidiuretic state and his daily urine volume has decreased to 720 mL. If his serum creatinine is still 1 mg/dL, his U/P Cr will be 200/1 or FEH₂O of 0.5%. After the big test, he goes on a 24-hour drinking binge at the local pub. His urine volume now quadruples to 5,760 mL/24 hours. Now his U/P Cr is 25/1 or FEH₂O of 4%.